Local and national emissions inventories in the

Transcription

1 Local and national emissions inventories in the UK: variations, uses and limitations Lucy Sadler London Research Centre, 81, Black Prince Road, London, UK Abstract Emissions inventories are schedules of emissions of pollutants over a specified area. They have many uses in the review and assessment of air quality, as required in response to the EU Directive on Ambient Air Quality and Management. In the UK this is implemented as the Environment Act 95. Emissions inventories allow potential air quality hotspots to be identified, source apportionment, and policy scenarios to be tested. They are also used as inputs into dispersion models to predict pollutant concentrations. Several emissions inventories have been compiled in the UK for the major urban conurbations. They use locally available data, and provide emissions data that can be investigated and scenarios to be tested at a local level. The UK also has a national emissions inventory, produced using national data, geographically apportioned using proxy data together with some activity data. This enables areas without a local inventory to undertake a screening estimate of emissions within their area. The EU also holds an emissions inventory on a 50 x 50 km basis. The major source of most pollutants is road traffic, but the importance of different sources varies from region to region. This highlights different problems the areas have. Not all pollutants are as easily estimated, and ideal data is often not available. These limitations have implications for air quality modelling, and policy decisions. This paper discusses some of the findings from the local inventories, and some of the uses and limitations of emissions inventories.

2 578 /4/ 1 Introduction Air Quality policy decisions need to be informed and backed up by research and information. One way of identifying sources of pollutants is an emissions inventory. An emissions inventory is a schedule of the sources and quantities of a pollutant or pollutants, released into the atmosphere across a specified area. It is important to note, that these are emissions, and are not pollutant concentrations. They also do not usually cover sources outside the area. Emissions inventories are compiled for different purposes and also on different bases. An inventory could give simply a national total of emissions from national production figures, or concentration on a local area - or even a single process - using local activity data. EU member states are required to report an emissions inventory to the European Commission on a 50 x 50 km square basis, with a set reporting structure. These are called CORINAIR. CORINAIR gives default emissions factors and methodologies, for use where more local information does not exist. CORINAIR allows comparison across member states, and assists community policy decisions. Different member states compile differing amounts of data on emissions, and unfortunately the CORINAIR data will vary across the EU. The UK has an extensive programme of emissions inventories. A national inventory covers the country on a 1 x 1 kilometre square grid, based on national data geographically apportioned with surrogate statistics, together with the larger point sources and more major roads. There are local inventories for selected urban areas'*. Local inventories take local data, and are more appropriate where detailed work is required. Local inventories allow data to be altered where appropriate, and contain more detailed information, including the activity data, than a national inventory could. An example of the output from a local inventory is presented in Figure 1, on a 1 x 1 kilometre square grid. The major roads and point sources can be identified. More important than the picture maps, are the data that lies behind them.

3 Air Pollution 579 Figure 1. PM,,, from all sources, for Greater Manchester. Uses of an Emissions Inventory Emissions inventories are an important tool within local review and assessment of air quality, which is required within the EU Directive on Ambient Air Quality and Management. This is especially so for urban or industrial conurbations, where air quality problems are greatest. The data behind the maps can be used to help identify air quality hotspots, to assess the relative importance of different sources, as inputs to dispersion models to predict pollutant concentrations, or to test different scenarios. They provide data and maps to guide policy decisions, and also help illustrate air quality problems to non-specialists. Limitations When compiling an emissions inventory, ideally one would take measurements of each pollutant from every activity. Monitored emissions

4 580 X/ are always preferable, but obviously this is not possible for every vehicle that passes along a road, so often activity data are taken together with emissions factors to estimate emissions; - i.e., Activity data x Emissions factor = Estimated emissions. For these cases, if either activity data or emissions factor are not available, emissions are difficult to estimate. For example, emissions from tilled land is a source for which factors are not yet available for Europe. The only factors in existence are from the USA, where the different weather conditions make them unusable in most of Europe. Activity data for construction work is subject to much uncertainty, both over the amount of work of different types, and about its geographical location as construction work, by its nature, moves frequently. Although accuracy and error estimates are important issues, it can be difficult to estimate these within the context of a local emissions inventory. When compiling an emissions inventory, there are often not several sources of data that one can use to corroborate the data, in order to estimate its accuracy. The data used is rarely designed for compiling emissions inventories - but is the only information available - and is not always as comprehensive as one would ideally want. This either requires further assumptions to be made, or will leave gaps unfilled. Some pollutants are easier to estimate than others. Pollutants that are particularly difficult to estimate include benzene, and particulates - or more particularly PM,,, or smaller. To some extent, this is because they are pollutants that have become of concern relatively recently, and there have been few measurements taken, as yet. PM,,, is also difficult to estimate for various other reasons: 1) much ambient PM,,, may be from secondary or sources outside the inventory area; ) many sources cannot be estimated due to lack of either emissions factors or activity data; ) the paucity of emissions factors that are available for PM, ; 4) the speed of knowledge change - as much research is being undertaken presently; 5) that emissions are given in tonnes per year of PM,<,, not smaller particles or in terms of the number of particles emitted - which is of importance in terms of the health effect.

