National Air Quality Reference Laboratories and the European Network AQUILA:

Transcription

1 National Air Quality Reference Laboratories and the European Network AQUILA: Roles and Requirements for Measurement Traceability, Accreditation, Quality Assurance/Quality Control, and Measurement Comparisons, at National and European Levels Version 2, December 2009 This document can be found at: 1

2 National Air Quality Reference Laboratories and the European Network AQUILA: Roles and Requirements for Measurement Traceability, Accreditation, Quality Assurance/Quality Control, and Measurement Comparisons, at National and European Levels Table of Contexts Chapter 1: National Air Quality Reference Laboratories and the AQUILA Network: Introduction, General Objectives, and Terms & Definitions Overview Drivers for the Establishment of National Air Quality Reference Laboratories, and Requirements for the AQUILA Network, arising from European Union Directives National Reference Laboratories: What are they? How are they appointed? What are they responsible for? General Objectives and Main Aims of AQUILA 9 Chapter 2: The Applications and Role of Measurement Traceability, Traceable Calibration Standards, and Certified Reference Materials, in Ambient Air Quality Measurements Introduction What is Traceability? The Realization and Dissemination of Traceable Measurement Standards at a National Level: General Certified Reference Materials and their Roles in Quality Assurance and Quality Control: Definitions and Roles The Realization and Dissemination of Traceable Standards and Certified Reference Materials at a National Level: Ambient Air Overview Methods for the Production of Calibration Standards Traceable Directly to SI Units: Ambient Air Overview Preparation of Gaseous Calibration Standards Purity Requirements for Diluent Gases, Calibration Gas Species, and Zero Gases Methods using Certified Reference Materials, and their Role in Quality Assurance and Quality Control: Ambient Air Calibration Methods that use Primary Measurement Techniques Not Directly Traceable to SI Units: Ambient Air Methods not Traceable to SI Units or Primary Methods, Realized by Convention or Definition as Reference Methods: Ambient Air Methods for Disseminating Traceability by Means of National or International Standards or Reference Methods: Ambient Air Overview Dissemination for Methods Using Calibration Standards Produced with Concentration Values Linked Directly to SI units: Ambient Air Dissemination for Methods that Use CRMs for Quality Control and Quality Assurance: Ambient air Dissemination Using Primary Methods: Ambient Air 30 2

3 Table of Contexts (continued) Dissemination using Reference Methods Designated by Convention or by Regulation: Ambient Air International Comparisons to demonstrate Comparability of Measurements between Countries Overview Summary of Comparisons Organised by the EC Joint Research Centre International Comparisons carried out by the CCQM and Associated Organisations Background International Comparisons of Ambient-Air Calibration Standards.33 Chapter 3: Interpretation, Design, and Implementation, of Quality Systems for European National Reference Laboratories Initial Background Development of the New European and Worldwide Quality-Assurance Standard General Quality Assurance and Quality Control Requirements for European and National Reference Laboratories Additional Responsibilities of NRLs that Relate to Quality Assurance at a National Level.43 Chapter 4: Type Approval Testing and Product Certification of Automated Instruments used for Ambient Air Quality Monitoring Definitions and Background Reference Methods/CEN Standards and Type Approval Requirements CEN Standards with Requirements for Type Approval Summary of Type-approval Requirements within the CEN Standards Requirements for the Certification of Type-approved Analysers Overview EN Part 1: Certification of automated measuring systems General principles Need for Certification across Europe Scope of the EN Part 1 Standard EN Part 2: Certification of automated monitoring systems Initial assessment of the AMS manufacturer s quality management system, and post-certification surveillance of the manufacturing process Possible Roles for an NRL Towards a Harmonised European Approach 51 Chapter 5: Quality Assurance and Quality Control in Ambient Air-quality Monitoring Networks at a National Level, Operated for EU Regulatory Purposes Introduction General Principles of Quality Assurance and Quality Control as Applied to Air Quality Networks Requirements of the CEN Standard Methods Type Approval of Analysers Field Operation and on-going QA/QC Activities Suitability Evaluation and Initial Installation Requirements for Ongoing QA/QC.56 3

4 Table of Contents (continued) Data Handling Calculating the overall Expanded Uncertainty of the Measurement Results Traceability and Other Requirements of the EN ISO Standard Further Requirements of QA/QC Systems for EU Regulated Pollutants QA/QC Activities Linked to the Collecting and Reporting of the Measurement Data Demonstration of Equivalence Inter-laboratory Comparisons at a National Level Background to the Requirements Types of Inter-Laboratory Comparisons Evaluation of the Results of the ILC at a National Level Links to ILCs at a European Level Examples of Specific Recommendations for NRLs at a National Level.62 Chapter 6: Intercomparisons Carried Out By the EC s Joint Research Centre, Italy Overview Examples for Inorganic Gaseous Compounds: Ozone, Carbon Monoxide, Nitrogen Oxides, and Sulphur Dioxide Examples for Organic Gaseous Compounds: BTEX and VOC Ozone Precursors BTEX VOC Ozone Precursors Examples for PM Constituents: Heavy Metals and EC/OC Heavy Metals EC/OC Example of PM10 and PM2.5 Comparisons.70 References.71 Annex 1: Organisation of Intercomparison Exercises for Gaseous Air Pollution for EU National Air Reference Laboratories and Laboratories of the WHO EURO Region.72 ANNEX 2: Background to the Requirements for Quality Systems from the Previous Directive 1996/62/EC 81 4

