WMO/CEOS REPORT on a STRATEGY for INTEGRATING SATELLITE and GROUND-BASED OBSERVATIONS of OZONE

Transcription

1 WORLD METEOROLOGICAL ORGANIZATION GLOBAL ATMOSPHERE WATCH No. 140 WMO/CEOS REPORT on a STRATEGY for INTEGRATING SATELLITE and GROUND-BASED OBSERVATIONS of OZONE JANUARY 2001

2 WORLD METEOROLOGICAL ORGANIZATION GLOBAL ATMOSPHERE WATCH No. 140 WMO/CEOS REPORT on a STRATEGY for INTEGRATING SATELLITE and GROUND-BASED OBSERVATIONS of OZONE WMO TD No. 1046

3 List of Contents Foreword... iii Executive Summary... v Milestones in the History of Ozone... ix 1. Introduction The IGOS Strategy The Ozone Project Requirements and Data Sources The Objectives of the Report User Requirements Sources of Information and Definitions Relationships between Applications and Requirements The Requirements Available and Planned Measurements Introduction Non-Satellite Measurements Satellite Measurements Harmonisation of Provisions and Requirements Introduction Total Column Ozone Ozone Vertical Profile Meteorological Parameters Related Chemical Constituents Calibration and Validation Introduction Calibration and Validation Approach Algorithms and Radiative Transfer Ground-based Observations Validation of Trace Gases Scientific Analyses Principles and Recommendations for Calibration and Validation Implementation Strategy Recommendations Introduction Algorithms and Calibration Implementation Recommendations for Additional Space-Borne Measurements Advisory Body for the Ozone Project Concluding Remarks i

4 Annex A: Lists of Scientists and Experts Consulted Annex B: Tables of User Requirements Annex C: The Data Records of Regularly Reporting Ground-Based Ozone Stations Annex D: Examples of Airborne Research Campaigns Annex E: Other Space-Based Instruments Annex F: Acronym/Abbreviation List ii

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7 EXECUTIVE SUMMARY Introduction CEOS and WMO recognize the need for better integration of the major satellite and ground-based systems to provide highly accurate, global environmental observation of the atmosphere, cryosphere, oceans and land in a cost effective fashion. To satisfy this objective, a framework for compiling user requirements, coupled with an overarching strategy for making global observations is the goal of the new IGOS (Integrated Global Observing Strategy), set up by a number of international bodies including WMO and Space Agencies. This report is a contribution to the international effort. It proposes the better integration of the various systems used to monitor ozone, including related key atmospheric parameters, and will contribute to the objectives of the IGOS within a general IGOS theme on atmospheric chemistry. This will assure the most effective use of available resources for global observations, although priorities must be established for upgrading existing and/or establishing new systems and provide a framework for decisions to ensure: the long term continuity and spatial comprehensiveness of key observations the research needed to improve understanding of Earth processes so that observations can be properly interpreted. The project will build upon existing and planned international global observation programmes (e.g. METOP, NPOESS, WMO-GAW and NDSC) and identify deficiencies in the current and planned systems. This report and its recommendations were compiled by a collection of clients, space agency representatives and a cross section of experts and specialist in atmospheric research. The list of contributors to this report and their institutions appears in Annex A The Ozone Project aims to develop the foundations of an integrated ozone measurement strategy. This strategy reflects the need to understand variations of ozone in the troposphere and stratosphere because of the central role the gas plays in several major environmental problems: total column ozone is a controlling factor in determining levels of biologically damaging ultraviolet radiation reaching the Earth s surface; ozone is an oxidising pollutant that is harmful to humans, animals and vegetation and degrades man-made materials; ozone is an active component of tropospheric and stratospheric photo-chemistry; ozone is a greenhouse gas that contributes to the Earth s radiative balance. The project covers primarily the observational requirements associated with the "Montreal" Protocol of the Vienna Convention. One of its specific objectives is to document the requirements for observations of ozone and associated parameters needed to properly interpret the ozone observations. These are then reviewed in the light of provisions for data acquisition with the focus on the observing community and the steps needed to meet user requirements. The project recognises the need and existence of appropriate numerical chemical and transport models used to interpret the observations. Grateful acknowledgement must be made to the many scientists and institutions (listed in Annex A) who have contributed to the production of this report both by participating in the workshops and by written contributions. Without this support the production of this report would not have been possible. v

8 Requirements The Ozone Project has compiled a list of user requirements from the scientific community (WMO-GAW, SPARC, IGAC) and existing measurement programmes from space and the ground 1 have been documented. From an analysis of the provisions and requirements, a set of recommendations for establishing an integrated global ozone observing system is proposed. This strategy distinguishes measurements that are needed continuously from those that are only needed occasionally. A well supported and on-going validation programme coupled with a data quality control programme is essential. As data sets improve, planning for the reprocessing and the distribution of data is a major objective. In addition to ozone itself, an array of chemical species and other geophysical parameters must be observed. These include long lived source gases, reservoir species, radicals and several closely associated meteorological variables such as temperature and winds to at least the same spatial and temporal resolution as the gases. Aerosols play an increasing role in the stratosphere and troposphere for chemistry and climate research so their characteristics must also be measured. In addition, the total and spectral solar irradiances must be observed in order to be able to interpret climate and ozone changes. Available and Planned Measurements A broad range of operational and research observations are underway and are planned from both space and the ground. Data from Nimbus, TOMS, SAGE, SBUV/2, UARS, ERS-2, WMO-GAW and NDSC, as well as many aircraft and balloon missions, have led to an improved understanding of relevant atmospheric processes and provided a baseline for assessing needs for future data sets. Research missions such as ENVISAT, EOS-Terra and EOS-Aura, and operational missions such as METOP and NPOESS, will provide platforms to ensure the continuation of baseline measurements though they only partially satisfy the requirements. A major concern is the provision of data in the longer term (after the ENVISAT/EOS-Aura era) when only those from METOP and NPOESS will remain available. TOMS type data sets are assured (though not TOMS itself) through EOS Aura, but there is a potential gap between EOS-Aura and NPOESS until the advent of NPOESS which will continue these measurements. Follow-on SAGE missions are assured although the exact platforms are at this time somewhat uncertain. UV-VIS-NIR backscatter measurements will continue with GOME-2 on METOP. GCOM and follow-on ADEOS will also provide collaborative data from space. The ODIN, ACE and SABRE research missions will compliment the larger research and operational missions. To date chemistry measurements have been made from low Earth orbit, but upcoming missions must take advantage of new strategic orbits such as the geostationary and L1 orbits to observe short term diurnal variations. Ground observations (surface, balloon, and aircraft) must continue and be expanded to provide correlative and validation data for the satellite missions as well as conducting essential research observations. The networks such as NDSC and GAW (e.g. ozone sondes, Dobson/Brewer and in-situ source gas observing stations)need to continue to provide data as part of a better integrated system. Aircraft missions should continue to conduct extensive campaigns to study processes with high spatial resolution. The commercial airlines also have a role in providing platforms for routine observations (e.g. MOZAIC). Calibration and Validation Another major concern is the continuation and consolidation of calibration and validation activities as these are critical to assure the scientific value of observations. They are essential for deriving climate quality data sets. The space faring nations have and must continue to allocate resources for the calibration and validation of Earth science missions. Both Europe and the United States are now planning operational satellite systems that will carry ozone sounders to extend the vi

9 long term record already produced by national research and operational missions. Japan is also committed to fly atmospheric chemistry missions. However, despite the fact that the major space agencies have embarked on these missions, no concurrent long term validation programme is being planned nor is there any assurance that the existing ground-based infrastructure will be in place when it is needed. Satellite systems can only meet the established requirements if they are supported by correlative data of known quality and continually challenged by reliable ground-based observations and quantitative science. Based on the experience gained from past satellite missions, an end-to-end approach for calibration/validation, supported by a fully integrated global observing system including both ground and space-based elements, must be established. For satellites this approach includes the internal calibration programmes, post-launch calibration employing on-board systems, external validation programs using highly controlled correlative measurements, subsequent algorithm refinements and scientific analyses of the data to ensure consistency with the best understanding of atmospheric processes and conditions. This is of particular importance given the existence of parallel streams of the national missions, e.g. the European METOP and the US NPOESS ozone instruments. Recommendations As discussed above, many of the identified requirements will be met by the existing and planned measurements from ground and space. However, there remains the problem of a lack of formal co-ordination among the space faring nations to optimise the deployed systems and to assure compatibility for international users. In addition, there must be formal recognition and support for the international community who are providing critical data from ground-based systems for the calibration and validation of the space-borne systems. The recommendations contained in the report (Chapter 6) make specific proposals for remedying the missing components of the upcoming systems. They also describe improvements that are required in existing systems and current procedures. The following is a summary of these recommendations: Establish a co-ordinated validation activity that extends over the entire lifetime of satellite sensors that encompasses all elements of the IGOS system and takes maximum advantage of concurrent national validation activities. Extend the coverage of ground-based (WMO-GAW and NDSC) systems particularly in the tropics and the Southern Hemisphere and designate a carefully selected subset thereof as permanent, long term ground "truthing" facilities. The space agencies that require validation data must provide sustained support for the ground networks to insure data availability and quality. Improve and/or provide additional measurements resulting from a survey of existing and planned measurements. There is a particular need for measurements in the lower stratosphere and troposphere. The validation process is iterative and resources for reprocessing data must be made available to ensure that users have access to the highest quality data. Standardise data formats and encourage the synergistic use of data supported by accessible archives and proper provision for reprocessing. Improve national radiometric standards and sensitise the user community to calibration issues. Encourage international co-operation in the development of algorithms employed by similar instruments and pool knowledge of radiative transfer physics. Establish a body of scientists, engineers and managers to provide technical support to funding agencies to ensure compatibility and completeness of the systems. vii

10 There is also a practical incentive for swift action. Several satellite missions with ozone instruments on board are scheduled for launch during this decade. The recommendations in this report attempt to co-ordinate these missions and to remedy those areas that remain deficient in the present and planned observing systems. Data collected following this approach will have the necessary quality to enable the state of the atmosphere to be reliably monitored and changes understood, thereby providing a basis for formulating sound environmental policies. viii

11 MILESTONES IN THE HISTORY OF OZONE 1839 Discovery of the ozone as a permanent atmospheric trace gas by C.F. Schonbein Surface ozone started to be measured at hundreds of locations Strong absorption band of solar radiation between 200 and 320 nm attributed to upperatmosphere ozone by Hartley Proof from UV measurements that most ozone is located in the stratosphere First quantitative measurements of the total ozone content Six Dobson ozone spectrophotometers are distributed around the world for regular total ozone column measurements The Umkehr method for vertical ozone distribution is discovered and determines the ozone maximum is lower than 25 km Photochemical theory of stratospheric ozone formation and destruction based on chemistry of pure oxygen Ozone sonde on balloon confirms maximum concentration at about 20 km Global network of ozone stations proposed for the International Geophysical Year (IGY) WMO establishes standard operating procedures for uniform ground-based ozone observations and the Global Ozone Observing System (GOOS) established First ever satellite for total ozone measurement launched by US Department of Defense Photochemical theory of ozone with destruction by HO x radicals First total ozone measurements from satellites Ozone destruction by NO x mechanism proposed First consideration of CIO x chemistry as an ozone-destroying mechanism Human-produced CFCs recognized as source of stratospheric chlorine WMO conducts first international assessment of the state of global ozone Plan of Action on Ozone Layer established by UNEP in collaboration with WMO NASA s Nimbus-7 launched carrying ozone and other atmospheric instruments Scientific assessments of the state of the ozone layer issued in 1981, 1985, 1988, 1991, 1994, and 1998 by WMO in collaboration with UNEP and national research agencies The US s NOAA commits to operational stratospheric ozone monitoring on polar orbiting satellites (POESS followed by NPOESS) NASA-SAGE I: Stratospheric ozone profile measurements through solar occultation Unusually low (-200 m atm cm) total ozone at Syowa, Antarctica, in October 1982, first reported at the Ozone Commission Symposium in Halkidiki, but its significance was recognized only the next year Vienna Convention for the Protection of the Ozone Layer concluded and data from Halley station on the existence of an ozone hole during Antarctic springs since the early 1980s published by the British Antarctic Survey NASA s Nimbus-7 TOMS maps Antarctic ozone whole which covers millon square kilometers 1983 Analysis of Montsouris (Paris) surface ozone ( ) indicates levels then were less than half of the present. ix

12 1984 Montreal Protocol on substances that deplete the ozone layer concluded under UNEP auspices and basic assessment of the state of the ozone initiated by the International Ozone Trends Panel Decrease of ozone concentrations by 10 percent per decade in the lower stratosphere documented; proof from NASA Antarctic Campaign that active chlorine and bromine byproducts of human activities are the cause of the Antarctic-spring ozone hole London amendment to strengthen the Montreal Protocol by phasing out all CFC production and consumption by The WMO/UNEP Ozone Assessment 1991 reveals ozone is declining not only in winter-spring, but all year round and everywhere except over the tropics; very large concentrations of CIO measured in the Arctic confirms concerns for potential stronger ozone decline NASA s Upper Atmospheric Research Satellite launched 1991 Quantified global and seasonal column ozone trends from TOMS Copenhagen amendment further strengthened Montreal Protocol by phasing out CFCs by the end of 1995, adding controls on other compounds Extremely low ozone values (-100 m atm cm) during Antarctic spring and largest area 24 m km 2 covered; also the lowest ever ozone values measured during the northern winter-spring seasons indicates increasing destructive capability by increasing chlorine and bromine concentrations in the stratosphere WMO/SPARC/IOC/GAW assessment of trends in the vertical distribution of ozone using SAGE, balloon, and umkehr data Europe s Eumetsat commits to operational ozone monitoring 1995 Nobel Prize for work on catalytic chemical destruction of ozone by Molina, Rowland, and Crutzen 1995 European Space Agency launches first mapping hyperspectral instrument (GOME) on ERS-2 to measure atmospheric composition 1995 Record low ozone values (exceeding 25 percent below long-term average) observed January to March over Siberia and a large part of Europe Complete ban on industrial production of CFCs 1996 Japan launches the ADEOS series and plans follow on GCOM missions to measure ozone and atmospheric chemistry 1996 CEOS initiated IGOS The Ozone Project as one of six pilot projects 1997 First Limb-scatter measurements of ozone throughout the Stratosphere from Space Shuttle Upper Atmospheric Research Satellite measured chlorine amounts in upper stratosphere leveling off resulting from Montreal and follow on protocols 2000 WMO/CEOS Report on a Strategy for Integrating Satellite and Ground Based Observations of Ozone x

13 1.1 The IGOS Strategy 1. INTRODUCTION The IGOS (Integrated Global Observing Strategy) is intended to combine data from major satellite, airborne and ground-based systems to provide global environmental observations of the atmosphere, the cryosphere, the oceans and the land in a cost effective fashion. A fundamental issue for IGOS is the identification of what it can contribute that cannot be achieved through existing national and international mechanisms. In short the added value of IGOS has to be demonstrated. To satisfy this objective IGOS must provide a framework for the formulation of a coherent set of user requirements to which providers can respond. It must formulate an overarching strategy for global observations, allowing those involved in their collection to improve their contributions and to make better decisions on the allocation of resources to meet priorities, taking advantage of better international collaboration and co-ordination. To facilitate the most effective use of available resources for global observations, priorities need to be established for upgrading existing and/or establishing new systems. IGOS must therefore provide a framework for decisions intended to ensure: the long term continuity and spatial comprehensiveness of key observations; the scientific research needed to improve understanding of Earth processes so that observations can be properly interpreted. It must build upon the strategies of existing international global observation programmes focusing additional efforts in areas where satisfactory international arrangements and structures do not currently exist. It should aim to exploit international structures that successfully contribute to current provision of global observations, rather than create a new centralised decision making organisation. The unnecessary duplication of observations must be avoided. IGOS is intended to help provide governments with improved understanding of the need for global observations and the deficiencies of current systems. Allied with this, opportunities must be identified for capacity building, assisting countries to obtain the maximum benefit from the total set of available observations. Situations where existing international arrangements for the management and distribution of key global observations and products could be improved must be identified. IGOS also seeks to stimulate the creation of improved high level products by facilitating the integration of multiple data sets from different agencies and national and international organisations. It assists the transition of systems from research to operational status through improved international co-operation. In striving to respond to these principles, contributions to IGOS should help ensure: the long term continuity of measurements of key variables; adequate archiving and access capability for all data sets; consistency of data quality even when there are disturbances in the data record, e.g. due to new technology; an active and co-operative validation programme extending over the entire life of the satellite sensor or measurement system to ensure the integrity of the space-borne data: sufficient ancillary data to enable users to judge the data quality and to properly interpret the results. Within this overall context the Committee for Earth Observation Satellites (CEOS) decided to establish six Pilot Projects to assess the feasibility of achieving the objectives of IGOS. One of 1

14 these was the Ozone Project which is the subject of this report and which includes observational requirements arising out of the Montreal Protocol of the Vienna Ozone Convention. 1.2 The Ozone Project Knowledge of the amount and distribution of ozone (and changes in total levels) in the Earth s troposphere and stratosphere is important because of the central role ozone plays in several important environmental problems: First, ozone, through its absorption and emission of solar and terrestrial radiation, contributes significantly to atmospheric temperature structure and the radiative forcing of the troposphere-stratosphere system. Secondly, the total column amount of ozone in the atmosphere is a major factor in determining the amounts of biologically damaging ultraviolet radiation that reach the Earth s surface, as well as the photochemistry of the troposphere. Thirdly, near the Earth s surface ozone is an oxidising pollutant which is harmful to humans, animals and vegetation as well as contributing to the degradation of manmade materials. As such it influences much of the photochemistry that occurs in the troposphere. Knowledge of the distribution of ozone is also important to the operational meteorological community both through its role as a contributor to the Earth s radiative balance and through its use as a motion tracer. Advances in meteorological modelling are demonstrating that the inclusion of ozone can lead to improved weather and climate forecasts and, as a result, ozone is beginning to be assimilated in meteorological models. Operational agencies are also increasingly being asked to predict levels of ultraviolet radiation reaching the surface; knowledge of ozone amounts is essential for this purpose. Changes in the distribution of ozone in response to human activity have been anticipated for some time and over the past decades such changes have actually been observed. Figure 1 (from NASA's Goddard Space Flight Center) illustrates predicted and TOMS measured global ozone trends. Predictions indicate a recovery in the near future, however these must be confirmed with measurements with TOMS-like precision. The predicted changes in ozone distributions are due to several factors. Emissions of industrially-produced chlorine (Cl) and bromine (Br) containing molecules into the atmosphere lead to destruction of ozone in the stratosphere due to the catalytic properties of chlorine and bromine. In addition, there are natural sources of bromine in polar regions that may also contribute to the catalytic destruction of ozone. Emissions of nitrogen oxides, hydrocarbons, and carbon monoxide change the photochemistry of ozone in the troposphere and increased emissions of these species, associated with human activity (burning of fossel fuels and biomass), have led to increases in tropospheric ozone amounts. Evolutions in climate also have the potential to change both tropospheric and stratospheric ozone in ways that are complex and not yet well understood. In addition to the most spectacular such effect, namely the seasonal decrease in total ozone which takes place over Antarctica every spring (with the near-total removal of ozone in some altitudes), there has been a gradual decrease in total ozone amounts over much of the midlatitudes. Most recently, there have been some significant instances of late winter/spring-time ozone depletion in the Arctic (most markedly in the winter of ). Satellites and balloons have shown that while most of this decrease has taken place in the lower stratosphere, there have also been some important decreases in ozone levels in the upper stratosphere. Figure 2 (from KNMI) illustrates very low ozone amounts over high latitudes of the Northern Hemisphere during for April1997, as observed by GOME. Normally ozone near the pole reaches a maximum value at this time 2