5 Air Pollution 581 Different sources can be estimated with differing certainty. Table 1 gives some of the more significant emissions sources for PM,,,, with an indication of their importance and the quality of the emissions factors available, and the ease of estimating emissions from them. Table 1. Emissions sources, their significance, and ease of estimation. Pollutant(s) of Significance* significance Source Road Traffic Part A' s (larger processes) Part B's (smaller processes) Oil boilers Coal burning Gas combustion Construction Shipping Aircraft Diesel railways Agriculture Waste transfer and disposal Road maintenance *0(n/a)-(good) NO,, PM,,, CO, Benzene, 1,-Butadiene, VOC's NO,, PM,o, SO,, Benzene, 1,-Butadiene, VOC's PM,,, SO,, VOC's PML, SO,, NO, PM,n, SO,, NO, NO, PM,, NO, PIVL, VOC's PM,o PM,,, VOC PM,o PM,n Quality of emissions factors* Availability of activity data moves frequently moves frequently Modelling studies with the West Midlands emissions inventory suggests that as little as 0% of ambient PM,,, is estimated with an emissions inventory'. Much PM^ may be secondary, or from outside the area covered by the urban inventory. Also, this inventory was the first in a series, and later inventories will include information that was not available at the time of the West Midlands inventory.

6 58 /(/' Although there are many limitations with emissions inventories, it is useful to have a starting point when estimating air quality. Emissions inventories give relative values more accurately than absolute values, so they are particularly useful in estimating the effects of policy changes. Unfortunately one cannot afford to monitor everywhere, and monitoring data itself can be highly site dependent, so a method of estimating emissions where one does not monitor is needed. Emissions inventories, often in conjunction with dispersion modelling, can give these estimates. Also, with the exception of some PM,^, monitoring does not give an indication of the pollutant source, which an emissions inventory can provide. 4 Findings from the UK inventories The proportion of different pollutants from each source type differs greatly for each area, as Figures and show. Each pollutant varies differently, and PM,,, and NC\ are shown here. Figure shows the proportions of PM,<, from different source categories, for the urban and national inventories. This highlights how the importance of each source category varies with geographical area, giving a guide to their important sources. For example, London has less industry than other areas, together with a higher proportion of diesel vehicles. Manchester is greatly affected by a large coal fired power station on the edge of the conurbation, so has a large proportion of PM,,, emissions from public power. The national proportions are also shown. These are again different, with a more even spread from each of the source categories, as it combines many different areas. As the national inventory includes rural as well as industrial areas, it also covers sources that are rarely found within urban areas, such as quarries or agriculture. Figure shows the emissions of NO^ by source category, per square kilometre, together with the relative magnitudes of emissions from each area. Again the difference in each of the areas can be seen, as well as moderate differences in the amounts of NO^ produced per square. It is important to note, that the area with the highest emission per grid square will not necessarily be the area with the highest pollutant concentrations, as the sources have different emission characteristics. Emissions emitted at ground level (e.g. from road vehicles) have a different effect from elevated emissions (e.g. from a power station stack).

7 Air Pollution 58 Pub. power S Com & dom comb M Ind. comb D Ind. proc. H petrol veh. diesel veh. U Other West Midlands Manchester London Swansea Southampton Mersey Bristol National 0% 10% 0% 0% 40% 50% 60% 70% 80% 90% 100% Proportion of emissions Figure. Proportion of PM,,, by source for urban and national inventories. I Pub. power H Com & dom comb M Ind comb D Ind. Proc. CO petrol veh. diesel veh. H Other West Midlands tonnes per year per km" Figure. Emissions of NO, by source, per km' for the urban inventories. Ground level emissions tend to have a much greater effect on the surrounding air quality than elevated emissions. This is important to note - for example when identifying air quality hotspots - that the street topography also has a large importance on the air quality of two streets with the same emissions. This is the interface between emissions inventories and air quality modelling, which will incorporate emissions, emission height, topography and, of course, meteorology.

8 584 /(/ 5 Comparison of local and national emissions inventories While the importance of each source category is different for the national inventory compared to the local inventories, within a particular urban conurbation - and even often within each grid square - the inventories compare well. The level of agreement depends on the pollutant considered, and the quality of the data available on the local level. As the local inventories are able to consider more processes as points and roads as individual links, there are fewer emissions to apportion using surrogate statistics. This can mean that emissions show a good level of agreement for the whole of an urban area, but may show a geographical pattern of differences within the area. For example, between town centres and residential areas, or trunk and other roads. Other differences can result from the local inventories having more detailed information about, for example, the road speed or the small industrial process. A few sources are more comprehensively covered by national inventories, like fugitive emissions or CO, from industry. Fugitive emissions are affected by details such as which seal is used on the storage container, so are hard to estimate locally. CO, emissions are often not reported, as it is not seen as an important pollutant for local air quality management. National estimates can be made for both of these sources, and then apportioned geographically. 6 Conclusions Different types of emissions inventory have different advantages. Those on a national scale allow screening, especially important for rural areas which would not justify a local inventory, and are also useful for informing national policy. Local inventories can be of use in more highly polluted areas, and are more responsive to local changes and scenarios. Although emissions inventories have limitations, they still provide a valuable tool for assessing air quality. It is important to understand their limitations, so that the information can be most appropriately used. Methodologies are data sources for emissions inventories are constantly

9 585 improving, with continuing research, and with this, many of the limitations are reducing. Further work, especially into PM,,,, and activity data for the smaller processes, and the standardisation of the collection of much of the data in different regions is especially welcome. References [1] Hutchinson, D. and Clewley, L., West Midlands Atmospheric Emissions Inventory, London Research Centre, London, [] Buckingham, C., et al, Greater Manchester Atmospheric Emissions Inventory, London Research Centre, London, [] Buckingham, C., et al, London Atmospheric Emissions Inventory, London Research Centre, London, [4] Buckingham, C., et al, Atmospheric Emissions Inventories for four urban areas, London Research Centre, London, 1997 [5] NETCEN, The First Phase Review: A Summary, Report produced for the Department of the Environment, Transport and the Regions, London, 1998.