Background concs    Road sources    Domestic fuel    Part A and B processes    Uncontrolled + fugitive emissions    Combined source impacts
Second Stage Review & Assessment
  1. The Second Stage review and assessment can be largely based upon the application of simple screening methods, for example the Design Manual for Roads and Bridges (DMRB) model for roads, and the Environment Agency's Guidance for Estimating the Air Quality Impact of Stationary Sources9 for industrial processes. Wherever possible, local measurements should be taken into account to refine the estimates of both existing and future air quality.

    Background PM10 concentrations

  2. There are clearly uncertainties associated with the 2004 background PM10 maps, particularly where the predictions have been based upon interpolations of data to areas where there are no measurements. For the Second Stage review and assessment, authorities are recommended to use actual measurement data wherever these are available. However, the quality of these data should be verified before use, and authorities are advised to take account of the guidance set out in LAQM.TG1(98).
  3. The contribution from different sources will not remain uniform between the current monitoring year and 2004. It is therefore not possible to apply a simple extrapolation to existing monitoring data to predict future levels - the data must be divided into the separate source categories ('primary', 'secondary' and 'coarse') and treated separately. Only the 'primary' component is of significance in terms of local emissions - the 'secondary' and 'coarse' components can therefore be removed and added back in once the future predictions from local sources have been performed.
  4. The predictions of background PM10 in 2004 may be carried out in several different ways depending upon whether monitoring data are available, and whether they are TEOM or gravimetric based. A suggested procedure for using measured background concentrations is described in Box 7. The calculated or estimated 2004 background concentrations should then be used in all of the subsequent predictions. A worked example is provided in Annex 1. If the estimated annual mean background concentration in 2004 exceeds 28 mg/m3, gravimetric, authorities may proceed directly to the Third Stage review and assessment.
  5. It should be noted that the procedure in Box 7 can also be used to correct monitoring data collected at a roadside site. However, in such cases, it will also be necessary to remove the roadside contribution to the primary PM10 in the monitoring year, and this will involve an additional stage of modelling. This is described in further detail below.

    Road traffic sources

  6. The annual mean PM10 concentration should be estimated for 2004 at relevant locations (see para 12) close to those sections of the road network identified from the First Stage review and assessment as being of potential concern. The predictions may be carried out using the Design Manual for Roads and Bridges (DMRB), or any other appropriate screening model. Guidance on suitable models is provided in LAQM.TG3(98).
  7. The precise relationship between tailpipe emissions, and TEOM and gravimetric measurements is at present uncertain. Due to the method of measuring tailpipe emissions, there is the potential for loss of some volatile components, but this has not yet been quantified. In addition, the recent APEG report has identified the important contribution of coarse particle resuspension to roadside PM10 concentrations. Both of these factors could lead road traffic models such as DMRB to underestimate the predicted concentrations.

    Box 7: Calculation of 2004 Background PM10 for Second Stage Review and Assessment

  8. However, a conservative approach has already been taken with regard to the relationship between the annual mean and the 90th percentile, and the 2004 background PM10 concentrations which are based on atypical meteorology. For these reasons, it is not recommended that a 1.3 adjustment factor is applied to the DMRB at this stage.
  9. If the DMRB model is used for the Second Stage review and assessment, it is recommended that the background concentrations given within the model are ignored for the purpose of this assessment, and that the model is used to predict the annual mean contribution from the road(s) alone for 2004. The predicted annual mean PM10 contribution from the road(s) in 2004 should then be added to the estimated background concentration in 2004 derived from the procedures in Box 7.
  10. If local monitoring data from a roadside site are available, then these can be used to supplement other background data, and may be more suitable. In this case, the DMRB (or other model) should be used to estimate the roadside primary PM10 contribution for the monitoring year (as well as for 2004) and this value should be subtracted from the measured concentration, along with the secondary and coarse contributions, as shown in Box 7. A number of worked examples are provided in Annex 1.
  11. Where the total annual mean concentration is predicted to be greater than 28 mg/m3 there is a potential for exceedance of the objectives, and the authority should proceed to the Third Stage review and assessment.

    Domestic solid fuel use

  12. An assessment of the impact of domestic solid fuel use can be carried out from existing black smoke data, based upon the empirical relationship described in Figure 5. Black smoke data should be available within all areas where domestic solid fuel use is likely to be a problem. The procedure is as follows;
    • Determine the annual mean black smoke concentrations for the most recent year
    • Determine the 1996 annual mean background secondary PM10 concentration for the area, using the Internet maps (/netcen/airqual/)
    • Locate the intercept point on Figure 5. If the point is below the line, it is unlikely that the objectives will be exceeded in 2004, and there is no need to proceed further. If the point is on or above the line, then further investigation is necessary, and the authority is advised to proceed to a Third Stage review and assessment.

    Part A and Part B authorised processes

  13. Predictions of PM10 concentrations arising from controlled industrial sources may be carried out using the Environment Agency's Guidance for Estimating the Air Quality Impact of Stationary Sources (GSS). Annual total emissions for PM10, stack heights, plume heat content and plume efflux velocity should be obtained or estimated for 2004 for each significant Part A or Part B source identified. There is the potential for loss of volatile components during the determination of PM10 stack emissions, but this is not well quantified. A conservative approach has already been taken with regard to the relationship between the annual mean background concentration and the 90th percentile, and the 2004 background PM10 concentrations are based on atypical meteorology. For these reasons, it is not recommended that a 1.3 adjustment factor is applied to the emissions at this stage.
  14. The assessment may be carried out as follows:
  15. If the predicted total 90th percentile concentration exceeds 50 mg/m3 the authority should consider undertaking a Third Stage review and assessment. A worked example is provided in Annex 1.
  16. It should be noted that the GSS model cannot be applied under the following circumstances:
    • stacks lower than 20m or higher than 200m;
    • stacks with adjacent tall buildings (defined as 40% of the stack height);
    • non-buoyant releases (where the plume is not heated above ambient temperature); and
    • stack exit velocities outside of the range 10 to 25 m/s.
  17. In such circumstances, it will be necessary to consider the use of an alternative suitable screening model, or to proceed to the Third Stage review and assessment. Guidance may be sought from the Modelling Helpline on the most appropriate manner in which to proceed.

     Uncontrolled and fugitive dust emissions:

  18. Due to the considerable uncertainties in emissions estimates for uncontrolled or fugitive dust releases, there is no suitable screening approach which can be confidently applied to the Second Stage review and assessment. In the absence of local monitoring data, authorities may need to undertake a detailed Third Stage review and assessment where such sources are considered to represent a problem.
  19. However, where local monitoring data of a suitable quality have been collected, and which coincide with the nearest relevant location (see para 12), these can be used to indicate the likelihood of a problem. Provided no significant changes to the operations are expected, it can be assumed that PM10 emissions will remain constant until 2004. It is suggested that existing monitoring data are compared with the proposed objectives, although this is a conservative approach as the background component will decline by 2004. Where concentrations exceed the criteria, authorities should proceed to a Third Stage review and assessment.

    Combined source impacts

  20. There are circumstances in which it may be necessary to take account of the potential combined impact of different sources, eg an industrial process adjacent to a major road, or a major road within a coal burning area. The process of combining these source impacts is relatively straightforward, and a number of worked examples are provided in Annex 1:
    Combining the impacts of an industrial process adjacent to a major road
     Combining the impacts of a major road within a coal burning area
    First Stage          Third Stage
    Report prepared by Stanger Science and Environment
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