15 Figure 1: Measured and Predicted Ozone Trends (Courtesy, Goddard Space Flight Center) Figure 2: Northern Hemisphere Assimilated Total Ozone (Courtesy, Royal Dutch Meteorological Institute) 3

16 Most of these changes have been attributed to long term increases in the concentrations of halogen-containing source gases whose breakdown products can destroy ozone through rapid catalytic processes. The decrease in ozone amounts in the lower stratosphere coupled with increases in greenhouse gases, have led to small but significant decreases in temperatures in the lower stratosphere over much of the Earth. These are amongst the most significant temperature changes that have been attributed to human activity. The ozone-temperature linkage in the stratosphere is therefore a critical one and detailed understanding of the feedback between changes in these two quantities is a priority. At the same time that ozone levels have been decreasing in much of the stratosphere, there has been an increase in ozone amounts in much of the troposphere stemming partly from an increase in combustion activities at the surface of the Earth, including both fossil fuel combustion and biomass burning. However, tropospheric ozone concentrations are much more variable in composition than the stratosphere with some regions showing increases in ozone levels while others do not. Increases in tropospheric ozone, especially in the radiatively important upper troposphere, can have a significant impact on radiative forcing and must therefore be considered in studies of climate forcing and atmospheric response. Figure 3 (from the Harvard University GEOS-CHEM model) illustrates calculations of monthly mean afternoon surface ozone concentrations (1-4 p.m.) in July. Particularly noteworthy is pollution over industrial areas in the US, Europe and Asia, with enhancements in Asia due to burning. Satellite observations are needed to provide a global perspective of regional to intercontinental transport of pollution phenomena. Figure 3: Modelled Global Distribution of Surface Ozone (Courtesy, Harvard University) 4

17 A problem peculiar to the tropopause region is that of emissions from aviation. Aircraft emit particles and gases affecting ozone, methane, and cloudiness. The emissions from aircraft are released directly into the free troposphere and lower stratosphere. At present the impact of NO x emissions on ozone formation near the tropopause and methane reduction and subsequent climate effects can be quantified only with large uncertainties. In addition, the chemical and radiative effects of contrails and cirrus clouds, and the role of water vapour emissions in the stratosphere are far from being understood. The relative climate impact of these emissions compared to that of CO 2 has to be determined as prediction of the impact of aviation is currently limited by the general understanding of air chemistry, cloud physics and related processes. Changes in the ozone profiles (in both the stratosphere and troposphere) can also have implications for global tropospheric chemistry because changes in levels of stratospheric ozone can affect the ultraviolet radiation flux into the troposphere which, together with ozone itself, is responsible for much of the photochemistry that takes place in this region of the atmosphere. In part, this photochemistry produces hydroxyl, a free radical that initiates the decomposition of many trace gases in the atmosphere as well as the formation of some types of aerosol particles. In parallel to these changes in ozone amounts and distribution, there are evolutions in the physical state of the atmosphere. The increases in carbon dioxide and other radiatively active gases are altering the temperature structure of the atmosphere which, over the longer term, may be associated with significant changes in the nature of the meteorological processes that occur in the troposphere, as well as in the properties of the tropopause region and the dynamical coupling between the troposphere and stratosphere. These can affect the transport of energy and momentum within the entire global atmospheric system together with the transport of ozone, its photochemical precursors and the agents of its catalytic destruction. Temperature changes will also directly affect the rates of chemical reactions involved in ozone photochemistry. Any long term changes in the structure and dynamics of the tropopause regions could have large impacts on the distribution of ozone in the stratosphere by changing, for instance, stratospheric water vapour amounts, formation conditions for PSCs (polar stratospheric clouds) and/or aerosols and the forcing of large scale stratospheric waves from the troposphere. Changes in the region of the tropopause will also affect levels of tropospheric ozone as the flux of ozone across the tropopause (from the stratosphere to the troposphere) is a major source of ozone to the troposphere. The ability of the scientific community to understand the observed changes in ozone and predict future evolutions, especially in the context of an atmosphere whose physical state is changing due to climate change, is critically dependent on the availability of comprehensive models capable of properly simulating both the chemical and physical evolution of the atmosphere and the linkages between the two. The further development of these models draws on both advances in modelling capability and their critical evaluation and validation. For this the provision of the broad range of representative and reliable observational data, considered in this report, is essential. 1.3 Requirements and Data Sources General Requirements It is clear that knowledge of ozone concentrations and its distribution is of fundamental importance given the pivotal role ozone plays in the climate system. Human-induced changes in ozone levels combine to make the accurate long term measurement of ozone a priority for policy makers as well as for the scientific and environmental communities. This places strict demands on measurement systems as they have to be capable of characterising long term trends in the presence of the very large variability that exists on several temporal scales. 5

18 These include diurnal cycles, day-to-day meteorological, seasonal and inter-annual (quasibiennial oscillation, El Niño/Southern Oscillation, North Atlantic Oscillation) variability as well as the 11-year solar cycle and sporadic events such as volcanic eruptions and solar proton events. It must also be possible to accurately differentiate between changes in ozone and those of other atmospheric parameters (e.g., temperature, aerosol loading) that may affect its direct measurement or retrieval via remote sensing techniques. Furthermore, full global coverage is essential so the measurement system addressing these needs must be able to observe from the tropics to the poles. Moreover, as changes in the stratosphere and troposphere may be quite different (indeed of opposite signs), the accurate characterisation of both regions as well as their combined effect, is essential. To interpret observed changes in ozone it is not enough to measure ozone alone. In addition to ozone itself, several atmospheric chemical species, meteorological (including aerosol) and solar parameters must also be observed. Without such information it will be difficult to understand why observed changes are taking place, making it impossible to forecast future developments and hence to assess the effectiveness of proposed (or current) control measures. For some applications, such as the prediction of the levels of ultraviolet radiation at the Earth's surface, the use of ozone data will be inadequate unless accompanied by knowledge of other quantities such as the distribution of aerosols, clouds and their respective optical properties. This means that three general groups of parameters will have to be measured, namely ozone itself, several closely associated meteorological variables and a number of chemical parameters. These are summarised in Tables 1.1 and 1.2 which list the various geophysical variables separated according to the above criteria. Table 1.1 also indicates whether they are observed by current systems, classifying them into one of three subgroups, namely: source gases - species having long lifetimes; typically produced by biological and/or industrial processes at the Earth's surface; reservoir species - species having intermediate lifetimes; typically formed in the atmosphere as a result of the breakdown of source gases, although some are directly emitted from the Earth's surface; free radicals - species having unpaired electrons and short lifetimes; often formed photochemically from source gases or reservoir species. In addition, meteorological information (such as temperature and winds) is needed to set the observations into a proper context and, in some cases, for inclusion in the algorithms used to derive concentrations of trace constituents from observed radiances. In compiling the lists of user requirements for observations of chemical species throughout the atmosphere it is important to recognise the breakdown between the different classes as summarised in these two tables. A distinction is made between parameters whose distributions need to be measured regularly over long periods of time over a broad range of geophysical conditions (Table 1.1), and those whose concentrations only need to be measured on either a limited number of occasions (though over a similarly broad range of geophysical conditions) or regularly but at a limited number of locations (Table 1.2). As far as this report is concerned the former are classified as being of primary importance and are the only ones considered further in this document. In this report, carbon dioxide and other greenhouse gases were considered only with regard to their direct or indirect relevance to ozone so there is no detailed discussion of measurement requirements arising as a consequence of the Kyoto Protocol. 6

19 Table 1.1: Parameters which must be observed regularly over long periods of time over a broad range of geophysical conditions PARAMETER CLASS SURFACE TOTAL COLUMN LOWER TROP. UPPER TROP. LOWER STRAT. UPPER STRAT. & MESO. AVAILABLE MEASUREMENT PLATFORM GBIS GBC GBP BBIS SBC SBP O 3 Mon./Trends A A A A A A P P P P P P O 3 Oper. Met. A A A P P P P O 3 Air Quality A N N P S O 3 UV Forecasts A A S P S P S Temp. Met. Variable A A A A A P S P P Wind Met. Variable A A P S P Tropopause Met. Variable A A P S Cloud Tops Met. Variable A A A P S P H 2 O Source Gas A A A A A A P S P P P N 2 O Source Gas A A A P S S S CH 4 Source Gas A A N N A A P S S CO Source Gas A A A A P S S P CO 2 Source Gas A N P S HCl Reservoir A A A P P HNO 3 * Reservoir A A A S P BrO Free Radical N N S P P S ClO Free Radical N A A S S P P NO 2 Free Radical A A N N A A P P S P P NO * Free Radical A A N N P P P Aerosol Pres. Met. Variable A A A A A P P P P P Aerosol Char. Met. Variable A N N A S S P S P PSCs Met. Variable A P P S S P UV Met. Variable A A S P S P Note * - not all of these are required everywhere; in some situations only one or two of them may be needed Key A = available N = needed GBIS = ground-based in-situ BBIS = balloon-based in-situ SBC = space-based column P = primary role S = supporting role GBP = ground-based profile GBC = ground-based column SBP = space-based profil 7

20 Table 1.2: Atmospheric trace species that only need to observed on either a limited number of occasions (though over a similarly broad range of geophysical conditions) or regularly but at a limited number of locations. CLASSIFICATION TRACE SPECIES Source Gases CFC-11, CFC-12, CFC-22, CH 3 Cl, CH 3 Br, H1201, H1311, CF 4, SF 6, Reservoir HBr, ClONO 2, HOCl, OClO, H 2 O 2 Free Radicals OH, HO 2, NO Data Sources To characterise the distribution of ozone and the associated parameters that affect it (listed in Table 1.1), as well as their short and long term variations, the capabilities of groundbased, in-situ, airborne and space-based systems all have to be exploited. Each type of platform should make an unique and complementary contribution to the overall data requirements. Thus, surface-based in-situ systems observe the concentrations of long-lived source gases, whose concentrations help drive both the chemistry and the radiative forcing of the atmosphere. Surfacebased remote sensing instruments can provide (often to very high accuracy and long-term stability) estimates of column amounts (and in some cases vertical profiles) of both industriallyproduced source gases and their breakdown products, as well as of ozone, aerosols and radiation. Balloon-borne instruments, especially ozone sondes, can provide unique, high vertical resolution, information on the distributions of variables from the surface up through to the middle stratosphere. They can also provide data below clouds which cannot be penetrated by most space-based instruments. This is especially important in the tropics where a high tropopause and persistent cloudiness frequently makes significant regions of the troposphere inaccessible to most space-based systems. Airborne systems have similar capabilities though geographic coverage is limited. Generally, space-based systems can provide global coverage, coupled with accurate longterm observations of global distributions of many important parameters, as well as measuring the solar radiation entering and leaving the Earth's atmosphere. These data are especially important over uninhabited regions of the Earth (or over developing countries) where only limited surfacebased data are available. 4-D data assimilation is increasingly being used to optimise or derive global distribution fields for a number of species. More localised, process-oriented observations exploiting comprehensively instrumented balloon-, aircraft- and space-based systems or exploratory satellites are also needed to quantitatively test understanding of the chemical, meteorological and transport processes affecting the distribution of ozone and related parameters. Currently models generally do an adequate job in predicting ozone levels except in the lower stratosphere in the Northern Hemisphere at mid latitudes where ozone is decreasing faster than model predictions. The results of such experiments are important in helping to clarify and reduce requirements for data by extending the capabilities of the models used to forecast future evolutions in ozone levels and reducing their data requirements. The needs of such process studies are not considered in this report as they fall outside the context of the Ozone Project (see next section). 8

21 1.4 The Objectives of the Report To meet the scientific and user requirements in as cost effective and efficient fashion as possible, it is essential to adopt an integrated global observing strategy, as set out in Section 1. This involves the strategic combination of data from all observing systems (i.e. ground-based, space-based etc.; in-situ and remote). To help establish this philosophy the Committee decided to initiate a set of Pilot Projects one of which is the specific concern of this Report, namely the provision of long term observations of ozone. The CEOS therefore mandated a small group of scientists to produce a report on The Long Term Continuity of Ozone Measurements to lay the groundwork for the formulation of a strategy for atmospheric ozone and related parameters. The list of contributors and their institutions can be found in Annex A. It should be noted that two workshops have taken place; one in Tokyo in July 1997 and one in Geneva in May During these meetings experts were asked to clarify requirements and review the capabilities of current observing systems with the aim of highlighting deficiencies and indicating possible courses of remedial action. These are the origin of the various recommendations contained in this report. Noting the specific objectives of the Ozone Project and in line with the arguments presented in the previous section, it was decided to limit the list of variables (in addition to ozone itself) to those strictly required either a) to properly interpret the ozone observations or b) for use in the geophysical algorithms used to retrieve ozone distributions from space-based instruments. Therefore, this is a climatological, as opposed to a process study, oriented project. This limits the list of variables to be considered and hence the scope of the recommendations contained in this report. The need for category b) variables will vary with the measurement technique. Underlying this is an implicit assumption that the contributions of several relevant unmeasured parameters can be calculated from the measured distributions of a relatively small sub-set of parameters. This assumes the existence of appropriate numerical chemical and transport models which must be tested against comprehensive data sets obtained by research oriented balloon, aircraft and/or space-borne missions. A further point to note is that not all the requisite variables need to be observed frequently or globally. Those listed in Table 1.1 generally have to be observed frequently and globally over the long term and quite often information on their vertical distributions is required. They are the focus of this report. For the reservoir and radical species listed in Table 1.2, it could be argued that once the relationship between their distributions and those of their chemical precursors and/or related family members (listed in Table 1.1) is well established (on the basis of observations), regular long term measurements may no longer be required. It is assumed that the requirements to observe the source gases listed in Table 1.2 can basically be met by ground-based systems though satellite observations are required to ensure representative global coverage. It is important to note that a number of the existing programmes have already been specifically designed to make long term observations of ozone and related parameters including: The ground-based Dobson/Brewer/Umkehr network for total ozone and ozone profile measurements, as well as the other surface-based measurements associated with the Global Atmosphere Watch (GAW) network of the World Meteorological Organization The ground-based remote-sensing network of instruments associated with the internationally sponsored Network for Detection of Stratospheric Change (NDSC) Surface-based in-situ sampling associated with several nationally-operated (but globally distributed) programmes (under the umbrella of WMO-GAW) designed to determine surface-level concentrations of long-lived trace gases The balloon-based ozone sonde network of the WMO-GAW and NDSC programmes 9

22 Operational space-based measurement programmes involving mainly the US (TOM, SAGE and NPOESS) and Europe (ERS-2 and METOP), which include both long term measurement programmes and multiple instruments on different platforms sequentially in time. In many instances requirements are likely to be met by access to these existing continuous observing systems. It is also necessary to consider research programmes that are of sufficient duration to be able to contribute to the aims of the Ozone Project. Here a measure of selection is necessary as short term measurements, even if of high quality, cannot be expected to contribute to the long term monitoring of ozone. Those that currently satisfy this selection criteria include ENVISAT and EOS-Aura. In considering the development of a measurement strategy addressing the objectives of the Ozone Project the report recognises that the first priority for the use of the data is for climatological purposes, namely to assess and predict changes the Earth s radiative balance and the amounts of ultraviolet radiation reaching the Earth s surface arising from changes in the concentration and distribution of ozone. The primary concern is the role of ozone as an indicator of the net effect of a complicated set of chemical and dynamical processes, the exact details of which may be changing with time due to human activity. These data also have major applications towards air quality research and monitoring and to meteorological models, especially in the context of the assimilation of ozone in such models. It is recognised that developing interest in these additional uses, especially in meteorological data assimilation, is likely to require that more consideration be given to the implications of this increase, particularly on data continuity and time between observations and availability of processed data. 10

23 2. USER REQUIREMENTS In considering requirements for global observations of ozone and related species, it is important to be specific as the requirements can vary significantly from one set of users to another with regard to spatial coverage, accuracy, etc. User requirements (like the capabilities of any measurement system) vary significantly with height, so it is necessary to link requirements to altitude. In all cases the interest of the user is in end-to-end system performance set in the context of an integrated global observing system. 2.1 Sources of Information and Definitions The requirements presented in this report are derived from those included in the "User's Requirements Data Base" prepared by the World Meteorological Organization and the report of the ad-hoc Global Climate Observing System (GCOS) Atmospheric Chemistry Panel meeting (Toronto, Canada, May 23, 1997). They were reviewed by participants at the initial meeting for the CEOS Ozone Pilot Project held in July, 1997 in Tokyo, Japan and during the Ozone Project Consultative Workshop held in May, 1999 in Geneva, Switzerland. The views of SPARC and IGAC have also had a strong bearing on the compilation of the requirements. Two levels of requirements have been derived for each parameter, namely: The "target" set of requirements - defined as the set of requirements that satisfy the needs of most (if not all) of the user community. The "threshold" set of requirements defined as the minimum set of requirements which satisfy the needs of at least one set of users. A system that did not meet the threshold requirements would be very difficult to justify but, on the other hand, to attempt to fully satisfy the target requirements is often unrealistic. Thus, this report (notably Chapter 4) mainly focuses on threshold requirements. In generating the tables (see Table 2.1 and Annex B) which summarise the requirements great reliance has been placed on "quantitative science", i.e. on measured concentrations, on published trend assessments and on known concentration differences in the vertical and horizontal distribution of the stated parameters. The target values are derived from user observation criteria (as used in atmospheric chemistry, trend analyses, etc...) and substantiated by "local" observations which exploit the best available technology. This means that, based on anticipated performance and target and threshold values, the benefits associated with the deployment of specific systems will be identifiable. Since requirements vary with height, it is logical (albeit a little controversial) to link and thereby generalise them to some broad pressure/altitude regimes, notably: Total Column Lower Troposphere Upper Troposphere Lower Stratosphere Upper Stratosphere and Mesosphere 0 to 5 km 5 km to Tropopause Tropopause to 30 km > 30 km 11

