Following the declaration of an AQMA the responsible authority is required to develop an air quality action plan outlining the measures that it and relevant stakeholders propose to undertake to help improve air quality within the AQMA and work towards the attainment of the Air Quality Objectives.
In order to do this effectively, the authority requires to get a better understanding of the air quality issues within the AQMA and where possible, determine:
How much of an improvement in air quality would be needed to achieve the air quality objectives within the AQMA
Identify the key sources of air quality pollution within the AQMA (source apportionment) so that the air quality action plans can be appropriately targeted
To undertake any additional air quality monitoring or modelling required to substantiate the declaration of the AQMA.
Prior to 2014, such work was undertaken in 'Further Assessment Reports'. Local authorities are no longer required to prepare a Further Assessment Report, however, authorities developing an air quality action plan are still expected to undertake the work outlined above as part of this process as this is crucial in informing the development of a targeted air quality action plan.
Source Apportionment
The Source Apportionment represents one of the most important components in the development of an air quality action plan. The Source Apportionment should identify the principle sources of pollution contributing to the local air quality problem. An important initial separation, in most cases, include the consideration of:
Regional background - over which the authority has little influence;
Local background - which the authority have some influence over;
Local sources - which will add to the background to give rise to the hotspot area of exceedences.
Given that the action plan should focus on the principal emission sources over which the authority has some influence, it is also important for the source apportionment to separate emission from the local sources into:
Stationary sources (if relevant)
Vehicle type e.g. split into cars, light goods vehicles, heavy goods vehicles, and heavy duty vehicles (buses and coaches)
Vehicle emissions split between moving and stationary traffic in situations where congestion is a significant factor
Other relevant factors
When undertaking a source apportionment exercise to inform the development of an air quality action plan authorities must show that they have calculated, in percentage terms, the extent to which different sources are responsible for any exceedences of the relevant air quality standard. This will allow consultees to form a view on whether the action plan is proportionate and properly targeted.
If the authority identifies that a source out with it's control is primarily responsible for the observed exceedence of the air quality standard(s) (e.g. air traffic emissions), it should not seek to implement disproportionate emissions reductions from other sources in pursuit of the objectives. Instead the authority should note that it has done all it reasonably can to bring about proportionate reductions in emissions from the sources over which it has some control.
Local authorities should utilise their local knowledge and exercise their judgment to help identify significant factors related to local emission sources. The preferred approach to apportionment of local sources is to use dispersion modelling. Where a detailed modelling approach is not feasible, source apportionment may be undertaken using a simple spreadsheet approach.
The Emission Factor Toolkit EFT can be used, the most up-to-date version of the spreadsheet can be downloaded from the LAQM Tools page.
A short video on how to use the EFT tool can be viewed below.
A second tutorial on source apportionment using the tool can be viewed below.
Further details on how to undertake a source apportionment study are provided in Chapter 7 of LAQM.TG(16) and some good examples of source apportionment studies can be found in existing action plans which are available:
Determination of reduction in emissions required to attain air quality standard(s)
The authority must also calculate the reduction in emissions required to achieve the air quality standard(s) of concern, as this will enable the authority to consider whether the measures proposed to achieve these reductions are proportionate and cost effective.
In theory, this stage involves a simple calculation that can be derived from the Council's Detailed Assessment Report which led to the AQMA declaration, and which involves the comparison of the objective with the maximum predicted pollutant concentration.
For NO2 close to roads, the required reduction should be stated as the μg m-3 reduction in the NO2 concentration required to attain the objective. However, the required percentage reductions of local emissions should be expressed in NOx. This approach is necessary as the primary emission is of NOx and there is a non-linear relationship between (NOx) and (NO2).
Calculation of the NOx reduction requires the calculation of the current (road NOx), as in the difference between total NOx and the (local background NOx). An Introduction to using the NOx to NO2 Calculator video provides guidance on this and can be viewed below.
The next step is to calculate the (road NOx) required to give 40 μg m-3 (road NOx-required) which can be achieved using the NO2 from NOx calculator available on the LAQM tools page. A short video on how to use the NO2 distance from road calculator can be viewed below.
Further guidance regarding the calculation of the reductions in emission needed to attain the objective are available in Section 7.34 of LAQM.TG(16).
By what date will the objectives be met?
Within the action plan, local authorities are also expected to provide an indication of when they anticipate the Air Quality Objectives to be met. This can be approached by modelling for future years or by following the guidance provided in Paragraphs 7.70 - 7.76 of LAQM.TG(16). However, in recent years, it has been generally accepted that this is a challenging ask and is informed by many assumptions.
