5: Air quality assessment following AQMA declaration
5.1 Once an AQMA has been declared, an assessment to provide the technical justification for the measures an authority intends to include in its action plan will normally be required. This will allow authorities to:
- calculate more accurately how much of an improvement in air quality will be needed to achieve the air quality objectives within the AQMA;
- refine their knowledge of the sources of pollution so that air quality action plans can be properly targeted;
- take account of national policy developments which may come to light after the AQMA declaration;
- take account as far as possible of any local policy developments which are likely to affect air quality and which were not fully factored into earlier assessment. These might include, for example, changes to national or local planning policy, the implications of any new transport schemes that are likely to be implemented in or close to the AQMA, or of any new major housing or commercial developments;
- carry out new or additional real time monitoring; and
- respond to any comments made by statutory consultees on any aspect of the AQMA declaration process, particularly where these have highlighted that insufficient attention has been paid to, for example, the validation of modelled data.
5.2 In many cases, authorities will already have done some of the necessary work as part of routine review and assessment or specific studies undertaken to inform AQMA declaration. They may already have a reasonably clear idea of which sources are responsible for the problem, and may already have calculated how much of an emissions reduction from each would be necessary to achieve compliance with the objectives of concern. In these cases, relatively little additional work will be required, although authorities will still be required to show that they have considered the possible impact on the AQMA of subsequent local and national developments.
5.3 This assessment work should be taken forward in parallel with the development of the action plan, allowing authorities to model the likely effects of particular policy measures, such as the introduction of a low emission zone or other traffic management scheme. As well as modelling the impact of particular measures on emissions and ambient air quality, authorities should also show that they have given due consideration to their likely costs and benefits. The assessment should demonstrate that authorities have considered a range of options and chosen the most cost effective solutions.
Calculating how much of an improvement is necessary inside an AQMA
5.4 A local authority must also show that it has calculated the reduction in emissions required to achieve the objective/s of concern. Having done this, the authority will be better placed to consider whether the measures proposed to achieve these reductions are proportionate and cost effective. It is important to note that a reduction of 10% in total emissions will not necessarily result in a 10% improvement in ambient air quality, because this fails to take account of background concentrations and also the complex atmospheric chemistry involved in, for example, the conversion of NOx to NO 2.
5.5 One of the most important elements of the technical assessment is the consideration of the extent to which different sources contribute to the problem. Is road transport entirely to blame for the exceedence of an NO 2 objective, or is there a significant contribution from an industrial source? To what extent do other sources contribute (for example, aircraft or train movements)? Within the road transport sector, to what extent are different classes of vehicle responsible for the emissions? Does the traffic in the whole urban area contribute more to the exceedence than the nearby road? Are sources outside the authority's immediate area contributing to any significant extent?
5.6 Only when an authority has a reasonably clear idea of the total breakdown of emissions from all sources can it draw up a sensibly targeted action plan. It will not always be possible to do this with absolute precision, and between year meteorological conditions will also have an effect on the relative contribution from different sources. But authorities must show that they have calculated, in percentage terms, the extent to which different sources are responsible for any forecast exceedences. This will allow consultees to form a view on whether the action plan is proportionate and properly targeted.
5.7 If a source over which an authority has little control (such as aircraft movements within the periphery of an airport) is responsible for a significant percentage of local emissions, an authority should not demand disproportionate emissions reductions from other sources in pursuit of the objectives. Instead it should note in its action plan that it has done all it reasonably can to bring about reasonable and proportionate emissions reductions from those sources over which it has any influence, but that further emissions reductions are required from other sources before the objectives can be achieved.
Taking account of policy changes
5.8 In many cases, central government policy developments may affect the designation of an AQMA or the extent to which local actions are necessary to achieve the prescribed air quality objectives. Possible examples include:
- changes to nationally prescribed air quality objectives, which may themselves reflect changes to European Union limit values;
- scientific and technical developments, such as changes to the emissions factors to be used in certain calculations;
- major policy developments such as those encouraging take up of renewable energy or new planning policies;
- the introduction of new powers for local authorities;
- measures to promote the use of cleaner fuels, which might affect the composition of the total vehicle fleet by the compliance date;
- decisions on major planning cases, such as the location of a new bypass, or the siting of new airports or runways; and
- developments in the industrial pollution control framework.
5.9 In many cases, local authorities will have based their AQMA designation on the results of a relatively short monitoring period, or entirely on the results of their modelling. In practice, even where at least twelve months' worth of monitoring data were available at the time of designation, there may still be uncertainties associated with the results (were the meteorological conditions typical, for example?). Sometimes, authorities will have diffusion tube data covering a long period, but only a limited set of results from a continuous monitor. Local authorities may therefore wish to carry out additional monitoring at key points to validate earlier findings and/or commit to ongoing monitoring as part of the action plan.
5.10 Following on from this, the designation of an AQMA will often have been based on a large number of assumptions (such as that traffic flows along a particular road are at a particular level). Additional assessment during action plan preparation is an opportunity to test these assumptions, in order to ensure that they are as accurate as possible.
Costs and benefits
5.11 A key element of the action plan will be an estimate of the costs, benefits and feasibility of different abatement options to allow for the development of proportionate and effective measures. The cost benefit analysis should cover both health and environmental considerations, besides the financial cost of any measures for the local authority and other affected parties. The options selected for taking forward in the plan should be reported on in action plan progress reports.