24 Table 2.1: Target and threshold requirements for ozone (O3 ) - greenhouse gas, ultraviolet shield and air pollutant. Target requirements for bias error and RMS error are consistent with trend requirements. Threshold requirements satisfy the needs of at least one user group. REGION HORIZONTAL RESOLUTION (KM) VERTICAL RESOLUTION (KM) RMS ERROR (BY VOLUME) BIAS ERROR (BY VOLUME) TEMPORAL RES. (OBSERV CYCLE; HRS) TREND DETECTION (WITH CONTINUITY) Thresh Target Thresh Target Thresh Target Thresh Target Thresh Target % per year Lower Troposphere 250 <10* % or 4 ppb 3 % or 1 ppb 30% or 6 ppb 5% or 2 ppb Upper Troposphere % or 4 ppb 3 % or 1 ppb 30% or 6 ppb 5% or 2 ppb Lower Stratosphere Upper Stratosphere /Mesosphere % or 100 ppb % or 75 ppb 3% or 20 ppb 3% or 20 ppb 20% or 150 ppb 20% or 100 ppb 5% or 40 ppb 5% or 30 ppb Total Column % or 6 DU 1% or 3 DU 5% or 6 DU 1% or 3 DU Total Column (Troposphere) % or 6 DU 5% or 3 DU 15% or 6 DU 5% or 3 DU Note * - Lower range due to air quality user/process study requirement 12

25 2.2 Relationships between Applications and Requirements To illustrate the way requirements vary with application and to set the scene for the listing of requirements, some of the principal applications for the ozone data are discussed in this section. The focus of this report is on ozone, reflecting its central role in atmospheric chemistry and the atmosphere's radiative balance. However, requirements are also established for related chemical and meteorological parameters which are either required to help interpret ozone observations or else for use in the derivation of relevant geophysical variables (see Chapter 1). Table 1.1 provides a list of all the variables considered in this report Climate and Radiation The radiation balance (and hence climate variability) is very sensitive to variations in the concentration of ozone with height so vertical resolution can be important, especially in the upper troposphere/lower stratosphere where vertical gradients can be quite steep. This means that for investigations into climate variability (and radiation balance) vertical profiles of ozone are also required. For work on radiation balance this must be coupled with an horizontal resolution compatible with that used in models (though this is not a critical issue for the study of ozone trends). In the stratosphere, above the peak of the ozone layer, the requirements placed on vertical resolution are generally less severe as gradients tend to be smaller. However, the ability to make measurements over fairly narrow latitude ranges is important as fairly strong horizontal gradients can exist across some of the so-called atmospheric "transport barriers" (e.g. polar vortex/midlatitudes, mid-latitudes/tropics). The same is true of water vapour (and some other variables) for which tropospheric and stratospheric amounts are usually very different (though in the opposite sense to ozone for which amounts are higher in the stratosphere and lower in the troposphere, while the converse is true for water vapour). Therefore, the ability to observe rapid changes in mixing ratios with altitude is essential. For many purposes (the same is true for ozone) long term measurement accuracy and precision is important so if multiple instruments are used there must be good consistency between them. For its use in long term studies of surface ultraviolet radiation, the main requirement placed on observations of ozone column amounts is the combination of quite high horizontal resolution with good precision and long term stability, i.e. minimal instrumental drift. Where a network of instruments is used this means that the absolute calibration of each instrument must be high enough to ensure there are no unknown station-to-station biases. Measurements must span a range of solar zenith angles and should be valid in the presence of clouds (especially broken clouds) and aerosols. In many cases, for the data to be quantitatively useful in calculating surface ultraviolet fluxes, information on these potential sources of interference will be required. For this, daily coverage of the sunlit Earth is almost a prerequisite. Although, the data must be of high quality, delivery times for trend and climatological studies can generally be quite slow. However, the use of ozone column data in forecasting levels of surface ultraviolet radiation and other meteorological applications presumes the existence of a capability for rapid delivery and processing. This is in line with the need to ensure the rapid turnaround of visual descriptions of the total ozone field, especially during times of significant ozone depletion in the Antarctic and the Arctic Meteorological and Other Applications Ozone data in the stratosphere and around the tropopause are finding increasing use in operational meteorology. The assimilation of ozone observations into numerical meteorological 13

26 models helps to consolidate information on atmospheric motion and the characteristics of the tropopause region. For this application the real-time or near real-time delivery of data is essential. For the moment the main focus is on total column amounts but the need for information on vertical profiles can also be anticipated. Near the tropopause ozone amounts vary significantly with atmospheric structure. Valuable insights into the evolution of meteorological situations can be obtained by examining ozone data. These data can also be used in combination with information on the presence of clouds and aerosols to forecast surface ultraviolet radiation and to help establish boundary conditions for tropospheric air quality forecasts. Other data, which are typically obtained along with ozone data (notably observations of aerosols, in particular those of volcanic origin), may serve as the basis for advice on how to avoid hazards or to improve estimates of radiative balance (essential for long term forecasting). For all these applications the rapid delivery of data is essential. Ozone is also one of the key parameters when considering air quality in the lower troposphere. For this both high precision and long term stability are essential if the significance of both spatial and temporal variations in measurements is to be established. Air quality programmes require knowledge of the distribution of ozone at the surface and as a function of altitude in the lower and middle troposphere. Knowledge of the concentrations of key ozone precursors (e.g. carbon monoxide, nitrogen oxides and hydrocarbons) and radiation levels [J(O i D) and J(NO 2 )] is essential. Pollution events have strong daily variations therefore diurnal variations of ozone and its precursors must be made available in near-real time. In regions for which there can be variable contamination from human activity, higher measurement frequencies are required to help characterise the relative contributions of polluted and unpolluted air masses. Where data are used for trend and climatological studies, its rapid availability is not of critical importance, but if they are to be used to check compliance with air quality standards or to forecast air quality, rapid availability is again a priority. Profile information separating the boundary and the free troposphere is essential. 2.3 The Requirements In this section the detailed requirements are presented, largely in tabular form (see Annex B). Table 2.1 for ozone and the tables in Annex B for other atmospheric parameters summarise the requirements for data on surface level concentrations, total column amounts and vertical profiles using the altitude regions (where applicable) defined in Section 2.1. In reviewing measurement requirements for atmospheric trace constituents, it is helpful to follow the classification introduced in Chapter 1 and to consider them as falling into one of three subgroups, namely source gases, reservoir species or free radicals plus pertinent meteorological information required to set the observations into a proper context or for use in retrieval algorithms. Generally, requirements vary from parameter to parameter and from region to region. They are less well established in the mesosphere than for other parts of the atmosphere. Thus, requirements in these parts of the atmosphere should not be considered as drivers for determining observation requirements. This is reflected in later chapters of this report where needs are assessed against provisions. For ozone, the primary quantity of interest in this document, detailed requirements are provided in Table B.1 in Annex B. Tables B.2 detail the requirements for the "source gases" listed in Table 1.1 (i.e. water vapour (H 2 O), nitrous oxide (N 2 O), methane (CH 4 ), carbon monoxide (CO) and carbon dioxide (CO 2 )); Tables B.3 the requirements for the "reservoir species" listed in Table 1.1 (i.e. hydrogen chloride (HCl); nitric acid (HNO 3 ); Tables B.4 the requirements for the "free radicals" listed in Table 1.1 (i.e. bromine oxide (BrO), chlorine monoxide (ClO), nitrogen dioxide (NO 2 ) and nitric oxide (NO)). Specific requirements for information on temperature and wind are summarised in Tables B.5 and those for aerosols and polar stratospheric clouds in Table B.6. 14

27 2.3.1 Ozone (Table 2.1 and B.1) Ozone plays a key role in atmospheric chemistry and the radiative balance of the atmosphere. In the stratosphere it is the main absorber of ultraviolet radiation. This absorption is responsible for the increasing temperature above the tropopause. In the lower stratosphere and upper troposphere it becomes a powerful greenhouse gas and forcing function for climate change. In the lower troposphere it is a pollutant and is created through complex chemical reactions with anthropogenic gases and sunlight. This means that the observational requirements vary considerably. Table B.1 attempts to satisfy most user requirements and to some degree it is a compromise. Because of the key role ozone plays in this report a more detailed justification for the table of requirements is provided below. General circulation models currently use a grid size of 1 o x 1 o, i.e. a horizontal resolution of about 250 km. This was set as the horizontal threshold. The horizontal target value for the lower troposphere was set to 10 km based on the requirement of the air quality user community to resolve the horizontal ozone gradient within and downwind of major population centres. Above the planetary boundary layer the horizontal ozone concentration gradient is less pronounced allowing a relaxation of the target value to 50 km. For the total column density, the threshold requirement of 100 km is based on the Dobson/Brewer user community constraining the representivity of their vertical "point" measurement to about this value. However, a much higher horizontal resolution will be required to fully meet user requirements; thus the target value of 10 km. The target value of 0.5 km for vertical resolution meets the modelling community requirement. Current regional climate and chemistry models which operate with vertical resolutions of this order of magnitude and observations confirm that the vertical ozone gradient does change significantly with altitude on this scale. Of particular importance to climate modellers are the ozone changes in the 8-12 km range (where an increase in ozone is postulated) and in the km range (where a decrease in ozone concentration occurred). The values for vertical threshold reflect the requirements of other user groups (air quality, climate and chemistry modellers, trend analysts) who also need ozone profile information. The target values for bias and RMS errors reflect the ozone concentrations observed within the stated vertical regions of the atmosphere and the issues that the different user groups need to resolve. For the troposphere, and particularly for the planetary boundary layer, the air quality community routinely demands an accuracy of 5% or 2 ppb and a precision of 3% or 1 ppb (always the larger of the two numbers). For the lower stratosphere, several issues are important which have to be considered in setting the target value, notably ozone increase due to air traffic (8-12 km), ozone destruction at higher levels due to heterogeneous reactions (< 20 km), global ozone decrease due to CFCs and ozone depletion in the Antarctic and Arctic regions. Since the ozone concentration above the tropopause increases significantly (by an order of magnitude) to attain a peak value at about km and decreases thereafter, different target and threshold requirements have been forwarded for these altitude regimes by the user communities, reflecting their interest in specific scientific or policy issues. With about 90% of the total ozone residing in the stratosphere, the total trend in column ozone is governed by changes in this region (mainly in the lower stratosphere). Between January 1979 and May 1994 total ozone (60 o N to 60 o S) showed a decline of 2.9% per decade. Consequently the target value for total ozone (column) trend detection was set to 0.1 % per year and for the lower/upper stratosphere to 0.3 % per year. Ozone trends in the troposphere have been studied by many workers, but remain uncertain in large regions of the globe due to the lack of reliable long term data sets. The atmospheric chemistry user group required a target value of 0.5% for both planetary boundary layer and free troposphere. 15

28 2.3.2 Other Chemical Species a) Source Gases (Tables B.2) The presence of source gases (produced both naturally and by human activity) can have significant impacts on the global atmosphere because of the radiative and chemical effects associated with their presence, in particular their role in influencing the distribution of ozone. Their long chemical lifetimes means that long term global measurements of a small number of them as a function of altitude, are sufficient to provide insights into atmospheric transport as well as providing a dynamical context for the measurements of ozone and other parameters. This means that the accurate long term observation of some of these species is a critical requirement - the most important of which is water vapour. Surface level measurements will be the most critical for establishing long term variations in the concentrations of many of these constituents which can evolve significantly with time but which cannot be predicted with any certainty. Classes of compounds for which such measurements are needed include halides and halocarbons from both natural and anthropogenic sources. Some of these are included in Table 1.2. The observation of the more chemically-active source gases are specified in this report and are listed in Table 1.1. The justification for these observations are discussed below. Nitrous Oxide (N 2 O) and/or Methane (CH 4 ) - it is useful to monitor one or more of the long life tracers to help clarify the dynamical context of the tropospheric air masses associated with observations of trace species. Two of the most commonly used tracers are N 2 O and CH 4. This reflects their differing lifetimes (which facilitates their use in transport studies), as well as the fact that they are among the more easily observed source gases. Ideally both should be observed as their lifetimes in the atmosphere are sufficiently different to provide complementary information. Both gases also play important roles in the stratosphere in the catalytic cycle of ozone and, furthermore, of water vapour through the oxidation of methane. N 2 O and CH 4 are also greenhouse gases. Carbon Monoxide (CO) - this is an important gas in the budget of tropospheric ozone as the oxidation of CO in the presence of NO x leads to the production of ozone. In NO x -poor regions CO oxidation results in the loss of ozone. CO also serves as a tracer for tropospheric air transferred into the stratosphere, notably associated with deep convective activity which, above continental regions, often penetrates into the stratosphere. Carbon Dioxide (CO 2 ) - this is one of the most well-known end product of the burning (oxidation) of fossil fuels and biomass. Associated with increasing industrial activity, levels increased dramatically during the last century and are expected to continue to increase well into the future. CO 2 is an important greenhouse gas, having little interaction with solar radiation but absorbing infrared radiation from the Earth's surface. Increasing CO 2 levels are expected to lead to tropospheric warming, with model predictions of increases over the next century in the global average surface temperature ranging between one and a few degrees. The large-scale long term monitoring of CO 2 is of critical importance. 1 The concentrations of source gases can vary significantly with height in regions where they photolyse. This means that for observations of these species to be useful they must have a vertical resolution that is no worse than the scale height (6-8 km). However, in general, a vertical resolution of at least 2-3 km will be required and an even higher resolution (~1 km) would be very 1 In Table B.2e target values in the troposphere are set to meet the most stringent requirements for trend detection (currently 0.36 ppm/year and only detectable through surface-based observations). Target values for horizontal resolution (10 km) are set to allow detection of "hot spots" of CO 2 emissions from satellites (total column). Lower stratospheric CO 2 measurements are important for obtaining the seasonal cycle of CO 2 which has an amplitude of about 4 ppm in the tropics (transport process studies). Upper stratospheric CO 2 measurements reflect only the annual increase. In addition, height resolved stratospheric CO 2 measurements are used for deriving temperature. 16

29 useful. As spatial and temporal variations are important, high horizontal resolution, coupled with high precision will be essential. b) Reservoir Species (Tables B.3) The concentrations of reservoir species in the stratosphere will tend to reflect the total burden of a given class of constituents. Thus, for example, hydrogen chloride (HCl) is a good indicator of the total chlorine burden in the atmosphere. This means that a high priority for the measurement of reservoir species lies in absolute accuracy (so that burdens can be compared with those suggested by summing concentrations of source gases) and long term stability. This is especially true for measurements of total column amounts and vertical profiles made in regions of small vertical and horizontal gradients (e.g. hydrogen chloride in the stratopause region). The highest priority reservoir species for long term measurement are the hydrogen halides and nitric acid. The former provide the best indication of the total halogen burden in the stratosphere, which is expected to change with time (and recently indicated in satellite data) as the surface concentrations of CFCs and related molecules decrease in response to The Montreal Protocol on Substances that Deplete the Ozone Layer. Nitric acid is the dominant reservoir for inorganic nitrogen in the stratosphere and is subject to loss from the gas phase through incorporation into polar stratospheric clouds or aerosols: Hydrogen Chloride (HCl) - this reservoir species is the ultimate fate of chlorine species in the stratosphere and near the stratopause; essentially all the chlorine is in the form of HCl. It is important therefore to ensure the long term provision of measurements of the vertical profile of HCl in the stratosphere to complement the ground-based total column measurements provided by the NDSC. This is especially true for the stratopause region. Nitric Acid (HNO 3 ) - this is an important atmospheric trace gas which serves as a reservoir for reactive nitrogen in both the troposphere and stratosphere. It is highly soluble and can be absorbed on ice as well as by water, so that its distribution tends to follow a downward motion in the atmosphere whether associated with precipitation (rapid) or the sedimentation of hydrometeors (slow). Particularly in the polar stratosphere, this leads to denitrification which has the consequence of reducing the uptake of reactive chlorine into the chlorine nitrate reservoir, ultimately enhancing the ability of chlorine to catalyse ozone destruction. Also in the polar stratosphere, HNO 3 is a constituent of Type I polar stratospheric clouds. c) Free Radicals (Table B.4) The concentrations of free radicals vary significantly with tropospheric and stratospheric conditions as well as with time of day. A key requirement, therefore, is for measurement techniques to be able to handle very large variations in observed concentrations. Long term precision is probably of less interest than short term accuracy, as the need is to be able to test consistency between the observed distributions of radicals and their precursors using atmospheric models. The most important free radicals to observe in the stratosphere are chlorine monoxide (ClO), bromine monoxide (BrO) and at least one (preferably both) of the simple nitrogen oxides (i.e. nitric oxide NO; nitrogen dioxide - NO 2 ). The measurement of BrO is especially challenging given its low concentrations. The need for observations of the hydroxyl radical (OH) depends on the validation of current hypotheses. If these are confirmed this variable will not need to be observed directly as it will be possible to derive it from other observations: Chlorine Monoxide (ClO) - this is one of the free radicals most closely associated with the destruction of odd oxygen. Its presence indicates on-going ozone destruction via the reaction Cl+O 3 ClO+O 2. ClO reacts rapidly and releases Cl, firstly via a reaction with atomic oxygen forming Cl plus O 2, and secondly via a reaction with NO, forming Cl plus NO 2, (this also constitutes an important coupling with the nitrogen cycles). 17