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6 Chapter 1: National Air Quality Reference Laboratories and the AQUILA Network: Introduction, General Objectives, and Terms & Definitions 1.1 Overview This Document provides the background, the context, and certain regulatory and scientific information, on the European Network of National Air Quality Reference Laboratories, known as AQUILA, which has been active since its constitution in December The AQUILA Network is made up of National Reference Laboratories from European countries. This document also defines what constitutes National Air Quality Reference Laboratories, and specifies their roles and responsibilities in a number of important scientific and technical areas. 1.2 Drivers for the Establishment of National Air Quality Reference Laboratories, and Requirements for the AQUILA Network, arising from European Union Directives Air quality across the whole European Union (EU) is a field where the European Commission (EC) has been proactive in defining a comprehensive strategy and an implementation methodology. Recently, this has resulted in the plans and initiatives developed under the Clean Air for Europe (CAFE) programme. This Programme have also involved the publication, or the revision, of a range of EU directives, concerned both with ambient air quality and atmospheric pollution emissions to the atmosphere. The directives currently of most relevance to this document are: 2008/50/EC The new Directive of the European Parliament and of the Council on ambient air quality and cleaner air for Europe (Official Journal of the European Union L ). 2004/107/EC - 4 th Daughter directive: relating to arsenic, nickel, cadmium, mercury and polycyclic aromatic hydrocarbons in ambient air; In addition, for completeness, it should be considered that this document covers the requirements of the previously published directives listed below, which will be repealed by June 2011 as the new Directive is required to be transposed in all Member States: 1996/62/EC - Air Quality Framework directive on ambient air quality assessment and management; 1999/30/EC - 1 st Daughter directive relating to limit values for sulphur dioxide, nitrogen dioxide and oxides of nitrogen, particulate matter and lead in ambient air; 2000/69/EC - 2 nd Daughter directive relating to limit values for benzene and carbon monoxide in ambient air; 2002/3/EC - 3 rd Daughter directive relating to ozone in ambient air; The above directives all on ambient air quality, and particularly the recent Directive 2008/50/EC, have a number of general objectives, which may be summarized as: 6

7 - define objectives for ambient air quality to avoid, prevent or reduce harmful effects on human health and the environment as a whole, - assess ambient air quality in Member States on the basis of common methods and criteria, - obtain information in order to combat air pollution and monitor long-term trends, - ensure public information, - maintain good air quality and improve in other cases, and - promote cooperation between Member States in reducing air pollution. The requirement for ensuring measurements of known measurement uncertainty that are comparable across EU is crucial for ensuring that appropriate abatement measures are triggered, where necessary, and that a level-playing field exists for any potential enforcement of the provisions of the ambient air quality Directives. 1.3 National Reference Laboratories: What are they? How are they appointed? What are they responsible for? The terms National Air Quality Reference Laboratory or the alternative National Reference Laboratory, and thier abbreviation NRL, have been used colloquially for a number of years, but there has been no explicit internationally-accepted formal definition laid down of what constitutes an NRL, or who exactly is responsible for specifying this, particularly in the context of the European Union s ambient air-quality directives outlined above. A brief explanation and interpretation of this is therefore provided here, particularly from the context of the most recent EU Directive 2008/50/EC. As is now well known, Article 3 of Directive 2008/50/EC, Responsibilities, (as well as the previous directive 1996/62/EC) states: Member States shall designate at the appropriate levels the competent authorities and bodies responsible for the following: (a) assessment of ambient air quality; (b) approval of measuring systems (methods, equipment, networks and laboratories); (c) ensuring the accuracy of measurements; (d) analysis of assessment methods; (e) coordination on their territory if Community-wide quality assurance programmes are being organised by the Commission; (f) cooperation with other Member States and the Commission. Where relevant, the competent authorities and bodies shall comply with Section C of Annex 1 (NOTE 1: The terms competent authority and competent body might be viewed in the above as being used interchangeably. However, for the purposes of this document the term competent authority is used to mean the national government ministry or the national agency that has been empowered to make the designations listed above, whilst a competent body is one of the organisations that are so designated by the relevant competent authority.) 7

8 (NOTE 2: In Directive 2008/50/EC, and the previous ambient air directives listed above, the term Community-wide intercomparisons is used. It is felt, however, that the word intercomparison is editorially unnecessary in this document in English vocabulary, and instead the word comparison is used, whenever this term in the directive is not being specifically quoted.) A number of points may be concluded and emphasized concerning Article 3 of Directive 2008/50/EC. Many of these might already be clear to the reader of this document. However, they are emphsized below for completeness and in order to provide a better uniformity of the intended concepts and conclusions across the AQUILA Network and other relevant organisations: (i) It is the responsibility of the competent authority of the specific Member State, to designate or appoint relevant organisation(s) to carry out all the tasks listed in (a) (f) above, where these competent authorities in each country are as defined above generally national government ministries or national agencies. (ii) It may not be practical for all of the tasks listed above to be carried out by one organisation in many Member States. For example, in some of the larger Member States there may be different organisations that cover different pollutants, in others all may be covered by one organisation. In addition, some of these tasks are broader or different than those required of an NRL (see below). This may also allow implicitly for subcontracting of certain tasks to other organisations or individuals, but it clearly leaves the designated body with its responsibilities outlined above and discussed in more detail later in this Document. These issues are all for the competent authority to define. (iii) Some of the above tasks comprise mainly scientific and technical experimental work associated with the requirements of Annex 1 Section C, whilst others do not. This enables us to differentiate whether there are requirements for the designated bodies for a particular task to be accredited to the EN ISO standard discussed in Annex 1 or not application of EN ISO is relevant to those organisations that have involvement with scientific and/or technical experimental quality assurance work, and not to the other tasks. (iv) The competent bodies that are designated for the tasks involving the scientific and technical activities associated with quality assurance and quality control of the accuracy of the results of all the regulated ambient-air pollutants at a national/member State level and that are involved in participating in, or coordinating, Community-wide quality assurance programmes being organised by the Commission (Article 3) should be considered as NRLs (v) The NRL(s) in a given Member State are also those that take part in the Community-wide intercomparisons covering one or more of the pollutants regulated by the directive, as stated in Annex 1 Section C, and these require accreditation to the EN ISO standard. The exact scope of the accreditation required is discussed below. (vi) There are a number of ambient air pollutants that are covered by Directive 2008/50/EC (and others covered by directive 2004/107/EC the 4 th daughter directive ). It is therefore possible that all the scientific and technical quality assurance tasks that are required to be implemented might be carried out by different organisations within a Member State, and then there will be more than one designated NRL in any Member State to cover all of these. This, as noted above, is the responsibility of the competent authority of a Member State to define. 8

9 (vii) There may be a multi-level or tiered system of quality assurance and quality control to ensure the accuracy of the measurements in a given Member State. This is acceptable within the requirements of the directive(s), and this is again a decision of the competent authority in that Member State. (viii) Where there is a multi-level or tiered quality assurance system for ambient air quality measurements relevant to the Directive 2008/50/EC (and by implication to directive 2004/107/EC), the definition and designation of an NRL, and the issues related to accreditation to the EN ISO standard apply only to the national (highest-level) organisation that also participates at the Community-wide intercomparison, unless national transposition of the Directive requires otherwise. However, in this case it should be understood that the competent authority and/or body still has certain responsibilities for ensuring the accuracy of all relevant measurements within the Member State as discussed further below (Section 3.4). (ix) The competent authority of a given Member State may, in principle, designate an organisation outside of that Member State, where it can be assured of the quality and accuracy of the measurements at a national or other level. This might occur if there is no competent body with suitable expertise within the Member State. (x) The competent authorities in Member States are, as stated previously, generally national government ministries or national agencies, and these may choose not to designate permanently any given organisation or laboratory as the competent body for a given range of the ambient air activities defined in Article C. Instead it may wish to retain this designation formally for itself. This may be, for example, because it allocates its scientific and technical, and other tasks, to different organisations regularly on a competitive-tender basis, and thus no one organisation remains permanently responsible. In this case it is anticipated that this competent authority will designate the organisation currently responsible for a range of quality assurance and quality control activities, and where possible these therefore become NRLs, and thus participate in AQUILA and the other required activities, and these will need to gain accreditation to the EN ISO quality-assurance standard with a scope that is described in detail in Section 3.3 of this document. 1.4 General Objectives and Main Aims of AQUILA A formal European Network has been established, with support from the EC Directorate-General Environment, and the Joint Research Centre, Ispra, Italy, - comprising about 40 organisations across Europe. AQUILA is now the formally constituted Network and is open to all National (Air Quality) Reference Laboratories across Europe. AQUILA, as the European Network of National Air Quality Reference Laboratories, has taken on a number of roles, responsibilities and tasks, with the following overall objectives: 1. Provide a forum for the regular exchange of scientific and technical information between the National Reference Laboratories (NRLs), in order to improve their knowledge, enhance monitoring methods, improve the accuracy of the results, and harmonise quality-assurance and quality-control practices, across Europe; 2. Provide coherent, expert, internationally agreed judgements and advice, on issues related to measurements and their strategy at a EU level; 9