30 A third rapid process retrieving Cl from ClO is photodissociation. The catalytic cycles involving Cl and ClO which destroy ozone can be stopped by the (slower) reactions of Cl with hydrogen compounds, most importantly methane, forming the reservoir species HCl. Another important channel for removing active chlorine is the reaction of ClO with NO 2, forming the reservoir species ClONO 2 (which again can dissociate in the ultraviolet). ClO is therefore a key species in active stratospheric chlorine chemistry. Bromine Oxide (BrO) - despite its much smaller abundance in the stratosphere compared to that of ClO, the presence of BrO, is highly significant for ozone destruction because of the "per-atom" effectiveness of bromine in destroying ozone. Recent evidence also seems to suggest that, in addition to its role in the polar stratosphere, the presence of BrO in the troposphere during the polar spring is important and is highly synergistic with ClO. The catalytic cycle involves the ozone depleting reaction Br+O 3 BrO+O 2 and the recycling of Br from BrO. This can be effected (similar to the ClO cycle) by reactions with atomic oxygen or NO, as well as with another BrO molecule. A strong synergy is achieved if ClO and BrO appear together as they accelerate Br and Cl retrieval through the reaction BrO+ClO Br+OClO Br+Cl+O 2. Nitrogen Dioxide (NO 2 ) and Nitric Oxide (NO) - NO 2 (along with its sister species NO) plays an important role in atmospheric chemistry. In the stratosphere it participates in the catalytic destruction of ozone, while in the troposphere its presence largely determines the rate of in-situ photochemical ozone production. Conversely, the sedimentation of polar stratospheric cloud particles containing nitric acid may enhance the future loss of ozone by reducing the conversion of ClO to ClONO 2. Further uncertainties are associated with factors such as the production of NO x by lightning, aircraft emissions and the convective transport of surface level pollutants. This means that the global budget of reactive nitrogen is both uncertain and variable in time, especially in the vicinity of the upper troposphere/lower stratosphere. Observations of NO 2 and its sister NO are essential to contain this uncertainty Meteorological Parameters (Table B.2a and Tables B.5) To obtain a full understanding of the distribution and concentration of ozone and associated trace species in both the troposphere and the stratosphere, knowledge of certain meteorological parameters is essential. Some of these are also required to drive the algorithms used to retrieve ozone and other variables. The list of relevant meteorological parameters includes vertical profiles of temperature and water vapour, the height of the tropopause, cloud information and wind profiles. Wind profiles are needed to take proper account of atmospheric dynamics and transport mechanisms, especially in the upper troposphere and in the stratosphere, when interpreting ozone observations. Most of the requirements for knowledge of meteorological parameters, as defined by the Upper Air Project in support of IGOS, also encompass the needs of the Ozone Project (in fact many actually exceed them). Thus, assuming that these requirements are met, here in this report the focus is on the two notable exceptions, namely tropopause height and levels of water vapour in the region of the upper troposphere/lower stratosphere. In both instances the requirements of the Ozone Project are more stringent. The actual requirements for tropopause height are for knowledge to 0.1 km (target) and 0.2 km (threshold) assuming the WMO definition of tropopause (based on thermal stability). The detailed requirements for water vapour are listed in Table B.2a and those for temperature and wind in Tables B.5. The more exacting requirements associated with tropopause height arise primarily from its use as an indicator of changes in ozone column amounts. The need to accurately characterise water vapour levels in the upper troposphere and lower stratosphere (which can be very sensitive to the temperature of the tropopause) stems from the important role that water plays in controlling the chemistry, radiative balance and particle formation in this part of the atmosphere. Here 18

31 concentrations are very small from a meteorological point of view and the requirements defined by the Upper Air Project do not sufficiently constrain the distribution of water vapour for the use of these data in chemistry studies Aerosol (Tables B.6) The dominant source of stratospheric aerosol are the major volcanic eruptions that deposit volcanic ash and aerosol precursor gases (i.e. sulphur dioxide that is rapidly oxidised to sulphuric acid and nucleated into fine particles) into the stratosphere where it can reside for up to 3 years, depending on its location above the tropopause. This means that the stratospheric aerosol burden is essentially determined by volcanic activity. The properties of stratospheric aerosol, which are relevant to the stratospheric ozone budget and essential for retrieving ozone concentrations, are the extinction coefficient (Table B.6 b) and (derived from this information) the size distribution and surface area/volume (Table B.6 c) and aerosol backscatter (Table B.6 d). These properties are measured or inferred from a few ground-based stations (aerosol lidars) or (occasionally) from aircraft or balloon platforms. The accuracy and precision levels listed in these tables as "target" are indeed achievable with these systems and comfortably meet the target requirements stated by the user, in particular for addressing the issue of ozone destruction on aerosol surfaces and for incorporating the required aerosol parameters in the ozone retrieval algorithms. Some ozone sensors (limb viewing) cease operation when optically dense aerosol is present Since aerosols exhibit very strong vertical (and horizontal) gradients, the vertical resolution has been set rather tightly allowing, for example, the assessment of the impact of the presence of aerosols on ozone concentration. In addition, consideration was given to the climate forcing of stratospheric aerosol which requires not only extinction/backscatter measurements approaching the stated target values, but also the ability to detect long term trends (~1% per year) in both the stratospheric aerosol burden and in the total aerosol column. The threshold requirements (of about 20 % per year) are essentially determined by the need to detect the presence of large quantities of volcanic aerosol after major eruptions. The requirements to be able to detect the presence of polar stratospheric clouds (PSCs) responsible for the processes that lead to ozone destruction and ultimately to the annually recurring ozone hole phenomena, are summarised in Table B.6 a. Because of the relatively high optical density of PSCs, their detection from space and ground seems to be feasible if the measurements lie within the limits stated in Table B.6 d. The importance of tropospheric aerosol is related to a lesser degree to ozone itself, but more to the measurement of UV-B fluxes at the surface, to aerosol forcing (Earth radiation budget) and most importantly to air quality issues. Regional haze originating from fossil fuel combustion or biomass burning and dust storms are but a few of the issues that challenge the science community. To satisfy their respective needs requires that the target values listed in Tables B.6 b- d are closely met, not only with regard to RMS and bias error but also for vertical resolution. At a minimum, measurements must differentiate between the aerosol residing in the planetary boundary layer versus that in the free troposphere. Unlike stratospheric aerosol, tropospheric aerosol is highly inhomogeneous, both in space and time, and, to complicate the situation further, in chemical composition and physical characteristics Spectrally Resolved Solar Ultraviolet Irradiance Accurate knowledge of levels of solar ultraviolet radiation, which drives atmospheric photochemistry and provides the photons that ultimately reach the Earth's surface, is essential if quantitative knowledge of the relationship between atmospheric ozone levels and surface ultraviolet radiative fluxes is to be obtained. Some information can be obtained based on the use of proxy quantities for solar variability and observed relationships between spectrally resolved solar ultraviolet radiation and these proxies. However, the possible variation in the wavelength 19

32 dependence of solar output over multiple solar cycles indicates that the direct measurement of solar ultraviolet spectral irradiance must be carried out on a regular basis. a) Top of the Atmosphere For top of the atmosphere (TOA) solar irradiance measurements the key requirement is a daily measurement of about 30 minutes per day (total and spectral). The wavelength range coverage needed to link atmospheric photochemistry and surface ultraviolet radiation extends from approximately 200 to 400 nm (wavelengths shorter than 200 nm are mainly important in controlling the extent and temperature of the mesosphere and the thermosphere). Wavelengths in the near infrared up to about 2000 nm are needed for the study of water vapour absorption and cloud processes. A wavelength resolution of the order of 0.5 nm is required, especially in the UV-B region, where the absorption cross section of ozone exhibits significant wavelength dependence which will have a major impact on the surface flux of ultraviolet radiation. The specific requirements for solar spectral irradiance observations are based on known cyclical variations and their expected effects on atmospheric chemistry and radiation. For the wavelength range 200 to 2000 nm, absolute accuracy should be 0.03% with a relative accuracy of 0.01% per year. For shorter wavelengths, absolute accuracy and relative accuracy should be better than 5% and 1% per year, respectively. Wavelength resolution at shorter wavelengths (but greater than 200 nm) should be higher (approximately 0.2 nm) but can then be reduced to 30 nm above 1000 nm. b) Surface Measurements For surface measurements of ultraviolet radiation the requirements are for observations longer than 290 nm because radiation of shorter wavelengths are filtered out by ozone. For the rest of the spectrum spectral resolution requirements are similar to those noted above. However, the required frequency of observation is much greater as there are many short term variations in surface ultraviolet flux associated with variations in cloud amounts, as well as in aerosol amounts and the overlying ozone column. There is also a change in solar zenith angle over the course of the day that affects surface ultraviolet fluxes. As there can be rapid changes in radiance due to the overhead passage of clouds, it is important that ultraviolet spectral measurements are made continuously during these periods to ensure that temporal and wavelength variation are discernable. They must also provide good spectral discrimination as there is an enormous variation in the surface ultraviolet flux with wavelength due to the existence of a sharp cut-off in ozone absorption in the atmosphere. For studies of surface ultraviolet radiation under cloudless conditions, ozone profiles (in stratosphere and troposphere) are needed. Aerosol optical depth are also controlling factors and should be measured. 20

33 3. AVAILABLE AND PLANNED MEASUREMENTS 3.1 Introduction In this chapter, the sources of observations of ozone, associated meteorological parameters, other trace constituents and related parameters (discussed in Chapter 2) are considered within the context of the concerns of the Ozone Project. The emphasis is on ground-, balloon-, airborne and space-based measurement systems capable of the routine operational provision of these data. Little attention is given to process-oriented measurements such as those involving research aircraft or balloons, which typically do not provide data sets relevant to the study and interpretation of long term global trends which are the concern of this report. Requirements for some parameters traditionally measured by the operational meteorological agencies have been identified in this report. The provision of these data is considered in Chapter 4, The Harmonisation of Provisions and Requirements. The focus is on instances where the data normally provided operationally by meteorological agencies are not likely to meet the requirements for long term ozone monitoring (see also Chapter 2). In addition, it is important to note that some measurements of clear relevance to the long term monitoring of ozone, most notably surface level ozone measurements, are made through networks operated by air quality-oriented agencies. The Global Atmosphere Watch (GAW) and its surface ozone data base can play a critical role in assuring the availability, representativeness and uniform data quality of such observations. It is important that such measurement networks will continue to exist and be well maintained. The information presented in this chapter was obtained from the institutions, agencies, and programmes responsible for the observing systems described. Much of the material on spacebased measurements was taken from an article Summary of Space-Based Observations of Atmospheric Chemistry which appeared in a newsletter of the Stratospheric Processes and their Role in Climate (SPARC) subgroup of the World Climate Research Programme (WCRP). 3.2 Non-Satellite Measurements A wide variety of non-satellite instruments and platforms are available and in operation for making routine total column and profile ozone measurements. As many of these also make measurements of other trace constituents, instruments that measure ozone and related parameters are treated together. Some of these activities are incorporated into national and international networks such as the NDSC and the WMO GAW, while others make measurements primarily on a campaign basis. Both types of measurement can make valuable contributions to the provision of the data required to quantify and interpret changes in the global ozone distribution, as well as helping to validate the accuracy and precision and hence the stability of satellite observations Ground-based in-situ Measurements The ground-based in-situ measurements most relevant to the interpretation of ozone observations concern surface-emitted, long-lived source gases that give rise to chemically active species (including those containing chlorine and bromine) in the atmosphere, or are radiatively active in climate forcing. Several well-established, geographically distributed networks exist for monitoring the long term evolution of the concentration of these species, notably the Advanced Global Atmospheric Gases Experiment (AGAGE) network of the US National Aeronautics and Space Administration and the flask sampling network of the US National Oceanic and Atmospheric Administration. A map of ground-based in-situ sampling stations which have long term data records of trace constituent composition is shown in Figure

34 These networks place substantial emphasis on the high quality, consistent calibration of instruments' accuracy and precision over the long term. They routinely observe the full range of chlorofluorocarbons (CFCs) and related halocarbons (methyl chloroform and carbon tetrachloride), bromine-containing halons and methyl bromide, hydrogenated chlorofluorocarbons (HCFCs) and other CFC replacement compounds, as well as methane and nitrous oxide. In so doing, they cover all the source gases listed in Table 1.1 (and Table 1.2) for which the primary requirement is for regular observations at ground level. It is worth noting that the list of species whose concentrations are being monitored is slowly expanding to include shorter-lived species such as methyl bromide, as well as the long-lived species that were the original focus of these networks. The stations cover a range of geographic locations, including relatively unpolluted areas, so that the atmospheric background is well characterised and contamination from polluted urban air is minimised. Individual stations operated by scientists in other nations also exist, and the calibration of the measurement systems used in these has, in many cases, been compared with that of the AGAGE and NOAA networks. Table 3.1 lists all the trace species/parameters observed (though not all be every station). It will be noted that some also measure ozone profiles and column amounts Ground-Based Remote Sensing Measurements Ground-based remote-sensing instruments for atmospheric chemistry measurements can be viewed as falling into overlapping groups: those that have been designed primarily to make ozone measurements versus those that have been developed to provide a more comprehensive set of atmospheric observations those designed primarily for the measurement of total column amounts versus those designed primarily for the measurement of vertical distributions (this breakdown is not completely clean - for example the ultraviolet-visible and the Fourier transform infrared instruments (see below), which have been designed primarily to measure column amounts, also provide some information on the vertical distributions of a few constituents, notably NO 2 and CO, respectively). As indicated above, Table 3.1 lists the species (plus aerosols and UV flux) measured by the different ground-based remote sensing systems, including those for which profile and column amount information are available (and the source of these data). Table 3.2 summarises the instrumentation available in the NDSC, including both the primary (that contain the full range of NDSC instrumentation) and complimentary sites. A map showing sites currently affiliated to the NDSC is reproduced in Figure 3.2. Particular issues related to calibration, validation, and data management for these systems are discussed in Chapter 5. 22

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36 Table 3.1: The list of species observed by ground-based observing stations and (where appropriate) the technique used to observe vertical profiles or column amounts INSTRUMENT TYPE SPECIES LIDAR FTIR UV/VIS µwave Sondes Dobson /Brewer O 3 p p c p p p UV Spectrometer N 2O c p NO c NO 2 c p p NO 3 c HNO 3 p p HNO 4 N 2O 5 CFCl 3 CF 2Cl 2 c c CF 3CCl 3 CCl 4 c CH 3CCl 3 CH 3Cl HCl c p ClO p OclO p ClONO 2 HF CF 2O c c c CF 3Br CF 2ClBr CH 3Br BrO p H 2O p p p H 2O 2 p OH c HO 2 p CH 4 p CO p p SF 6 c CF 4 c OCS c SO 2 HCN c T p p Aerosols p p UV Flux p Note: "p" - profiles and column amounts "c" - column amounts only 24

37 Table 3.2 (a): Instrumentation at NDSC primary sites - at some stations instruments may only be operational during campaigns. (Notes - "O 3 (A)" indicates ozone lidar with aerosol channel; "O 3 (T)" indicates ozone lidar for troposphere (below 15 km) only; "A" indicates aerosol) STATION NAME COUNTRY LAT. LONG. ELEV. (M.) Arctic LIDAR FTIR UV/VIS µwave SONDES DOBSON/B REWER Eureka Canada O 3, A X O 3 B SPEC. UV Ny Alesund Norway O 3, A X X O 3, ClO, H 2O Thule Greenland to 220 A X X O 3 O 3 D Sondre Stomfjord Greenland to 300 Term 96 B Alpine Garmisch Germany A, O 3(T) X Zugspitze Germany X X Bern Switzerland O 3 Jungfraujoch Switzerland X X ClO Observ. de Bordeaux France O 3, H 2O Plateau de Bure France ClO Obs. Haute Provence France O 3, H 2O, A, X O 3 D O 3(A) Hawaii Mauna Kea USA ClO Hilo USA O 3 Mauna Loa USA O 3, A, O 3(T) X X O 3, H 2O O 3 D X Lauder New Zealand O 3, A X X O 3, H 2O O 3, A D X Antarctic Dumont d'urville Antarctica O 3, A X O 3 Arrival Heights Antarctica X X McMurdo Antarctica A Term 94 O 3, A Scott Base Antarctica ClO South Pole Station Antarctica -90 N/A O 3 D 25

38 Table 3.2 (b): Instrumentation at NDSC Complimentary Sites - at some stations instruments may only be operational during campaigns. (Notes - "O 3 (A)" indicates ozone lidar with aerosol channel; "O 3 (T)" indicates ozone lidar for troposphere (below 15 km) only; "A" indicates aerosol) STATION NAME COUNTRY LAT. LONG. ELEV. LIDAR FTIR UV/VIS µwave SONDES DOBSON SPEC. UV (M.) Scoresbysund Greenland X Andoya Norway A, O 3 X X Kiruna Sweden X Sodankyla Finland X O 3 Zhigansk Russia X Harestua Norway X Zvenigorod Russia A, O 3 X X Aberystwyth UK 52 4 A, O 3 X O 3 Moshiri Japan X X Toronto Canada A,O 3 Rikubetsu Japan X X Greenbelt USA A Wallops Island USA O 3 Mt. Barcroft USA X Billings (OK) USA X Tsukuba Japan O 3 Kiso Japan X Table Mountain (CA) USA O 3, O 3(T), A H 2O Kitt Peak USA X Tarawa Kiribati Rep X Bandung Indonesia X 26

39 Table 3.2 (c): Instrumentation at NDSC Complimentary Sites (continued) - at some stations instruments may only be operational during campaigns. (Notes - "O 3 (A)" indicates ozone lidar with aerosol channel; "O 3 (T)" indicates ozone lidar for troposphere (below 15 km) only; "A" indicates aerosol) STATION NAME COUNTRY LAT. LONG. ELEV. LIDAR FTIR UV/VIS µwave SONDES DOBSON SPEC. UV Reunion Island A X O 3 Durban South Africa -34.4??? X Wollongong Australia Campbell Island New Zealand Term 95 Macquarie Island X Faraday Antarctica Term 95 Rothera Antarctica X Syowa Base Antarctica X SA Antarctic Station Antarctica X Halley Bay Antarctica X 27

40 Figure 3.2: Map of primary and complementary sites affiliated to the NDSC 28

41 a) Column Measurements The Dobson Spectrophotometer has a history of high quality total ozone observations stretching back nearly 70 years and regular measurements have been made with it for about 40 years. The current WMO-GAW network includes approximately 70 stations with many of these concentrated at mid-latitudes in the northern hemisphere (e.g.33 in Europe). Almost all of these instruments have their calibration tied to a single standard Dobson instrument (#83) through regional and national standard Dobson instruments. Although the continuation of this network has often been clouded by uncertainty, the on-going efforts of the WMO-GAW programme and the importance of the long term data set based on this network, have allowed it to survive basically intact. A map showing the regularly reporting Dobson and Brewer Spectrometer stations, as well as those using other well-established techniques (such as filter instruments) to measure total column ozone is shown in Figure 3.3. Annex C provides further information on the data records available from regularly reporting ground-based ozone measuring stations. The Brewer Spectrometer is a high quality instrument which measures ozone, as well as several other constituents, by making spectral measurements in the UV-B part of the solar spectrum. Its high performance coupled with the feasibility of automating its operation, has led to the deployment of about 70 of these instruments in the network. As with the Dobson instrument, many are concentrated at mid-latitudes in the northern hemisphere (e.g. 20 in the U.S.). A reference triad of Brewer instruments is maintained at MSC Canada as a standard and, at least initially, the station instruments are linked to this standard. The Brewer is also a component of the GAW network and about 15 of them are included within the NDSC. There are also travelling standards. The Ultraviolet-Visible Spectrometer, of which the SAOZ is the most widely used example, is able to obtain total ozone amounts at low sun elevation angles when instruments such as the Brewer and Dobson are not capable of making measurements. This has led to them being mainly sited at the poles for use in polar winter conditions though they are also used in other latitudes. The calibration of these instruments is not as well established as is the case for the other two instruments but work within the NDSC should help remedy this situation. These instruments can also be used to measure other trace constituents with strong absorption in the visible and ultraviolet wavelength regions such as NO 2, NO 3, BrO and OClO. Fourier Transform Infrared Spectrometers (FTIR) can be used to make measurements of a whole host of atmospheric constituents in addition to ozone, including a number of long life gases (methane, nitrous oxide, water vapour, carbon monoxide and selected CFCs) and important reservoir molecules like hydrogen chloride, hydrogen fluoride, nitric acid and chlorine nitrate. Column amounts of such species measured over many years using FTIRs have been critical in documenting the increasing concentrations of halogenated species in the atmosphere. The nearly two dozen FTIR instruments affiliated with the NDSC are operating according to protocol rules established and up-dated by an ad-hoc NDSC working group. A mobile FTIR serves as a "reference" to ensure internal consistency for this type of instrumentation throughout the network. Furthermore, a strong effort is underway within the NDSC and WMO-GAW stations to ensure the provision of high quality measurements and, even now, FTIR ozone amounts are routinely compared with Dobson and Brewer observations. 29