10 3. To provide scientific and technical advice, where required, to NRLs that are less well developed and to other organisations (European Environment Agency, World Health Organization etc); 4. Provide appropriate scientific and technical advice to the European Commission to support current EU legislation and aid in the development of future policy; This includes, for example, the provision of quantitative performance data on new monitoring methods under a range of realistic operating conditions, in order to further environmental protection by substantially accelerating the acceptance and use of improved and cost-effective technologies that provide equivalent results 5. Provide technical advice to the Competent Authorities in the EU Member States, in order to assist with the prompt dissemination of EU policies and monitoring requirements, and ensure up-to-date, valid, and harmonised implementation of these, and where needed, provide interpretation; 6. To raise the profile of the roles and responsibilities of AQUILA and the NRLs, in order to make greater inputs and thereby improve, where required, scientific and technical activities within Member States; 7. Coordinate, or contribute to the coordination of, international and national intercomparison exercises carried out for the purposes of demonstrating the harmonisation of ambient air-quality measurements across Europe; 8. Participate, where possible, in European standardization activities in the field of relevant ambient air-quality measurements, and/or provide technical advice to these. This may be done by means of common representation of AQUILA members on the relevant CEN Working Groups, and/or through recommendations of the AQUILA group for consideration by the relevant CEN Working Group. 9. Act as a forum for the collation of practical experiences on published standards so that future revisions are based on sound and up-to-date data. The objectives listed above represent the main technical activities that the AQUILA Network has been, and is involved with. However, the AQUILA activities become broader in detail as time progresses. For example, the AQUILA Group is now considering the best methodology for handling and presenting monitoring data at close to the detection limits to ensure a harmonised means of applying this across Europe. The subsequent Chapters in this document provide background information on the selection and the responsibilities of National Reference Laboratories that make up the AQUILA Network, and also their requirements for accreditation as used in the context of the European ambient-air directives summarized above. The subsequent Chapters also provide details on the main scientific and technical requirements that the AQUILA Members must conform to, and some of the most relevant activities of AQUILA Network itself up to the present time are also discussed. 10

11 Chapter 2: The Applications and Role of Measurement Traceability, Traceable Calibration Standards, and Certified Reference Materials, in Ambient Air Quality Measurements 2.1 Introduction The concept of measurement traceability is generally well known within the metrological community. Within this community, it is generally known as metrological traceability as defined in Reference 1, but the term measurement traceability will be used in this document instead identically - for the benefit of some readers. In this Chapter a brief discussion of the general principles of traceability and its importance to AQUILA is provided first. This is followed by a discussion on what is meant by measurement traceability in the context of the ambient air quality measurements, of relevance to EU directives and related regulations in Europe. This measurement traceability is described in the specific context of its requirements to underpin, and hence support, the accuracy and comparability of measurements made by European NRLs, and/or similar institutes and/or associations of organisations, that carry out measurements of ambient air quality to meet the regulatory requirements in the relevant EU directives. (NOTE 3: In many Member States the NRLs do not carry out the actual measurements in the networks or do not make the associated quality-assurance/quality-control activities at the network sites that are required for regulatory purposes. Instead these are devolved to regional or other groups of laboratories or associations of laboratories. However, these NRLs nevertheless have responsibilities that are specified within the Directive(s) for ensuring the quality and accuracy of all such reported measurements in the Member State, as discussed in more detail in Section 1.4.) These network measurements are carried out to provide data in support of national regulations derived from the EU directives that are listed in below, are made using a number of techniques and range from, for example, continuous automatic monitors to discontinuous manual methods. Nevertheless, the requirements for measurement traceability of all of these (as defined in this metrological context) are given in Section C of Annex 1 of Directive 2008/50/EC: To ensure the accuracy of measurements and compliance with the data quality objectives, the appropriate competent authorities and bodies designated pursuant to Article 3 shall ensure that: - All the measurements undertaken in relation to the assessment of ambient air quality discussed in Articles 6 & 9 are traceable in accordance with EN ISO Section And: - The national laboratories appointed by the competent authority, designated within Article 3, that take part in Community-wide intercomparisons covering the regulated air pollutants, are accredited according to EN ISO for the Reference Methods given in Annex VI of the Directive. The need for traceability of measurements is also specified in the EN ISO standard as one essential aspect for fulfilling the requirements of that standard (e.g. see paragraph 5.6 of the standard). In addition, the concept of traceability to relevant (primary or other) standards of the SI system of units of measurement is introduced as a requirement in that standard, for use wherever practical. It should be understood that this is considered important because traceability should provide a rigorous and robust 11