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43 Filter Instruments have had a complex history due to problems with filter stability which limits the ability of such instruments to make accurate long term measurements. A network of 40 filter instruments, mostly in the Russian Federation, report data. Recent advances in the ability to make more stable filters leaves open the possibility that such instruments may, ultimately, be able to make important contributions to long term records of ozone column amounts (also aerosols). A notable example of such efforts is the Atmospheric Radiation Measurement (ARM) programme under the auspices of the US Department of Energy. Other filter instruments are also used extensively for the measurement of surface fluxes of ultraviolet radiation. Their long term stability remains to be proven, although preliminary indications suggest that it is much improved over that of its predecessors. Unless these filters have a fairly narrow spectral band pass (e.g.1-2 nm) there are additional complications associated with their use for ultraviolet trend measurements. b) Profile Measurements Dobson and Brewer instruments are both capable of exploiting the Umkehr technique to produce vertical profiles of ozone in the stratosphere. These profiles have an altitude resolution of 5 km or greater but suffer from the same geographical distribution constraints as the total ozone observations made with these instruments. Only a limited number of the Dobson locations (15) make profile observations and most of these are where the Dobson instrument has been automated. The Brewer is, by its very nature, an automated instrument so that its potential for profile observations is much greater than is the case with the Dobson, but at present only a limited number of profiles are being reported. Lidars are used to obtain profiles of atmospheric variables in both the stratosphere and troposphere with one to two kilometer resolution. In the stratosphere they are used to measure profiles of ozone and temperature at over 15 locations world wide. Most of these instruments are affiliated with the NDSC as primary or complementary sites. The NDSC has carried out several validation campaigns and concluded that these instruments produce valid data in the range of km for ozone and up to 80 km for temperature. In addition, a few tropospheric versions of these instruments are currently operating, mostly in a campaign mode, though nine are committed to long term operation as part of existing international networks including the NDSC. They are used primarily to measure tropospheric ozone profiles and, at some stations, water vapour profiles in the upper troposphere. Lidars also make important contributions to the long term observation of stratospheric aerosols. An international lidar network has been developed which provides very extensive spatial coverage, although as with all ground-based instruments, measurements are restricted to land-covered areas. Observations from developing countries and remote territories are much fewer than from more populated, developed areas. Microwave Radiometers are used to observe ozone profiles from the stratosphere up to the mesosphere and are able to make measurements under most weather conditions. They are currently being operated at several NDSC sites. The validity of their ozone profiles has been established through validation campaigns and by intercomparison with other profile measurements. As they observe in emission, these instruments can make measurements during both day and night (including the polar night). Microwave observations of diurnal variations and at the South Pole have been particularly important as such instruments can also be used to make measurements of a range of trace constituents, the most important of which are H 2 O and ClO, as well as long-lived molecules like N 2 O. The vertical resolution of these instruments is typically fairly broad (5-10 kilometers) which places constraints on the usefulness of their data in regions of strong vertical gradients. FTIR and UV/Visible Instruments can provide profile information on some gases although their primary use has been for determination of total column amounts. FTIR instruments make use of the pressure variation of the line width (and the temperature dependence of thermal emission) and have been shown to provide low vertical resolution information on species such as CO, CH 4, HCl, HNO 3, O 3 and H 2 O,. UV/Visible instruments can make profile measurements at twilight when the Earth s shadow line is scanning upwards in altitude. Such measurements have been made for NO 2, OClO, and BrO. 31

44 c) Balloon-Based Measurements For many years now balloon-borne instruments have been used to make the long term global observations of ozone required to monitor ozone trends (with the main focus in the past on the stratosphere). For this ozone sondes have been utilised and, at present, these sondes provide the bulk of the data on vertical profiles of ozone from near the surface to approximately 30 km. The observations derived from ozone sondes are of a very high vertical resolution which is unattainable by any of the existing (or near-term projected) satellite techniques. Regular soundings are currently made at about 40 sites. Although these tend to be concentrated at middle and high latitudes in the northern hemisphere, several of the more recently established sites are in the under-sampled tropical and subtropical regions. About half of the sites have records that extend 10 years or longer and most stations are affiliated with the WMO/GAW network; some to the NDSC. A map of regularly reporting ozone sonde stations is shown in Figure 3.4. Information on the available data from the different sonde stations (e.g. length of record, frequency of flights) is included in Table 3.3. An effort has recently been made to increase the frequency of ozone sonde launches in the tropics and southern hemisphere subtropics through the Southern Hemisphere Additional Ozone sonde (NASA/SHADOZ) programme. Long term information on stratospheric composition may be obtained through the judicious use of the results of long series of process-oriented balloon profiles (correcting for seasonal and geographic differences between flights). This has been achieved by relating concentrations of the more rapidly changing species (like the CFCs) to those of more slowly changing ones (like nitrous oxide) which can help establish a reference co-ordinate system for the measurements. Measurement validation has been through field intercomparison campaigns and most recently through the use of the simulation facility at the Research Centre in Jülich, Germany (WMO-GAW world calibration facility for ozone sondes). In addition, there are several programmes where ozone and other trace constituents are measured as part of a larger process-oriented balloon payload. These measurements are particularly useful as they are made with instruments capable of observing a complement of photochemical and tracer species in addition to ozone. Campaigns take place on a regular basis within the US and European (notably the French and German but see Table 3.4) balloon programmes. They include large payloads such as the Observations of the Middle Stratosphere (OMS) programme, as well as flights of payloads like the SAOZ, AMON and MIPAS instruments. The long duration Montgolfier balloon could be very promising and is planned to be flown in the next few years. Balloon flights also provide an important element of satellite calibration and validation programmes (e.g., UARS, ADEOS, and for the SAGE III and ENVISAT campaigns). d) Airborne Measurement Programmes The only currently operational programmes in which atmospheric trace constituents are routinely measured on board aircraft are the European MOZAIC and CARIBIC programmes, in which in-situ ozone photometers, water vapour and NO y measuring instruments have been placed on French and German airliners. These fly international routes between Europe and Asia, North and South America, and Africa, with the heaviest concentration of flights over the North Atlantic. The sampling is primarily in the upper troposphere but does include some stratospheric data. Profile information can be obtained during take off and landing. The MOZAIC programme dates back to 1993 and was preceded by the NASA/GASP in the seventies which may be viewed as the precursor to MOZAIC. In addition, an instrument suite is currently under development in the US for the routine measurement of ozone, water vapour, carbon dioxide and tetrachloroethylene. The long term goal of this activity is operational use on commercial aircraft, supplementing the MOZAIC and CARIBIC programmes. 32

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46 Table 3.3: The WMO-GAW data records of regularly reporting ozone sonde stations (note that some of these have operated only over limited time intervals and so their data are not suitable for trend calculations) STATION LOCATION COUNTRY LAT. LONG. START END SONDE TYPE Alert Canada Jan-87 ECC Eureka Canada Nov-82 ECC Ny Alesund Norway Oct-90 ECC Resolute Canada May-78 ECC 5-Jan Nov-79 BM Sodankyla Finland Jan-89 ECC Yakutsk Russia Jan-94 ECC Churchill Canada May-78 ECC 19-Oct Sep-79 BM Edmonton Canada May-78 ECC 1-Oct Aug-79 BM Legionowo Poland Jan-79 BM Lindenberg Germany Mar-92 ECC 14-Sep Feb-92 OSE de Bilt Netherlands Jan-93 ECC Uccle Belgium Jan-97 ECC 9-Nov Dec-97 BM Prague Czech Rep Jan-92 ECC 30-Jan Mar-91 OSE Hohenpeissenberg Germany Mar-65 BM Payerne Switzerland Nov-66 BM Haute Provence France Sep-90 ECC Sapporo Japan Dec-68 Japan Boulder USA Mar-79 ECC Wallops Island USA Jul-67 ECC Tsukuba/Tateno Japan Mar-68 Japan Kagoshima Japan May-68 Japan New Delhi India Nov-83 India Izana (Tenerife) Spain Nov-68 ECC Naha Japan May-68 Japan Taipei Rep. of China Jan-92 ECC Hong Kong China Mar-93 ECC Hilo USA Sep-82 ECC Petaling Jaya Malaysia Jan-92 ECC Kodaikanal India Jul-71 India Nairobi Kenya Jan-96 ECC Natal Brazil Aug-79 ECC Watukosek Indonesia May-93 Japan Ascension Island UK Jul-90 ECC Samoa USA Aug-86 ECC Reunion Island France Sep-92 ECC Pretoria/Irene South Africa Jul-90 ECC Easter Island Chile Jan-94 ECC Melbourne/Aspendale Australia Jun-65 BM Lauder New Zealand Aug-86 ECC Marambio Antarctica Mar-66 ECC Syowa Antarctica Mar-66 Japan McMurdo\Aug-Oct Antarctica Jan-88 ECC Neumeyer Antarctica Mar-92 ECC Amundsen-Scott Antarctica -90 N/A 1-Jan-86 ECC 34

47 Table 3.4: Some European balloon-borne experiments INSTRUMENT COUNTRY TECHNIQUE OTHER SPECIES Ozone sensors AMON France stellar occult., UV-VIS spectrometer NO 2, NO 3, OClO, OBrO, aerosol extinction BOCCAD France solar occult. and scattering, 4-l radiometer aerosols DOAS Germany limb viewing, UV-VIS spectrometer NOx, NOy, BrO, OclO LPMA France solar occult., FTIR spectrometer N 2O, NO, NO 2, HNO 3, ClONO 2, HCl, HF, H 2O, CH 4 MIPAS Germany emission, FTIR spectrometer N 2O, NO, NO 2, HNO 3, N 2O 5, ClONO 2, H 2O, CH 4 O 3 Semi Conductor UK in-situ, solid state sensor SALOMON France moon occult., UV-VIS spectrometer NO 2, NO 3, OClO, OBrO, aerosol extinction SAOZ France sun occult., UV-VIS spectrometer NO 2, aerosol extinction SPIRALE France in-situ, IR laser diode absorption spectrometer NO, NO 2, CO, CH 4 Other sensors ASTRID Germany in-situ, grab sampler N 2O, CFC-11, CH 4 BALLAD France limb viewing, VIS-NIR 3-l radiometer aerosols BROCOLI Germany resonance fluorescence ClO, BrO DESCARTES UK in-situ, cryogenic air sampler CFC-11, CFC-113 ELHYSA France in-situ, H 2O FISH Germany in-situ, Lyman-alpha hygrometer H 2O Filter Radiometer Germany in-situ, narrow-band UV radiometer photolysis rate of O 3, NO 2 Grab sampling Germany in-situ, air collection N 2O, CFC-11, CFC-12, CFC-113 LMD-Aerosol France in-situ, particle counter aerosol number Mass Spec Germany in-situ, mass spectrometer HNO 3, H 2SO4, HCl, HF MACSIMS France in-situ, mass spectrometer HNO 3, N 2O 5 RADIBAL, France solar scattering, NIR 2-l photopolarimeter aerosol model and extinction µradibal SDLA-LAMA France in-situ, NIR laser diode absorption spectrometer H 2O, CH 4 Research aircraft can carry comprehensive payloads for the study of the chemistry of the troposphere and stratosphere. Several aircraft/payload combinations exist and have been used extensively to investigate the stratosphere and upper troposphere. These aircraft programmes have made enormous contributions to the validation of satellite sensors and to our understanding of the chemistry of the stratosphere and upper troposphere, as well as of the relationship between chemical and transport processes. In some cases, the duration and spatial coverage of the sequence of missions is long enough to be relevant to studies of long term trend issues that are the focus of this report. Some examples of airborne research campaigns are provided in Annex D. 3.3 Satellite Measurements In this section, space-based systems capable of providing routine operational observations of the required parameters (see Chapter 2) are discussed and a summary of their capabilities presented. The space-based measurement systems are organised into two groups, namely those that are designed primarily for long term continuous operations and those that are planned as one-time experimental missions. Generally, the lifetimes of the latter are too short for their data to be relevant to the long term data requirements considered in this report. However, these groups are to a certain extent complementary and not necessarily exclusive, as some research-oriented satellites may operate for a sufficiently long time (e.g. UARS now has more than eight years of operation) that relatively long term studies can be carried out with their data. 35

48 Within the class of measurements designed for long term operation, presently-operating and future measuring systems are treated separately. Time lines for space-based measurement programmes contributing to the study of ozone are shown in Figure 3.5, while Table 3.5 shows the measurement objectives of the different space-based measurement programmes. In addition to the instruments and missions listed below, there are many others that provide information of relevance (e.g. insights into relevant processes) to the Ozone Project. These are listed in Annex E Currently Operating Operational Systems In this section five currently operating measurement systems, designed for the long term measurement of stratospheric ozone and related parameters, are considered. Additional long term measurement systems not yet in operation but planned are treated in Section 3.3.2, while relevant research satellite systems (both present and future) are considered in Section a) The Stratosphere Aerosol and Gas Experiment (SAGE II) series of instruments observe the absorption of visible and near-infrared radiation during solar occultation to determine the concentrations of ozone, water vapour, nitrogen dioxide, and aerosol extinction in the stratosphere and, for some parameters, in the upper (cloud free) troposphere. This technique is self-calibrating and can provide excellent accuracy/precision and vertical resolution (~1 km), although it has the usual spatial limitations associated with the solar occultation technique (i.e. two latitudes of observation per orbit corresponding to local sunrise and local sunset). The currently operating SAGE II instrument was launched in October 1984 onboard the Earth Radiation Budget Satellite (ERBS). The previous SAGE instrument (which could not be used to observe water vapour) operated from In both instances the satellites flew in inclined (~57 degree) orbits so their observations cover much of the Earth s surface (subject to the usual spatial sampling problems). A related instrument, the Stratospheric Aerosol Monitor (SAM II) made observations of PSCs from the Nimbus 7 satellite ( ) using a single near-infrared wavelength. This satellite flew in a polar-orbiting, sun-synchronous orbit so all the occultations were at high latitudes, which facilitated its for PSC studies. b) The Total Ozone Mapping Spectrometer (TOMS) series of instruments makes measurements of the total column amount of ozone using six ultraviolet wavelengths and the backscatter ultraviolet (BUV) technique. By exploiting cross-track scanning, the TOMS instruments typically obtain full daily coverage of the sunlit Earth. The horizontal resolution is typically 50x50 km 2 at nadir. Four TOMS instruments have flown - Nimbus 7 TOMS ( ), Meteor-3 TOMS ( ), Earth Probe TOMS ( present) and ADEOS TOMS ( ). An additional TOMS instrument is planned for 2000 on board the QuikTOMS spacecraft. With the exception of Meteor-3 TOMS, all the TOMS instruments have flown on board polar-orbiting, sun-synchronous satellites. Provided the equator crossing times are close to noon, solar zenith angles will be low and the orbit will be well tuned to the requirements of TOMS as atmospheric path lengths will be short. In addition to its measurements of total ozone, it has been shown that TOMS can provide information about tropospheric aerosols, stratospheric sulphur dioxide (when levels are high due to large volcanic eruptions), the surface flux of ultraviolet radiation, the ultraviolet reflectivity of the atmosphere (including the ground and clouds), and (this requires other data and depends on various assumptions) tropospheric ozone, especially at low latitudes. 36

49 Table 3.5 (a): Measurement objectives of the different space-based system (note that limb and occultation instruments measure predominantly stratospheric column not TOTAL column) Instrument Platform Ozone Column Ozone Profile Aerosol Column Aerosol Profile Constit. Column Constit. Profile Temp. Profile Winds Irrad. Surface UV TOMS Earth Probe X X SO 2 OMI EOS-Aura X X X SO 2, BrO, NO 2, CH 2O OMPS NPOESS X X X SO 2, BrO, CH 2O, OclO SBUV Nimbus 7 X X SO 2, NO (p< 1 mb) SBUV/2 NOAA-11, X X SO 2, 14, (POES) NO (p< 1 mb) SSBUV Shuttle X X SO 2, NO (p< 1 mb) GOME ERS-2 X X X SO 2, BrO, NO 2, CH 2O, OClO, H 2O SCIAMACHY ENVISAT X X X SO 2, BrO, NO 2, CH 2O, CO, CH 4, OclO, H 2O, N 2O GOME-2 METOP X X X SO 2, BrO, NO 2, CH 2O, OClO, H 2O SAGE I AEM-2 X X X X NO 2 NO 2 SAGE II ERBS X X X X NO 2, H 2O NO 2, H 2O SAGE III METEOR, X X X X NO 2, H 2O, NO 3, NO 2, H 2O, NO 3, ISS & TBD 1 OclO OclO UVISI MSX X X O 3, NO 2 O 3, NO 2 ACE SCISAT-1 X X X X About 30 About 30 species species SMILES ISS X X ClO, H 2O, H 2O 2, ClO, H 2O, H 2O 2, HCl, HNO 3, BrO, HCl, HNO 3, BrO, IMG ADEOS X X H 2O, CH 4, CO H 2O, CH 4, CO X X X X X UV/Vis UV/Vis /NIR UV UV UV UV/Vis UV/Vis /NIR UV/Vis X X X X X (Note 1) : TBD - to be determined) 37