12 means of determining the uncertainty of such measurements. This is discussed in more detail below in this Chapter, and in Chapter 3 of this document. The concept of measurement traceability also leads naturally to requirements for nationally- or internationally-traceable calibration standards. It leads, in some cases, to requirements for validated Certified Reference Materials (CRMs). The applications and roles of both of these types of calibration standards, and CRMs generally used for validation, are also summarized below. These are used to ensure the accuracy and international comparability of measurements, when monitoring ambient air quality pollutants that are regulated by EU directives. 2.2 What is Traceability? The concept of (metrological or measurement) traceability, as it is now generally used, was initially and generally defined in metrological terms in the International Vocabulary of Basic and General Terms in Metrology (VIM) revised in 2008 Reference 1. This (metrological) traceability can be interpreted or paraphrased as: The property of a measurement result (or the value of a standard) whereby the result can be related to stated references that are generally appropriate national or international standards, through an unbroken chain of comparisons (calibrations), all having stated (measurement) uncertainties that contribute to the overall measurement uncertainty of the result This requires an unbroken chain of measurements all with stated uncertainties - possibly from a primary or national realization of an SI unit, or another internationallyaccepted unit, to the final measurement result, which may itself be the result of many steps in the chain. From the above discussion it is clear from this that each step down the measurement chain results in a poorer level of accuracy, as is represented in Figure 2.1 To further assist with the interpretation of the above definition of traceability from VIM, it is useful also to note a further definition that is used in ISO standard Natural gas - Guidelines to traceability in analysis : Ability to provide evidence of the uncertainty attributed to the measurement results through documented calibrations, using measurement standards of known uncertainty, and comparison measurements of known performance This emphasises the procedure that is used to assign the overall uncertainty to the final result of the measurement process. It is also compatible with the above VIM definition. The concept of measurement or metrological traceability is illustrated in Figure 2.2 to demonstrate how it may be achieved in practice using different primary methods. A discussion on what may potentially constitute a primary method for the measurement of amount of substance, relevant to the monitoring of atmospheric pollutants, has been published (Reference 2). 12

13 SI Units c o m p a r i s o n s (Inter)national (Primary) Standards Secondary Standards Work Standards/In-house Standards Measurement/Calibration uncertainty Figure 2.1: Schematic of how the Hierarchy of Traceability is achieved, and the Resulting Measurement Uncertainties SI system of units primary direct methods (e.g. coulometry, FPD) pure materials primary ratio methods (e.g. IDMS) primary direct methods (e.g. gravimetry) calibration standards real sample or matrix reference material secondary methods real samples Figure 2.2: Examples of Practical Realizations of Measurement Traceability to the SI system of Units (FPD = flame photometric detection, IDMS = isotope-dilution mass spectrometer) 13

14 In addition, wherever the measurement results are to be described correctly as traceable it is essential to specify to which specific types and values of appropriate measurement standards the traceability has been established. This can be, for example, established either to: (a) One of the base system of measurement units (the International SI System of Units), such as a mass measurement; (b) A reference material, or similar, that has one or more of its properties certified by one or more internationally accepted measurement methods; (c) An internationally defined and recognised scale, such as a ph value; (d) A method defined in a widely recognised international standard; (e) A practical realization of a well established measurement method. (f) Traceability to a recognised national or similar laboratory (e.g. NIST USA traceable ) this is not an acceptable claim on its own in the context of this document. The above routes for achieving traceability have all been adopted and employed in the field of ambient air measurements, carried out for the purposes of compliance with the regulatory requirements in the EU ambient-air directives. These will be discussed more specifically in Section 2.5 below. However, we first present some GENERAL background on the methods whereby traceability is developed and disseminated in different countries worldwide in order to illustrate the concepts involved. These involve dissemination using one of the following methods: - At a national level through primary standards realized nationally or obtained from another National Metrology Institute (NMI) or, - At an international level through the application of appropriate Certified Reference Materials. This may not be an acceptable means of achieving rigorous traceability but may be necessary under some circumstances. - Or using one of the other primary or related methods given in (a) (e) above in this Section. 2.3 The Realization and Dissemination of Traceable Measurement Standards at a National Level: General Many countries worldwide have appointed Institutes in their country to take responsibility for the development, maintenance, and the dissemination of measurement standards in order to provide calibrations and references for industry, government, and the public. There are currently approximately 80 such Institutes worldwide, which are known usually as National Metrology (or Measurement) Institutes (NMIs) - also as National Standards Laboratories. These NMIs develop and maintain measurement standards at a national level that are considered important to national and international trade, the quality of life etc, and to which traceability of most of the measurements in that country can be derived, wherever required. Clearly it is difficult to develop and maintain in each country all of the measurement standards that are required and that exist within the SI system, and any other measurement systems as required, particularly in one institute. Therefore, most NMIs develop and maintain a subset of the total, which are considered to be the most important to that country, and they will then depend upon other NMIs in different countries to produce those that they require but do not maintain themselves, and for which national or international traceability is considered necessary. Alternatively, the 14