50 Table 3.5 (b): Measurement objectives of the different space-based systems (note that limb and occultation instruments measure predominantly stratospheric column not TOTAL column) Instrument Platform Ozone Column Ozone Profile Aerosol Column Aerosol Profile Constit. Column Constit. Profile Temp. Profile Winds Irrad. Surface UV ODUS GCOM-A1 X X SO 2, BrO, NO 2, CH 2O, OClO SOFIS GCOM-1 X X X X NO 2, CH 4, CFCl 3, NO 2, CH 4, CFCl 3, CF 2Cl 2, HNO 3, ClONO 2, CF 2Cl 2, HNO 3, ClONO 2, CO 2 CO 2 POAM II SPOT-3 X X X X NO 2 NO 2 X POAM III SPOT-4 X X X X NO 2, H 2O NO 2, H 2O X UV/Vis X LIMS Nimbus 7 X X NO 2, H 2O, HNO 3 NO 2, H 2O, HNO 3 SAMS Nimbus 7 CH 4, N 2O CH 4, N 2O X SOLSE/LORE Shuttle X X ATMOS Shuttle X X Close to 30 species Close to 30 species x MAS Shuttle X X ClO, H 2O ClO, H 2O X CRISTA Shuttle/SPAS X More than 20 species More than 20 species x MAHRSI Shuttle/SPAS OH, NO OH, NO CLAES UARS X X X X More than 10 species More than 10 species X ISAMS UARS X X X X H 2O, CH 4, NO, NO 2, H 2O, CH 4, NO, NO 2, X N 2O, N 2O 5,HNO 3, CO N 2O, N 2O 5,HNO 3, CO HALOE UARS X X X X H 2O, CH 4, NO, NO 2, H 2O, CH 4, NO, NO 2, X HCl, HF HCl, HF MLS UARS X X ClO, H 2O, HNO 3 ClO, H 2O, HNO 3 X HRDI UARS X SOLSTICE UARS UV SUSIM UARS UV 38

51 Table 3.5 (c): Measurement objectives of the different space-based systems (note that limb and occultation instruments measure predominantly stratospheric column not TOTAL column) Instrument Platform Ozone Column Ozone Profile Aerosol Column Aerosol Profile Constit. Column Constit. Profile Temp. Profile Winds Irrad. Surface UV ILAS ADEOS X X X X NO 2, CH 4, CFCl 3, NO 2, CH 4, CFCl 3, CF 2Cl 2, HNO 3 CF 2Cl 2, HNO 3 RIS ADEOS X Osiris Odin X X X X NO 2 SO 2, CH 2O, BrO, OclO, H 2O, NO NO 2 SO 2, CH 2O, BrO, OClO, H 2O, NO SMR Odin X X More than 10 species More than 10 species GOMOS ENVISAT X X x x NO 2, NO 3, H 2O NO 2, NO 3, H 2O X MIPAS ENVISAT X X X X More than 20 species More than 20 species X ILAS-2 ADEOS-2 X X X X NO 2, CH 4, CFCl 3, CF 2Cl 2, HNO 3, ClONO 2 NO 2, CH 4, CFCl 3, CF 2Cl 2, HNO 3, ClONO 2 HIRDLS EOS-Aura X X CFC11, CFC12, ClONO 2, H 2O, N 2O, NO 2, N 2O 5, HNO 3, CH 4 MLS EOS-Aura X X ClO, H 2O, N 2O, CO, SO 2 CFC11, CFC12, ClONO 2, H 2O, N 2O, NO 2, N 2O 5, HNO 3, CH 4 ClO, H 2O, N 2O, CO, SO 2 TES EOS-Aura X X About 30 species About 30 species X ACE SCISAT-1 X X X X About 30 species About 30 species X SMILES ISS X X ClO, H 2O, H 2O 2, HCl, ClO, H 2O, H 2O 2, HCl, X HNO 3, BrO, HNO 3, BrO, X X 39

52 40

53 c) The Solar Backscatter Ultraviolet (SBUV) series of instruments measures both total ozone amounts and the vertical distributions of ozone using the backscatter ultraviolet (BUV) technique. The SBUV instrument also measures spectrally-resolved solar irradiance from 180 to 405 nm with 1 nm resolution. Instruments in this series have included the original SBUV instrument which flew on-board the Nimbus 7 satellite and the SBUV/2 instruments which have flown aboard several meteorological satellites (afternoon equatorial crossing time) operated by the US National Oceanic and Atmospheric Administration (NOAA), including NOAA-9, NOAA-11, and NOAA-14. Unlike TOMS, the SBUV instruments are not capable of cross-track scanning, as they only view in nadir. Vertical profiling is 7 km in the middle and upper stratosphere, with little sensitivity in the lower stratosphere. The equatorial crossing time of the NOAA POES spacecraft have drifted which may limit the usefulness of some of these data for high accuracy trend studies (though this has been taken into account in characterising the data and the algorithm; future POES platforms will have stable orbits beginning in 2000). Like TOMS, SBUV can provide some information on sulphur dioxide levels when these are elevated following volcanic eruption. When operated in a spectral scanning mode it can also provide information on the column amounts of nitric oxide in the mesosphere and thermosphere. Help in calibrating the SBUV instruments during the period was provided by observations made by a Shuttle-borne version of the instrument (SSBUV), which made eight flights over this time period. The SSBUV flights provided a first order correction to the long term observations of the SBUV/2 series. The SSBUV observations were particularly important for calibrating the SBUV/2 solar irradiance measurements which correlated well with the UARS solar irradiance instruments. d) The Global Ozone Monitoring Experiment (GOME) instrument was launched on board the European Space Agency s Earth Remote Sensing satellite (ERS-2) in GOME uses a nadir-viewing geometry to measure total column amounts and vertical profiles of ozone and total column amounts of a wide range of trace constituents, including BrO, NO 2, H 2 O, SO 2, CH 2 O, and OClO (in the polar vortex), as well as providing information on clouds, aerosols and surface spectral reflectance (see Table 3.5). The ERS- 2 spacecraft flies in a polar sun-synchronous orbit which is well suited for such measurements. The instrument has a broad spectral coverage ( nm) which is coupled with an excellent spectral resolution ( nm). This enables it to exploit a combination of spectroscopic fitting and the backscatter ultraviolet (BUV) technique. Its horizontal resolution can vary between 40x80 km 2 and 40x320 km 2 ; on most days it operates at the latter spatial resolution. The resolution of GOME ozone vertical profiles is 7-10 km and, because of its multispectral capability, profile information can be derived in both the lower stratosphere and the upper troposphere unlike BUV measurements. Like SBUV it has been calibrated against the SSBUV as well as against other instruments (see Chapter 5, Calibration and Validation). The GOME has a multi-faceted calibration programme including views of the Sun and the Moon as well as an internal lamp. e) The Tiros-N Operational Vertical Sounder (TOVS) series of instruments flying aboard NOAA s operational meteorological satellites were designed primarily as a source of meteorological data (notably temperature and moisture). However, in addition, they do provide information on total column ozone amounts (see also SEVIRI). This measurement is made using the 9.6 µm channel of the High Resolution Infrared Sounder (HIRS) which forms part of the TOVS and which was originally included to remove ozone effects from the temperature sounding channels. 41

54 There is some uncertainty as to what the 9.6 µm channel actually senses in the upper troposphere and lower stratosphere though it is probably best characterised as an observation of lower stratospheric ozone. TOVS is quite insensitive to middle- and upper-stratospheric ozone. However, it does provide data during the polar night which is not possible with either TOMS or GOME because they require the presence of solar radiation to make measurements; TOVS exploits emission and can therefore work in darkness Currently Planned Operational Satellite Systems This section considers satellite programmes planned for the near future which are intended to help satisfy requirements for the operational provision of these data on a long term basis. It also outlines some variants of current experimental instruments which may find operational application. Additional SBUV instruments are planned for future NOAA polar orbiting meteorological satellites with afternoon equatorial crossing times; currently scheduled launch dates are shown in Figure 3.5. Utimately, these instruments will be replaced by GOME-2, OMI, and OMPS which have superior performance. Table 3.5 provides details of the species observed by each of the operational systems described below together with an indication of their performances. Again, in referring to this table, it is important to remember that system performance is very dependent on orbit characteristics. a) The Stratosphere Aerosol and Gas Experiment (SAGE III) is an improved version of the SAGE instrument, with higher spectral resolution and greater spectral wavelength coverage. It will also have a lunar occultation capability which will allow observations to be made over a broader range of latitudes than are available from solar occultation alone (especially in sun-synchronous orbits where solar occultations are confined to high latitudes). Its lunar occultation capability should enable it to make measurements of NO 3 and OClO which are present almost exclusively at night because of their rapid daytime photolysis (temperatures will not be available in this mode of operation). As compared with previous SAGE instruments, this version has an ultraviolet channel (290 nm), which can be used for the improved detection of ozone in the upper atmosphere, and an additional near-infrared channel (1.5 µm) that can provide information on the distribution of aerosol in the cloud-free troposphere. Temperature information is also included through the observation of the molecular oxygen A band (near 762 nm); these measurements will facilitate the conversion of measurements to the desired mixing ratio versus pressure co-ordinate system (rather than the observed number density versus altitude one). In order to provide a better geographic coverage (given the inherent coverage limitation of solar occultation), the goal is to have one SAGE instrument flying in an inclined orbit and another in a sun-synchronous polar orbit. Currently planned flights are aboard a Russian Meteor-3M (polar sun-synchronous orbit) in mid-2000 and the International Space Station (51.5 degree inclination orbit) in early A third SAGE III instrument is currently under construction for use aboard a platform still to be decided. b) The Global Ozone Monitoring Experiment (GOME-2) is similar to GOME with slightly better accuracy and better spatial resolution, but the same vertical resolution. The improvements mainly relate to improvement in performance (i.e. accuracy) rather than the number of atmospheric variables observed. Like GOME, the GOME-2 instruments will be ultraviolet/visible, nadir-viewing instruments exploiting a combination of the SBUV and spectroscopic fitting techniques to observe a range of atmospheric variables, far wider than any of the other instruments described above (see Table 3.3). The GOME-2 instruments will be flown on the METOP series of meteorological satellites which have a total planned life time of fifteen years. They will fly in the morning polar orbit, METOP being the European operational replacement for the current series of 42

55 NOAA operational meteorological satellites. This is a joint ESA/EUMETSAT programme with the launches planned for 2003, 2007, and c) The Infrared Atmospheric Sounding Interferometer (IASI) is also part of the core payload of EUMETSAT Polar System (EPS) METOP-1 and will contribute to the primary mission objective of EPS which is the assessment of meteorological parameters. The AMSU-A and MHS microwave sounding systems, the HIRS/3 infrared sounder and the AVHRR/3 imager are all companion instruments of the meteorological payload. It will operate from a low altitude, sun-synchronous polar orbit, over a 2000 km wide swath. The main focus of IASI is the provision of temperature profiles with improved accuracy and vertical resolution compared with the currently existing infrared temperature sounder HIRS on the NOAA polar satellites. To achieve this goal a high spectral resolution is required, and a novel instrument was designed based on a Michelson interferometer. It will cover the spectral range from 645 to 2760 cm -1 with a spectral resolution (unapodised) between 0.25 and 0.5 cm -1. Among the parameters that will be measured with IASI, either in a stand-alone or in a synergistic mode with other EPS instruments, are, in addition to temperature profiles, water vapour profiles, surface characteristics (i.e. temperature, emissivity), cloud parameters (top pressure and temperature, effective amount) and column integrated and vertical information on some minor constituents (O 3, CO, CH 4, N 2 O, SO 2 ). d) The Ozone Mapping and Profiling Suite (OMPS) is a two-instrument combination being planned for the US National Polar Orbiting Environmental Satellite System (NPOESS) series of polar orbiting spacecraft. The OMPS instrument is still under definition though provisional instrument specifications have been listed. These envisage the OMPS instrument providing full daily global mapping of total column ozone amounts with a horizontal resolution of 50x50 km at nadir (or better), and vertical profiling (no mapping) with a vertical resolution of at least 5 km; with an objective of 3 km vertical resolution. The vertical profiling resolution requirement rules out a nadir-viewing instrument; therefore the OMPS instrument will include both a nadir-viewing total ozone instrument (using a push broom technique) and a limb-viewing vertical profile instrument using the limb scattering technique in the ultraviolet, the visible and the near-infrared. The first NPOESS spacecraft with the OMPS instrument is not expected to fly before 2009, although the actual launch date could vary in the range depending on the operational conditions of the remaining NOAA polar orbiting spacecraft (such as NOAA-N). It is currently expected that NPOESS will continue the spectrally resolved and total solar irradiance measurements using the instrument currently operating aboard UARS and planned for the SORCE mission (see Annex E). e) The Stationary Visible/Infrared Imager (SVIRI) will fly on the Second Generation Meteosat (MSG) series of satellites; the first is due for launch in 2001 and they have planned lifetimes of 15 years. These will be operational meteorological satellites flying in geostationary orbit above the Greenwich meridian. They will replace the current series of Meteosat satellites. SVIRI will represent a significant advance on the current Meteosat imager, having 5 channels in the visible and 5 in the infrared. One of the latter (at 9.7 µm) is intended to be used to observe the distribution of ozone though not to the accuracy attainable with GOME or SAGE. It will have a horizontal resolution of about 3 km. 43

56 3.3.3 Current and Projected Research Satellite Systems This section outlines four major research missions of sufficient duration to provide long term data sets relevant to the Ozone Project. In addition to the actual provision of data, experimental satellite missions also point the way to the future by providing a test bed for new operational instruments. a) The Upper Atmosphere Research Satellite (UARS) was launched in September, 1991 to study the chemistry and dynamics of the Earth s stratosphere and mesosphere, as well as solar radiation and particle forcing of the Earth-atmosphere system. UARS has a total of 10 instruments, all but two of which are still working. UARS instruments may be broken down into several categories - atmospheric chemistry (Halogen Occultation Experiment - HALOE, Microwave Limb Sounder - MLS, Cryogenic Limb Array Etalon Spectrometer - CLAES, Improved Stratospheric and Mesospheric Sounder - ISAMS), atmospheric dynamics (High Resolution Doppler Interferometer - HRDI, Wind Imaging Interferometer - WINDII), solar irradiance (Solar-Stellar Irradiance Comparison Experiment - SOLSTICE, Solar Ultraviolet Spectral Irradiance Monitor - SUSIM, Active Cavity Radiometer Irradiance Monitor - ACRIM), and particle input (Particle Environment Monitor - PEM). All except two of these (CLAES, ISAMS) continue to operate after more than eight years. UARS provides continuous coverage equatorward of 34 0 but only views higher latitudes half the time (viewing northward or southward in approximately 36 day increments). The present focus of UARS is to document long term changes in the upper atmosphere together with solar and particle forcings. In particular, the continuing measurement of the vertical profile of ozone, key source and reservoir gases, and temperatures, as well as the UV solar spectral irradiance have provided valuable data sets for the study of both long term trends and interannual variability in the stratosphere. b) The ENVISAT mission will fly in 2001 and includes three instruments focused on atmospheric chemistry. These are the Scanning Imaging Absorption Spectrometer for Atmospheric Cartography (SCIAMACHY), the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS), and the Global Ozone Monitoring by Occultation of Stars (GOMOS) instruments. SCIAMACHY is a multi-wavelength ( nm, µm), multi-viewing geometry (limb/nadir/occultation) instrument designed to measure the column and profile distribution of a number of gases, including high horizontal resolution measurements of ozone. MIPAS, a high spectral resolution limb sounder, operating in the wavelength range from µm, will provide measurements of vertical profiles of more than 20 species (especially nitrogen-containing species), as well as pressure/temperature, aerosols, and PSCs. GOMOS will use the stellar occultation technique to measure ozone profiles and the possibility to retrieve NO 2, NO 3, H 2 O, and aerosol extinction profiles. Since there are very many stars to use as light sources, the stellar occultation technique has the potential to provide much more complete spatial coverage than is available from solar occultations, so measurements at nearly all latitudes are possible even though ENVISAT will be in a polar sun-synchronous orbit. An operational version of this instrument is under consideration (i.e. COALA). ENVISAT will fly in a high inclination, sun-synchronous orbit at an altitude of 800 km. The local mean solar time in the descending node will be 10:00 hrs. The planned mission duration is five years. Both MIPAS and SCIAMACHY will provide global coverage; the coverage of GOMOS will depend on the distribution of occultation targets. c) The EOS-Aura mission which is planned for launch in mid 2003 will have four instruments dedicated to the study of atmospheric chemistry. These include the Tropospheric Emission Spectrometer (TES), the Microwave Limb Sounder (MLS), and the High Resolution Dynamics Limb Sounder (HIRDLS), and an ultraviolet mapping spectrometer for study of total column and profiles of ozone known as the Ozone 44

57 Monitoring Instrument (OMI). TES is a high spectral resolution instrument designed to observe both tropospheric ozone and its precursor nitrogen dioxide. It will have both a nadir and a limb mode, and will also provide information on a number of constituents in the lower to middle stratosphere. MLS is a significantly enhanced version of the UARS MLS instrument and will measure a number of constituents in the stratosphere and upper troposphere, including OH. By virtue of measuring the distribution of OH, ClO, BrO, and several nitrogen oxides, MLS will provide the first global information on catalytic ozone destruction by all important chemical families in the stratosphere. HIRDLS is an infrared emission instrument designed to measure stratospheric trace constituent and temperature distributions at high horizontal and vertical resolution, and should provide information on small-scale variability in the atmosphere for use in transport studies. A primary focus of HIRDLS is the study of long-lived trace gases that most clearly reflect atmospheric transport processes, although HIRDLS will also measure several chemical reservoir species as well. The OMI is a hyperspectral nadir viewing instrument with daily global coverage and spatial resolution of 13x24 km for total ozone column. Additional parameters, such as those measured by GOME (see Section 3.3.1) will also be measured by OMI but at somewhat reduced spatial resolution over that which it achieves for total ozone. EOS-Aura will fly in a 750km orbit with a 1:45 PM equatorial crossing time. Since TES, MLS, and HIRDLS are emission instruments, coverage of the entire Earth will be provided by these instruments. OMI, which uses a scattering technique, only obtains data over sunlit areas so no coverage of polar night is provided. The instruments and spacecraft are designed for five years. d) The GCOM-A1 programme of Japan will consist of two instruments for atmospheric chemistry flying aboard a satellite in an inclined non sun-synchronous orbit planned for launch in The first of these instruments is the Ozone Dynamics Ultraviolet Spectrometer (ODUS), a grating spectrometer covering the wavelength range from 306 to 420 nm with 0.5 nm spectral resolution and ground resolution of 20x20 km at nadir. It is designed to measure the column amounts of ozone, aerosol, SO 2, NO 2, BrO, and OClO (similar to GOME and OMI). The second instrument is a follow-on to the second Improved Limb Atmospheric Spectrometer (ILAS-II) planned to fly aboard ADEOS-II in This follow on instrument will improve on the ozone, aerosol, and trace constituent profiles made by its predecessors. It is planned to complement the GCOM-A1 payload with a third instrument. The GCOM-A1 spacecraft is planned for operation until In addition to these three major missions there are several other missions that can provide information of relevance to the Ozone Project. These missions are more concerned with process studies than monitoring. They are listed in Annex E Observations from Non-Low Earth Orbits Finally it is relevant to highlight the role of space-borne instruments making observations from satellites flying on non-low Earth orbits as, in particular, they offer the opportunity to increase temporal coverage. This is important as there are large diurnal variations in the emission of pollutants which are associated with the photochemical production of ozone in the planetary boundary layer and free troposphere. The levels of ozone produced are high. The production of tropospheric ozone and photochemical smog typically peaks in the midto-late afternoon. NO 2 emissions follow traffic and energy use patterns. The time scale of the oxidation of SO 2 yielding H 2 SO 4 which acts as condensation nuclei for aerosol and cloud formation, is similarly short. Biogenic emissions also have complex emission patterns increasing when the temperature rises but also being strongly dependent on levels of humidity. Both determine the rate of opening of stomata. Comprehensive measurements of air quality on urban, 45