15 NMI in a given country may seek to identify and to designate another laboratory as its representative for a certain range of measurement activities, where it does not have the expertise, or the capacity to maintain itself in that country. The national standards developed in any specific NMI are generally maintained and retained within that institute, and not disseminated or distributed to outside customers or other interested organisations. Instead, there are generally two different mechanisms for disseminating the traceability from a nationally established primary measurement standard or reference measurement method. This is by: (1) Bringing the equipment, a measurement method, or some form of calibration artefact to the NMI, where it is calibrated with respect to the national standard by means of experimental comparisons that have known performance and measurement uncertainty. (2) The NMI producing and marketing calibrated artefacts or other measurement techniques, that has been calibrated with respect to the national standard held by the NMI, where the calibration has been shown to be stable over time. This then can therefore be provided for application by the end user externally to the NMI. These two mechanisms may give rise to nationally traceable calibrations that have different measurement uncertainties. Both of these mechanisms are used by NMIs worldwide. The mechanism selected depends on the quantity being disseminated and also on the optimum way that achieves the required traceability with the necessary measurement uncertainty. A schematic diagram of the general hierarchy of traceability applicable to many measurements is shown in Figure 2.1 above, indicating clearly that the measurement uncertainties increase as the measurement becomes further from the source of traceability. In addition, it is important to recognise that many NMIs (or other laboratories at a national level designated for a particular field of measurement expertise) participate regularly in comparisons, usually between each other. This is necessary in order to demonstrate the international comparability and accuracy of the national standards they maintain. Some of the methods for doing this are discussed in Section 2.7 below. Section 5 below summarizes the work that NMIs are carrying out in certain countries in the field of ambient air-quality measurements, and in particular for the provision of traceability to nationally maintained or internationally recognised standards, and/or using internationally-accepted Certified Reference Materials. This procedure for disseminating traceability is similar in principle to that shown in figure 2.1, and this is explained in more detail in Section 2.5 below. 2.4 Certified Reference Materials and their Roles in Quality Assurance and Quality Control: Definitions and Roles It is important to realise that in certain circumstances, the term Certified Reference Material and the term measurement standard are used interchangeably. This is not fully valid, and will not be used in this document. In addition, their roles and methods of use are usually also different. Nevertheless Certified Reference Materials may be used as methods of calibration, or methods of checking the validity of a calibration. They may be considered effectively as calibration standards where certain of their properties only are known, and they are therefore generally used differently to traceable calibration standards of the type discussed above. However, they may provide one method of supplying a methodology for establishing the comparability of measurement 15

16 results between one organisation or one country and another, with certain limitations, particularly where no fully traceable method exists. For example, in general, CRMs can only be considered calibrants where they are used in a manner such that they exhibit the certified value for the quantity of concern in the method being used, with no other effects due to their different composition sample matrix effects etc. Thus, where CRMs are used properly for calibrations they provide traceability but where they are used in quality control and quality assurance only they do not strictly provide traceability. A Reference Material (RM) may be defined as: Material, sufficiently homogeneous and stable with reference to specified properties, which has been established to be fit for its intended use in measurement, or in the examination of nominal properties (of a sample); This emphasizes that certain of the properties only may be homogeneous and stable. It specifies nothing about the other properties of the RM, or its matrix. This therefore means that it may be only applied where the effects of not knowing these other properties, or the effects of the matrix material(s), are not significant for the validity of the analysis being used. This also constrains how they should be applied. The activities whereby Reference Materials are produced characterised and certified is one of the key activities in improving and subsequently maintaining a comparable, coherent, stable, and accurate system worldwide for nearly all types of measurements. These result in what is known generally as a Certified Reference Material (CRM). The definition of this may be considered as: Reference material, accompanied by documentation (usually in the form of a recognised certificate) issued by an authoritative body using valid certification procedures and providing one or more specified values for specified properties with associated uncertainties and with specified traceabilities. The definition indicates some of the differences between nationally or internationally traceable calibration standards that are discussed in Section 2.3 above, and RMs and their associated CRMs: - Only certain of their properties may be homogeneous and stable, with no certification of the behaviour of the other properties of the CRM, which may give rise to interference effects; - The CRM is provided in a given matrix material (usually solid or liquid). This may or may not have an effect on a given analytical procedure, and this effect may give rise to a different result if the sample to be analysed is in a different matrix; - Certification is carried out on a specific and finite batch of the RM. When this is used up, a new (and different) RM must be certified and this may have different properties, interference effects, matrix effects etc; The properties and limitations summarized above, are also applicable to the CRMs used in the validation of the measurements of ambient air quality that are regulated by EU directives, as discussed in Section They are rarely used as calibrants in these ambient-air pollutant cases. 2.5 The Realization and Dissemination of Traceable Standards and Certified Reference Materials at a National Level: Ambient Air 16