58 regional, continental, and global scales impose stringent requirements on space-borne observations with a focus on the upper troposphere. High temporal and spatial resolutions are required to determine accurately the emission rates of pollutants from industrial pollution, biomass burning and biogenic emissions to the atmosphere. These measurements are also necessary to assess the impact of anthropogenic activity on air quality and for meteorological applications. Excluding latitudes above about 70, remote sensing instrumentation on sun-synchronous low Earth orbits (LEO) can only observe the same location at the same local time at best once a day. This may be contrasted with instruments mounted on geostationary platforms which can observe continuously approximately one third of the globe, from about 65 0 N to 65 0 S. A further observing location is the so-called L1 orbit, which enables the whole Earth to be observed about once a day. In order to obtain global coverage having the required spatial and temporal resolution a fleet of LEO satellite-borne instruments (approximately 14) is needed. The same data set can be generated by instruments aboard three geostationary platforms, separated by 120 in longitude. In theory one instrument in a L1 orbit can also provide the same information. On the downside it is important to remember that the geostationary orbit is approximately 40 times further from the Earth than the LEO and the L1 orbit is further away still. Thus a passive remote sensing instrument in L1 requires a relatively large telescope and launcher. The TRIANA mission now planned for 2002 launch will be the first Earth science mission to test this vantage point. Instruments in geostationary orbit are attractive for the transcontinental monitoring of pollution and biomass burning. Observations of quite high spatial resolution are possible from this orbit if the ability to stare continuously at a particular location is used to increase signal-to-noise ratios. A combination of three geostationary platforms, as has been employed for meteorological observations, would provide the requisite global coverage. This geostationary perspective has not yet been exploited to monitor air quality though several missions are now under study to evaluate this unique vantage point. These are described in Annex E. Currently there is an experimental TOZ total ozone column product available from the NOAA GOES platform which is similar to the TOVS product. It is only obtained in non-cloudy fields-of-view but it is produced many times a day. 46

59 4. HARMONISATION OF PROVISIONS AND REQUIREMENTS 4.1 Introduction In this chapter the user requirements (see Chapter 2) are compared with current data provisions (see Chapter 3). This assessment takes account, not only of the provision and capabilities of space-borne instruments but also those of the other elements of the observing network i.e. ground-based and airborne. Chapter 6 highlights the main conclusions and recommendations that emerge from this exercise. The provision of the large number of measurement systems described in Chapter 3 goes a long way towards helping to satisfy the requirements outlined in Chapter 2. However, in addition to addressing individual requirements, there is a critical overarching requirement that the Ozone Project must satisfy, namely the need for: accurate long term calibration. continuity of data provision with overlap in case of instrument change; The first requires regular calibration (traceable to international standards) and validation of derived geophysical parameters over the lifetime of a sensor or observing system. It is essential to avoid gaps in data streams, inconsistent calibration between instruments and long term drifts in instrument performance. It is also important to ensure the proper harmonisation of ground and space-based systems. The quality of ground-based observations and their spatial representivity must be documented. The second requirement represents a particularly challenging issue for space-based systems as it may prove prohibitively expensive to try to ensure the provision of systems sufficiently robust to safeguard continuity in case of failures (including launch, spacecraft and instrument failures) for anything beyond a very limited set of parameters. Some prioritisation of the need for continuity of observation of different parameters must be established. There are two different measurement protocols covering the required parameters, summarised as: monitoring long term continuous measurement by a series of closely related and regularly intercalibrated instruments; regular observation - continuity of measurement is desired but there is a much greater tolerance for gaps in data records. Thus, species such as the chlorofluorcarbons with very small spatial and temporal variation may only require periodic observation from space for long term characterisation, while a trace gas such as ozone must be continually monitored. The other point that must be highlighted is the need to view all the various components of the observing system. Thus, in the sections that follow, in addition to considering each system in its own right, the composite picture is assessed in the light of the requirements detailed in Chapter 2. The question of the calibration and validation of space-borne instruments is addressed in Chapter 5 Calibration and Validation. 4.2 Total Column Ozone Ground-Based Measurements The current ground-based measurement programme for total ozone column amounts is adequate for providing long term, well calibrated measurements of column amounts for monitoring stratospheric trends. However, there are difficulties in ensuring continued support for the operation of some of the existing stations as in several instances only minimal funding is available. Unless 47

60 this is corrected it will ultimately compromise the quality of the data, especially if resources become inadequate to support participation in calibration-related activities. The elements of the current network are deployed in areas where support from national agencies is available, that are logistically convenient for operation and, as a result, are not well distributed geographically. In particular, there is a lack of sites in the tropics and Southern Hemisphere. The provision of funding for establishing networks in the tropics and Southern Hemisphere is a challenge for the international community which must be resolved in the very near future Space-Based Measurements Space-based measurements of total and profile ozone amounts form one of the most important IGOS data sets although they mainly reflect changes in stratospheric ozone. No single observing technique fully meets user requirements so it is necessary to exploit the capabilities of complementary observing systems. For example, there is excellent complementarity between the TOMS technique which uses a limited set of wavelengths but provides data with excellent spatial resolution, and the GOME approach which can observe a much larger set of wavelengths but at reduced spatial resolution. Because of their enhanced spectral coverage, the GOME-type instruments have the potential to make more accurate observations than TOMS. The plans currently in place by the space agencies go a long way towards meeting the requirements listed earlier. Two major polar orbiting programmes are already in place: The US programme of TOMS (one flying now, one planned for launch in 2000), the Dutch OMI instrument on EOS-Aura in 2003 and the OMPS instrument planned for NPOESS (beginning in ). The European programme of GOME (currently flying on ERS-2), SCIAMACHY (planned for 2001 on ENVISAT) and GOME-2 and IASI (planned for 2005 and beyond on the METOP series of operational satellites). In addition, there is NOAA/SBUV/2 which is a major operational programme currently in place. METOP and NPOESS will be fully operational programmes (after the launch of METOP-2 there should always be a "hot" spare available in orbit) and ENVISAT will supply products operationally. The provision of operational products from EOS-Aura is being considered. A gap in the US series could occur prior to 2010 if the NPOESS OMPS instrument does not fly prior to the end of the expected period of operation of the OMI instrument on EOS-Aura (late 2007 assuming a late 2002 launch). In such a case, the global observing system would depend on the instruments flown on the METOP/GOME-2 and the POES/SBUV/2 which do not provide the spatial coverage to meet all requirements (Chapter 2). If the first NPOESS/OMPS launch does not occur till the end of its potential launch window an alternative strategy must be considered. One possibility would be an early flight of the OMPS instrument. The Japanese GCOM-A1/ODUS which is planned for 2006 to 2010 would be a good candidate to fill this gap. However, the GCOM-A1 orbit is not sun-synchronous and therefore cannot meet the spatial requirement. From geostationary orbit only Meteosat Second Generation (MSG) includes an ozone monitor (from 2001). This will be based on an infrared emission nadir-viewing technique and has quite a lot in common with TOVS. These data are not strictly compatible with those from ultraviolet-based nadir-viewing systems so intercomparisons will be essential. The inclusion of similar (or ultraviolet) instruments on some of the other geostationary satellites would be a very good idea as a combination of polar orbiting and geostationary systems is required to ensure the proper combination of geographic and temporal coverage. 48

61 Overall the situation is quite encouraging provided NPOESS OMPS is not delayed. Exploitation of geostationary missions will provide a unique opportunity to strengthen temporal coverage. 4.3 Ozone Vertical Profile Ground-Based Measurements In the stratosphere the ground-based measurements of vertical profiles of ozone which are of most relevance to the Ozone Project are the lidar and Umkehr observations (based on the Dobson and Brewer instruments), though currently only a limited number of Dobson instruments are being used for Umkehr observations. A first priority must be to ensure the continuation of Umkehr observations from those stations with long data records. However, the initiation of new Umkehr observations is also necessary as geographic coverage is currently limited. Lidar instruments have the potential to provide high resolution, well calibrated, observations of ozone profiles in the stratosphere and will undoubtedly increase in importance in the future. Many of the lidar observing systems are already affiliated with the NDSC. This is important as adherence to NDSC protocols undoubtedly helps to ensure the overall consistency of data quality. The current ozone lidar network is not well distributed geographically and the provision of additional lidar sites in the tropics and Southern Hemisphere is very important. In the troposphere, lidars are principally able to provide high resolution information on ozone profiles so, given the increasing importance of tropospheric ozone data, the provision of additional lidar instruments and their intercomparison must be viewed as a priority. These should also be affiliated with the NDSC and WMO-GAW. The contribution of these observing systems will undoubtedly increase as they become more widely distributed (possibly associated with their expanding use in air quality monitoring programmes). They would also prove very useful in helping to validate space-based tropospheric ozone profiles. The microwave spectrometers included in the NDSC can also be used to observe vertical profiles of ozone. These instruments are fairly unique as they can provide information day and night (this also makes geophysical validation easier). The provision of these data together with observations of some important related trace constituents, notably chlorine monoxide, water vapour and nitrous oxide, is of great importance. It is essential to ensure the continuation of high latitude observations made with these instruments (especially in winter) as they are the best source of these data. The main concerns here are to improve the geographic distribution of the lidar stations, notably in the Southern Hemisphere and in the tropics, and to increase the number of such stations affiliated with the NDSC Balloon- and Aircraft-Based Measurements An extremely critical element of an integrated system is the maintenance of the balloonbased ozone sonde programme, whose importance was noted earlier (Chapter 3). Apart from ensuring the continuation of the current system, the main priorities are the expansion of the network in the tropics and Southern Hemisphere, the continued focus on calibration and intercomparison according to the WMO-GAW programme (especially if new types of instrument are introduced) and the development of new improved instruments that correct some of the deficiencies of current instruments. Some progress has been made in improving the availability of ozone sonde data in the tropics but much of this is on a temporary basis. The development of a long term international plan to ensure the continued operation of ozone sondes in the tropics is a top priority as, at present, there is a strong possibility of the data base being terminated or drastically reduced. For ozone sonde data to be useful for trend determination, a measurement frequency of at least twice 49

62 monthly is needed, with weekly flights being preferable. The expansion of regular ozone sonde flights to several geographic areas, where there are few or no regular data (i.e. South America, Africa, all regions of the former Soviet Union and the Middle East), would be a major enhancement to the current international observing programme. Calibration is also an issue as the use of different types of ozone sondes in different locations, combined with the effects of variations in sonde preparation on their use, means that the different groups must regularly intercompare sondes (both in actual use and in controlled chambers). Support for this type of activity has been limited in the past. Steps must be taken to ensure their continuation and regular implementation. Intercomparisons with lidars is equally important. This topic is discussed in detail in Chapter 5. Finally, it is necessary to focus efforts on the development of improved ozone sondes, especially if they can be made smaller, simpler and/or cheaper, to facilitate increased use. This could be an excellent goal for technology programmes. Current problems include requirements for pump efficiency corrections and the need to normalise to Dobson by comparing integrated total ozone profiles with those obtained from nearby Dobson stations. Routine operational aircraft observations, such as those within the MOZAIC programme, should continue and be expanded because of their ability to characterise the tropopause region, especially at mid-to-high latitudes (the tropical tropopause is well above the altitude range accessible to today s commercial aircraft). Some expansion of the programme to improve the spatial distribution of measurements would be very useful, especially when this means that these data span several ozone sonde and/or lidar locations. Again the main concern is the lack of coverage in the tropics and in the Southern Hemisphere, though in assessing the requirement the provision of lidar stations must be taken into account. Routine operational aircraft observations should also be expanded and the development of improved ozone sondes considered Space-Based Measurements Space-based vertical profile measurements of ozone are not nearly as well in line with requirements as is the case for total ozone column amounts. The current situation is complex - several instruments associated with long term measurement programmes measure the vertical profile of ozone in the stratosphere and in the cloud-free upper troposphere, but none has all the desired characteristics (i.e. good vertical resolution, good spatial coverage). Candidate solutions for the stratosphere include ultraviolet/visible limb scattering (proposed for NPOESS) and microwave/infrared emission. In the troposphere proper, the problem is even more acute as only limited data are currently available and vertical resolution is coarse except near the tropopause. A pressing requirement is the development of instruments capable of redressing this deficiency. Those instruments with good vertical resolution (e.g. SAGE II and soon SAGE III) tend to have limited spatial coverage because of their reliance on solar occultation, while those with better spatial coverage (e.g. SBUV/2 and OMI) lack vertical resolution, especially in the troposphere and lower stratosphere. For the future, instruments based on the stellar occultation technique (e.g. GOMOS) may strike a reasonable compromise between these two extremes though, as indicated above, there are other possibilities including limb viewing infrared and microwave instruments. However, none of these techniques appears capable of meeting requirements in the troposphere. Many of the current instruments are operating well beyond their anticipated lifetimes, for example, SAGE II has been operating since SAGE III instruments are planned for launch but the first of these instruments (as the current POAM-3 instrument) will fly in a polar sunsynchronous orbit so that solar occultation will be limited to high latitudes. Lunar occultations made with this instrument will provide some tropical and mid-latitude data but these must be considered as experimental until successfully demonstrated using actual SAGE III data (SCIAMACHY should also help clarify the potential of this approach). 50

63 The next SAGE instrument in an inclined orbit, planned for the International Space Station (ISS), will not be launched until 2002 and will suffer from some loss of data due to downtime associated with Shuttle visits as well as the limited viewing capabilities available from the ISS. An additional SAGE III instrument is being constructed, but no flight opportunity has yet been identified. The SBUV/2 situation is mixed as the NOAA-14 instrument has problems with its grating drive and the solar diffuser is not operational on the NOAA-11 instrument (this limits calibration for ozone profiles). The SBUV/2 instrument launched in September 2000 will overlap NOAA-9 and NOAA-11, and will be major sources of archival information on ozone changes in the upper stratosphere. A significant contribution should ultimately be made by the GOME instrument on ERS-2 which is just starting to return ozone vertical profiles. Although these must still be considered as research products, operational products may shortly become available. Reprocessing should ensure the availability of these data back to 1995 and support the requirements for trend detection stated in Chapter 2. Mention must also be made of SCIAMACHY, OMI and GOME-2, especially the latter which will fly on an operational series of satellites. Together these instruments (also with OSIRIS and SMR on ODIN) should ensure data continuity from 1995 to at least However, given the importance of these data, there is still a need for a high vertical resolution (with good global coverage) operational ozone profiler, such as is planned for NPOESS (OMPS), that can be flown on a regular basis. Over the next few years a number of ozone-profile measuring research-oriented instruments will be launched. Several of these (notably MIPAS, HIRDLS and MLS) should provide the desired combination of vertical resolution and spatial coverage (though the troposphere will remain a problem). However, these instruments are all complex and not well suited to long term operational use. Significant effort will have to be devoted to the construction of a unified data set which includes data from these and any predecessor instruments that may overlap with them. SCIAMACHY, in its solar, lunar occultation and limb scattering modes, will provide profiles to the desired vertical resolution in the stratosphere and upper troposphere though, like SAGE, with limited geographic coverage. GOMOS has the potential to provide both good vertical resolution and reasonable geographic coverage. However, this will be the first routine implementation of stellar occultation (currently limited to a small number of observations by UVISI which are not generally available) so its role in ensuring a continuous data set cannot be taken for granted. An operational version (COALA) is on the drawing board which could be implemented if GOMOS lives up to expectations. Overall the situation is complex as there is no single instrument (or group of instruments) capable of fully meeting the requirements, notably for good vertical profiles in the troposphere. It is also disturbing that the flight opportunities required to exploit SAGE III are still not confirmed. 4.4 Meteorological Parameters Although another of the CEOS projects (i.e. the Upper Air Project) in support of IGOS will be providing requirements for most of the meteorological variables relevant to the Ozone Project (i.e. temperature, wind, cloud information, water vapour concentrations/specific humidity, etc.), it does not per se consider the requirements of the Ozone Project for these data. In most instances this does not pose a problem but there are some instances where the Ozone Project's requirements for meteorological information are stricter than those required to meet the objectives of the Upper Air Project. These are considered in this section but, without doubt, the most serious concern is the provision of adequate observations of water vapour in the upper troposphere and in the stratosphere, and the precise location of the tropopause. 51

64 The discussion in the ensuing sections does not implicitly refer to the contribution of the operational radiosonde network. These sondes are capable of measuring all these parameters to the requisite accuracy but are restricted in geographic coverage, notably in the tropics and in the Southern Hemisphere. Programmes such as MOZIAC help to resolve the coverage problem over the oceans in the Northern Hemisphere but the rest of the globe remains a problem. It is also clear that GNSS occultation data have a vital role to play, assuming these data are properly assimilated Upper Tropospheric and Stratospheric Water Vapour Although the Upper Air Project has listed requirements for water vapour profiles in the upper troposphere, these are inadequate for the Ozone Project. In particular, the climate and weather oriented focus of the Upper Air Project has led to these requirements being stated in terms of specific humidity. This makes sense in the lower and middle troposphere where water vapour amounts are reasonably large, but is of little use in the upper troposphere where the concentrations are extremely small (lower ppm-range). Here a small uncertainty in specific humidity can correspond to an enormous uncertainty in relative humidity. The stricter requirements placed on upper tropospheric and stratospheric water vapour near the tropopause (see Annex B; Table B.2a) impose a significant constraint on measurement systems. In particular, the large change in water vapour mixing ratio with altitude, especially near the top of the tropospause, means that high vertical resolution is of critical importance if the observations are to prove useful within the context of the Ozone Project. Here ground-based lidars, capable of measuring water vapour profiles, should play a very useful role. The operational space-borne meteorological sounders are not designed to meet these requirements. However, limb-viewing chemically-oriented profilers such as MIPAS, HIRDLS, SCIAMACHY and MLS do have the capability but all are non-operational instruments and long term data continuity is not anticipated. Another possibility is the combination of GPS/MET data with independent temperature observations which has considerable potential for meeting the ozone Project's requirements. To date the most important long-heritage measurements of water vapour in the lower stratosphere and upper troposphere are those emanating from HALOE (since September 1991) and SAGE II (covering mainly the pre-pinatubo period when aerosol contamination was small; reinitiation of the SAGE II water vapour measurements once stratospheric aerosol loading has declined (after 1995) should be feasible though the revised SAGE algorithm is not yet available. However, both these instruments are well past their planned lifetimes. The logical step would be to seek the continuation of one or both of these occultation-based measurements. The SAGE III instrument planned for the ISS should provide tropical and mid-latitude coverage (though viewing will be limited - see earlier). High latitude observations will be provided by SAGE III, POAM-3 and ILAS-2, but long term measurements are not guaranteed Stratospheric Temperatures The need for accurate temperature information throughout the stratosphere is clearly essential to the Ozone Project and here again the requirements are stricter than those formulated by the Upper Air Project. These temperature data are also required to convert the observations made by occultation-viewing instruments (they actually observe number density versus altitude) into the more scientifically useful mixing ratio versus pressure co-ordinate system. However, fortunately, most of the newer occultation sensors make simultaneous measurements of temperature (e.g. SAGE III, POAM-2) so normally there would be no need to look to externally supplied temperatures. 52