17 2.5.1 Overview This Section outlines some of the methods by which nationally traceable calibrations and/or calibration standards and Certified Reference Materials are realized in the field of ambient air quality, in the context of the requirements of the EU directives: - At a national level by the NMI concerned; - Or by the alternative laboratories that are designated by the competent authority (known then as National Reference Laboratories - NRLs); - Or a laboratory designated by the NMI, with specialist scientific and technical expertise in the ambient air quality field. The different methods of dissemination of these standards are also summarized below in Section 2.6, which include the distribution of what are sometimes described as Certified Reference Materials that are traceable to these national standards. The use of other Certified Reference Materials, that are internationally recognised and used widely across different countries, is discussed separately in Section 5.3 below. The methods by which nationally traceable calibrations and/or calibration standards are developed and maintained at a national level within the scope of this document fall into a number of different categories. These cover calibration standards that are: (1) Prepared in a manner that is traceable directly to SI units though mass, volume (length), flow (mass and time) etc. These usually apply to calibration standards for certain pollutant gases, although there are different means of realizing and disseminating these (see Section below); (2) Not able to be produced in a manner that is directly traceable to SI units but traceability can be achieved by means of the realization of a potentially absolute method such as optical photometry, when implemented under controlled conditions (see Sections & below); (3) Not able to be realized in a traceable manner, and not able to be realized as a primary or absolute method, but it is realized by convention or by definition as the reference method, preferably having the smallest measurement uncertainties achievable using that method (see section below); Methods for the Production of Calibration Standards Traceable Directly to SI units: Ambient Air Overview There are a number of methods for the preparation of calibration standards that are considered as traceable directly to the SI system of units. These, which are described in published EN ISO standards, are summarized below. They have been used by NMIs to develop and maintain their national/primary standards, and several international comparisons have been carried out between NMIs worldwide. The most important of these comparisons are summarized in Section 2.7.3, which demonstrate comprehensively the level of the comparability and accuracy of these nationallyrealized primary standards that have been developed and maintained using these primary methods. There are, however, a number of quality assurance requirements that are absolutely essential to implement if accurate and comparable standards are to produced and maintained. Many of these are specified in detail in the relevant EN ISO standards discussed below, but one common and important issue in the quality control is the 17

18 requirement in all the preparation methods for diluent gases with sufficient and monitored purity and this is discussed in Section below Preparation of Gaseous Calibration Standards The most convenient calibration standards that can be produced to provide traceability directly to SI units at a national level are those for the gaseous pollutants covered by the 2008 EU Directive: (a) Sulphur dioxide, nitrogen dioxide (and nitrogen monoxide), ozone, carbon monoxide, and benzene, directly regulated under this Directive; (b) Multi-component or single-component hydrocarbon ozone precursors as specified in Annex X of the Directive There are a number of primary methods that can be used for preparing these standards that are considered directly traceable to SI units as listed in Section 2.2 (a) above (where traceability is achieved directly to one of the base units of the SI system such as mass or length (volume), as well as to one or more of the derived SI units, such as pressure). These are as referred to in Section bullet (1). These have been developed into international standards (norms) through the International Standardization Organisation (ISO), and mainly through its Technical Committee TC 158. This has produced a suite of ISO standards specifying different methods for preparing and validating the prepared gas standards, as summarized in Table 2.1, presented for the example of NO x calibration standards. Outlines of the different preparation techniques listed in Table 2.1 are summarized below for clarity and completeness. For more detailed information, the relevant ISO standards given in Table 2.1 should be consulted: One of the methods used worldwide to prepare gas standards at all levels of concentration, for many different species, is to use gravimetry to weigh specific pollutant gases together with relevant matrix/diluent gases into high-pressure cylinders, and then to dilute these gas mixtures to the required low concentrations for ambient air concentration calibrations, as required. A number of NMIs and other designated laboratories, carry out this process. An advantage is that these gas mixtures can be prepared for use at the low concentrations required. A disadvantage is that the lower the concentrations prepared the more likely the gas standard is to be unstable for certain pollutant species. It is important to recognise, however, that comprehensive quality assurance and quality control procedures are required if valid standards are to be produced, and this becomes increasingly difficult at the low concentrations required for ambient calibrations. This method is an example of where gas standards are prepared that are directly traceable to the SI mass unit, via a number of accurate weighings (gravimetry), as stated in Section bullet (1). A further method, usable only for some pollutant species and concentration ranges, is to employ permeation tubes. These permeation tubes are designed so that a given mass of pollutant gas permeates from the tube per unit time, depending on the temperature at which the tube is maintained. Then, if this gas is mixed into a stream of diluent gas of known flow rate, a known concentration gas standard may be produced. This requires that the mass loss of the permeation tube is determined, and the flow of the diluent gas is measured. The mass loss is thus traceable to the SI unit of mass, and the dilution process is traceable either to pressure measurement, or it may alternatively be calibrated using gravimetry. Static injection can be used to prepare a known concentration in a vessel of known volume, with injections of known amounts of the pollutant gas into the vessel through a syringe or similar. The advantage is that the standard can be prepared 18