65 Several of the existing measurement systems can provide the requisite stratospheric temperature information, including lidars, GNSS occultation, radiosondes and essentially all infrared- and microwave-based satellite instruments (though not all on an operational basis) and a sensor onboard MOZAIC. The oxygen A-band absorption technique which is already being used on several satellite systems (e.g. GOMOS and SAGE III) will also provide temperature profiles. Also, if successfully implemented, ultraviolet limb scattering (SCIAMACHY) could also help ensure the provision of the requisite observations of stratospheric temperatures (air density and ozone must be retrieved together) as will research-oriented instruments exploiting emission techniques (e.g. HIRDLS, MLS and MIPAS). Given this multiplicity of sources it appears that most of the stratospheric temperature needs for ozone applications should in principle be met, at least during ENVISAT and EOS-Aura Tropopause Height and Temperature The interpretation of long term records of ozone amounts (especially total ozone column amounts) requires the height of the tropopause to be accurately known as there is a strong correlation between tropopause height and ozone column amounts. Even a very small change in tropopause height would, if it continued for an extended period of time, have an effect on derived ozone column amounts which might be confused with actual chemically-generated changes. The needed precision (i.e. tropopause height known to approximately 100 m; year-to-year consistency to about 50 m - see Chapter 2) requires the long term availability of lidar, radiosonde and/or GNSS occultation data. No other measuring systems can be expected to provide the requisite accuracy and stability of observation of tropopause height. As noted earlier, the geographic distribution of lidar stations is limited, especially in the tropics and Southern Hemisphere, though GNSS occultation should provide good global coverage. Continued support for both systems would ensure the availability of good global information to the requisite accuracy. The temperature of the tropopause is also a required parameter. This should be obtained to sufficient accuracy by any temperature profiling system meeting the requirements specified by the Upper Air Project (using the WMO definition of tropopause). Here specific mention must be made of the IASI and AIRS instruments. The former will fly on the operational METOP satellites Cloud Top Height and Cover As far as the Ozone Project is required, the main justification for observations of cloud top heights (and coverage) is for use in the accurate retrieval of ozone information from instruments with nadir-viewing geometry and for estimating UV fluxes at the ground. The most important requirement placed on observations of cloud top height and coverage is the co-registration of cloud top heights with ozone measurements (especially total column amounts). This can be achieved in one of two ways - either as part of the measurements made by the ozone instrument itself, for example, through measurement of the oxygen A-band at 762 nm or at shorter wavelengths (such as nm) through the Ring Effect or by the inclusion of another instrument (i.e. lidar) on the same platform as the ozone measuring instrument. Some of the available and projected instruments (e.g. SAGE III, GOME, OMI and SCIAMACHY) have sufficient spectral range and wavelength resolution for cloud top heights to be derivable from the instrument s data alone. The same will be true of GOME-2 on METOP, an operational system which includes IASI, another source of cloud height information. The imager on METOP will of course provide high quality images of clouds as will the NPOESS instrument. Cloud cover should prove no problem. 53

66 4.5 Related Chemical Constituents A list of related chemical constituents that are needed to interpret ozone changes was presented in Chapter 1. This was based on the assumption that the actual requirement is for continued observations of a small number of key parameters whose concentrations can be related to those of other constituents through chemical models. The requirements listed in Chapter 2 are, at least for the stratosphere, quite well addressed by planned space and ground-based measurement systems. However, there is a problem ensuring the long term continuity of observation of some of the parameters which will be measured from ENVISAT and EOS-Aura but not from the planned operational systems. Follow on research space platforms are only being considered. For many of these chemical constituents the observing requirements are more in the vein of continuous observations rather than monitoring (see earlier). This means that small gaps in data records, instrument-to-instrument variability and long term drift can be tolerated provided biases and precision are compatible with the detection of small changes. This is not generally the case for ozone column and profile measurements (or for temperature profiles) Associated Trace Constituents a) Surface-Based In Situ Measurements The major requirement that must be met by surface-based in situ measurements is the need for very accurate determination of concentrations of CFCs, halons, CFC replacements, other halocarbons (including methyl chloroform, carbon tetrachloride, methyl bromide) and other chemically and radiatively active source gases (e.g. nitrous oxide, methyl bromide). Existing long term networks, notably WMO-GAW (e.g. AGAGE and the NOAA/CMDL flask sampling network), perform well in this area and essentially meet the requirements listed in Chapter 2. However, geographic coverage remains a problem. It is clear that these activities, with their strong emphasis on ensuring consistency of calibration over both the short and long term must continue. This means that all sites attempting to document the long term evolution of surface concentrations of halocarbons and related species, must engage in calibration tests, intercomparisons and data quality control (see Chapter 5). Some expansion of the current network is essential if improved information about the longitudinal distribution of sources of long life gases in the Northern Hemisphere is to be derived from the surface concentration data (data from relatively industrialised areas in Europe and Asia would be especially useful additions to the overall international network). There is also a need to expand the data base in the tropics and in the Southern Hemisphere. b) Surface-Based Remote Sensing Measurements Through its combination of sensing instruments the NDSC can provide information on all the main trace constituents (see Table 3.1). This means that continued support for the NDSC is crucial to documenting the long term evolution of the distributions of trace constituents. It is especially true of total column amounts which are most easily measured with infrared and ultraviolet/visible instruments. Microwave radiometers also provide profile information on chlorine oxide (ClO) and other species. Again some geographic expansion of the current network would be desirable through the addition of complementary sites, especially in the tropics and Southern Hemisphere. However, it is important to remember that only those sites that pay sufficient attention to long term calibration can be considered as contributing to the overall requirement. 54

67 c) Space-Based Measurements The constituents considered in this section are those for which there is the greatest need for continuous measurement from space (these are listed in Table 1.1). Not all these need, in general, to be measured in the monitoring mode since their long term trend can be reliably assessed from an expanded ground-based network (i.e. CO 2, N 2 O, CH 4 and CFCs). Nitrous Oxide ( N 2 O) and Methane (CH 4 ) - Source Gases - currently planned instruments will provide quite a lot of information on these two trace gases. Thus, observations of CH 4 are currently being made with HALOE, and the planned MIPAS, SCIAMACHY, ILAS-2, HIRDLS, MLS and TES instruments will all make measurements of the vertical distribution of one or both of these constituents. The vertical resolutions of these instruments are more in line with the threshold requirement (3 km) than the target value (1 km). HIRDLS, because of its high vertical and horizontal resolution, probably comes closest to the requirements listed in Chapter 2, at least for the stratosphere and upper troposphere. In the lower troposphere where the variations in these constituents are fairly small against a large background, the planned measurement systems will do less well though both SCIAMACHY and MOPITT will observe CH 4 column amounts as will IASI on METOP. Post ENVISAT/EOS-Aura only the latter will continue. There are no specific NPOESS requirements for these measurements. Instruments exploiting infrared-based occultation have the capability to meet the requirements but again there are no firm plans to develop operational versions of these instruments, or for further flight opportunities. None of these instruments can meet the requirements for tropospheric data Carbon Monoxide (CO) - Source Gas - the most important historical set of measurements of CO were made using the MAPS instrument flying on the US space shuttle though only for short periods. Regular measurements from space will become available with MOPITT on TERRA, SCIAMACHY on ENVISAT, IASI on METOP and TES on EOS-Aura. Beyond that only IASI data will be available. Carbon Dioxide (CO 2 ) - Source Gas - CO 2 measurements from space have thus far been mainly used for determining atmospheric temperature profiles. With the possible exception of SCIAMACHY, the accuracy of space-based CO 2 observations is not sufficient to detect smaller, short term changes in CO 2, or to observe horizontal variations, though the larger changes (factor of two) expected over the next century should be detectable. For current trend detection, none of the sensors meet the requirements. Hydrogen Chloride (HCl) -Reservoir - the vertical profile of HCl is currently being measured by the HALOE instrument on UARS and will be measured by the MLS instrument on EOS- Aura. In addition there is the SMILES instrument on the International Space Station which can also observe HCl (though with limited viewing capability - see earlier). Beyond this the future is uncertain though the continued operation of a HALOE-like infrared occultation instrument or a related instrument, such as an infrared Fourier transform spectrometer, in an inclined orbit would go a long way towards satisfying the requirement. An alternative approach would be the periodic flight of a suitable instrument (probably exploiting infrared solar occultation) on the Space Shuttle, such as has been done with the ATMOS instrument. As there is relatively little seasonal or spatial variation in the distribution of this constituent near the stratopause, long term trends can in principle be determined from a series of intermittent measurements. Here the self-calibrating nature of occultation instruments would be a distinct advantage as this would help reduce the uncertainty associated with intermittent observations. 55

68 Nitric Acid (HNO 3 ) - Reservoir - so far satellite observations of HNO 3 have only been conducted on a limited basis, notably with HALOE on UARS. However these observations would be useful in helping to clarify questions relating to polar stratospheric denitrification. Some data should be provided by MIPAS on ENVISAT. The performance of TES on EOS- Aura will also be of interest as it should be able to observe HNO 3 from the surface to around 35 km. HNO 3 will be adequately observed from HIRDSC on EOS-Aura. Nitrogen Dioxide (NO 2 ) and Nitric Oxide (NO) - Free Radicals - currently operating and planned satellite systems should provide significant information on both NO and NO 2. HALOE and SAGE II use solar occultation to measure their profiles and GOME can be used to observe total column amounts. Vertical profiles of NO and NO 2 will also be measured by several forthcoming instruments, notably SAGE III, POAM-3, SCIAMACHY, GOMOS (+COALA), HIRDLS and TES. Total column amounts will be measured by GOME- 2, SCIAMACHY and OMI. For the long term, observations of column amounts is adequate but the same cannot be said of profile information at high latitudes. Denitrification at high latitudes is very important so it is vital to maintain profile measurements at high latitudes to complement the measurements made using occultation instruments in a polar sun-synchronous orbits. The current plan for this (involving POAM-3, SAGE III and ILAS-2) will provide some measurements, but this is not very well coordinated (as evidenced by the unfortunate separation between the visible/ultraviolet POAM-3 and SAGE III instruments and the infrared ILAS-2). ACE will now carry both an ultraviolet/visible spectrometer and an infrared interferometer focusing on occultation measurements on many gases related to polar processing. This is no assurance that these measurements will be continued beyond ACE. Chlorine Monoxide (ClO)- Free Radical - observations are currently being made with the MLS instrument onboard UARS and will be continued with its successor onboard EOS- CHEM. Additional observations should come from the microwave instrument aboard ODIN and SMILES on the International Space Station (though with limited viewing capabilities). However, given the importance of ClO, it is necessary to develop a long term plan for ensuring the provision of continuous observations of ClO. These need not be in the monitoring mode (as described above) as a continuous series of high quality observations (with occasional gaps) should prove adequate given the focus on examining inter-annual variations over the entire globe. The need for long term trend information can perhaps be met by ground-based microwave radiometers associated with the NDSC. Therefore, the major need for ClO is to develop a plan for the continuation of vertical profile measurements in the post EOS Aura timeframe. The requirement could probably be met by the provision of a relatively focused microwave instrument. Another possibility is SMILES on the International Space Station, though viewing will be limited and its orbit is incompatible with the need to observe ClO in the polar regions. Bromine Oxide (BrO) - Free Radical - the need for global observations of total column amounts can be met to a certain extent by making observations in the ultraviolet/visible as with GOME, SCIAMACHY and OMI. In the long term these data will be safeguarded by the provision of GOME-2 (and its successors) on METOP. 56

69 5.1 Introduction 5. CALIBRATION AND VALIDATION To assure the scientific value of remote sensing measurements, calibration and validation are critical activities; for deriving climate quality data sets they are essential. This is recognised by the space faring nations who have and must continue to allocate resources for the calibration and validation for Earth science research missions. For example, NASA s UARS programme set aside support for correlative measurements to validate most of its key data products. This effort provided the essential credibility for UARS data which led researchers to use the data to make some major scientific discoveries on the processes controlling stratospheric ozone. CNES played a major role in the UARS correlative measurements programme. Their validation activities continued in support of ILAS flying on NASDA s ADEOS mission with several multiple balloon flights involving European partners. ESA s ENVISAT mission, which includes three atmospheric chemistry instruments, has initiated an international effort to establish a comprehensive calibration and validation programme. NASA s EOS-Aura mission, also carrying an international payload, will initiate and support a global calibration and validation programme. These missions will rely primarily on the existing groundbased infrastructure (surface, balloon, aircraft and networks) to provide the needed correlative data. Both the European Community and the United States are now planning operational satellite systems that will carry the ozone sounders required to extend the long term record already produced by national research missions and the US NOAA operational system. NASA will also continue to fly ozone chemistry instruments on their ADEOS and GCOM series of satellites. However, despite the fact that the major space agencies have embarked on operational atmospheric chemistry missions, no unified concurrent validation programme has been established nor is there any assurance that the requisite ground-based infrastructure will be in place. Satellite systems can only meet the requirements listed in Chapter 2 if they are supported by correlative data of known quality and are continually challenged by reliable ground-based observations and quantitative science. An on-going effort at NASA s Goddard Space Flight Center has shown that the series of satellite BUV instruments onboard NOAA operational and NASA international research satellites can provide a continuous and accurate ozone data record of climate quality, satisfying assessment issues extending from 1970 to the present. However, this has only been accomplished by the comprehensive cross calibration and validation of satellite and ground-based observations. This effort has also included the concurrent development of improved radiative transfer models and the refinement of algorithms, and to a large degree has been necessitated and guided by the intercomparisons. It is quite clear that satellite and ground-based observations together form mutually supporting (and complementary) sources of information for quantifying changes in the global distribution of ozone throughout the atmosphere. Based on the experienced gained in these research satellite missions, an end-to-end approach to calibration/validation, highlighting the need for a fully integrated global observing system encompassing both ground and space-based, is clearly essential. This end-to-end approach must include the satellite's internal calibration programme, post-launch calibration (employing on-board systems), an external validation programme using highly controlled correlative measurements, subsequent algorithm refinements and a scientific analysis of the data to ensure consistency with the best understanding of atmospheric processes and conditions. These steps form the basis of the recommendations from this chapter which are summarised in Chapter 6. Although validation programmes following this approach are planned for upcoming national research missions, there are currently no calibration and validation programmes designed to guarantee the overall integrity of global measurements over long periods of time and which meet 57

70 the objectives of IGOS. This is of particular importance given the existence of parallel streams of the national missions (e.g. the European METOP and the US NPOESS ozone instruments). In addition, a realistic possibility remains that gaps in one or both streams will arise and that the systems may employ different wavelength ranges and techniques associated with significantly different vertical resolutions. The ability of the atmospheric chemistry user community to combine data sets from different remote sensing instruments will require that the calibration properties of the individual systems are well understood and that the validation programmes be placed on a common footing. This chapter defines the calibration and validation process and describes briefly the various systems available for validation. A set of principles and guidelines are listed to establish the basis of an international calibration/validation programme. Finally an implementation strategy is proposed. 5.2 Calibration and Validation Approach Satellite sensors represent an enormous investment of intellectual and economic resources but in return offer unique opportunities for observing the Earth, notably the ability to obtain essentially global coverage with a small number of well-characterised instruments. However, to satisfy the requirements listed in Chapter 2, of the scientific and the policymaking communities and potential commercial users, the geophysical products derived from satellite sensors must be of known quality and adequate for their intended use. The calibration and validation of satellite sensors establishes the foundation on which the integrity of all these data is based: Calibration involves the definition of a set of pre-launch and in-orbit operations (or procedures) to determine the relationship between the quantities derived from the output of the satellite instrument and the corresponding values available from a traceable national/international standard. Characterisation is the set of procedures used to quantitatively determine the sensor s response over the range of operating conditions experienced in orbit during its lifetime. Validation is the objective assessment of the accuracy of the observables (radiances) and retrievals of geophysical/atmospheric parameters from calibrated and well characterised instruments over a range of geophysical conditions. Experience has shown that several steps are required to produce validated data. These steps are illustrated in Figure 5.1 beginning with the space-based measurement. These data are converted to geophysical values, namely radiances commonly called Level 1 products, by means of pre-launch calibration and on-board systems which correct for time dependent changes. Radiance, Level 1 validation, can only be performed via comparisons conducted using instruments with overlapping wavelengths. This has been done in the past and should be feasible for the upcoming research and operational missions (i.e. ENVISAT, EOS-Aura, METOP and NPOESS) though the need to observe the "same" air mass (time and location) will pose the usual problems. Validation of Level 1 radiance data should be considered as a tool for isolating calibration errors from algorithm errors. Algorithms, based on the best understanding of radiative transfer properties of the atmosphere, convert radiances into estimates of atmospheric composition and physical parameters (Level 2 products). These Level 2 products are validated by means of correlative measurements and scientific analysis (to check for scientific consistency - this may require further refinements of the algorithms). Once this is completed, the data may have to be reprocessed to produce climate quality data sets. To realise the full potential of an instrument this iteration normally occurs several times over the life time of the sensor. Correlative measurements used for Level 2 validation come from many sources. These include operational and dedicated surface-based measurements, dedicated airborne measurement campaigns (aircraft and balloons), nationally and internationally co-ordinated field programmes and sensors on other spacecraft measuring the same parameter. Also important are national and 58

71 international networks such as WMO-GAW, the NDSC and the World Climate Research Programme (WCRP) Baseline Surface Radiation Network (BSRN). However, it is important to remember that in many cases correlative measurements are not the primary goal of the measuring network or programme, so their use in correlative measurement programmes must recognise their limitations for that purpose. One key consideration is the need to observe the "same" air mass (space and time). 59