19 A further method uses dynamic dilution of a higher concentration of the pollutant gas than required, usually prepared gravimetrically as above, blended with a diluent gas through flow controllers to achieve the required low concentration. The advantage is that the gas standard is possibly more stable, with the disadvantage that dilution is required. This is an example where the original standard is traceable through gravimetry directly to the mass unit, and the dilution process is traceable either to pressure measurements or it may be calibrated using gravimetry Table 2.1: Methods for preparation of Calibration Gases According to ISO Standards and other Guidelines: Example for NO x Standards Method NO a NO 2 Description Cylinder + + Gas cylinders containing nitrogen or synthetic air, as appropriate, at the required low concentrations for the direct calibrations of NO x ambient air analysers Permeation tubes - + Determination of the weight loss of a permeable tube containing NO 2 liquid and gas Static dilution + + Preparation by means of injecting known amounts of NO and NO 2 into a known volume Dynamic dilution + + Dynamic blending of cylinder of pollutant gas at a higher concentration than required with nitrogen or synthetic air as appropriate Standard or Guideline to be used ISO 6142 ISO 6143 ISO ISO 6144 ISO /7 Gas phase titration - + Conversion of NO containing gas with ozone to produce NO 2 ISO FDIS & VDI a +appropriate method, - not applicable These calibration standards are generally realized in different countries at a national level either by the relevant NMI or its associated designated laboratory, or by an appointed NRL (but not always see below). It is also worth noting that, in general, different laboratories in a given country may have responsibility for developing and/or maintaining the calibration standards for different pollutants covered by the EU ambient-air directives at a national level, since a large breadth of expertise is required, and this is not always available in any one laboratory.. Comparisons carried out between these laboratories, with other recognised laboratories either in Europe or worldwide, are used to demonstrate the international comparability of these national standards, and hence demonstrate their accuracy These have been carried out for nearly all of the pollutant gases at ambient concentrations discussed here (see Section 2.7 below). 19

20 The different methods used in different countries for disseminating traceability from these national standards to the individual network sites are also summarized - see Section 2.6 below. NOTE 4: It is worth clarifying in the context of definitions, that gas standards that are prepared by the primary methods discussed above, provide calibration standards that are directly traceable to one or more of the international units of measurements, (and hence may be used to provide robust calibration methods when applied correctly), could fall within the definition of Certified Reference Materials (CRMs) as defined above. The gas standards produced by these methods may therefore sometimes be referred to as CRMs. However, for the purpose of this document, and in relation to ambient air calibrations, a distinction is made between these calibration standards described above for ambient air, and the specific and different actual CRMs discussed below. These CRMs are employed in a different manner within ambient air monitoring applications Purity Requirements for Diluent Gases, Calibrant Gas Species, and Zero Gases In all the above gas standard preparation techniques, there are critical requirements to carry out analyses of the purity of all components used for the standard. There are a number of reasons that measurements of impurities in the diluent and calibrant components of the standard are essential, in order to ensure the accuracy of the calibration and/or ensure the stability of the mixture with time: (a) Impurities in the diluent gas may react with the calibrant gases and an incorrect concentration of the prepared calibration gas will result. Examples include: - trace levels of oxygen in the nitrogen diluent used to prepare nitrogen monoxide standards, which will result in conversion of some of the nitrogen monoxide to nitrogen dioxide; - water vapour levels in diluent air or nitrogen when used to prepare sulphur dioxide standards may result in mixtures that are unstable with time; - when there is a significant concentration of the calibrant in the diluent gas, then this will increase the concentration of the resulting calibration gas unless measured and corrected for. This is most important for low concentrations. (b) Impurities in the pure calibrant gases will give rise to incorrect calibrations unless the impurities are analysed and corrected for. One example is the presence in nitrogen monoxide of impurities comprising other oxides of nitrogen and nitrogen itself. In addition, it is clear that presence of the trace calibration gas in all zero gases used to provide the zero response of an analyser, or any other actively interfering gaseous species, will clearly result in incorrect zeroing of the analyser. It is therefore clear that all components used in the preparation of all calibration gases, and wherever zero gases are employed, must be analyzed with sufficient sensitivity and accuracy or incorrect results may occur. These activities should be one part of the NRLs accreditation to the EN ISO standard, wherever relevant. However, it is not necessary for the NRL to perform such analyses themselves, but these should then be obtained from a suitable source. Certified zero reference gases (usually of nitrogen or synthetic air) are available commercially with suitable levels of purity Methods using Certified Reference Materials, and their Role in Quality Assurance and Quality Control: Ambient Air 20