AQA Dispersion Model

AIR QUALITY DISPERSION MODELLING FOR PLANNING

Standard requirements for modelling operational impacts of developments within Air Quality
Assessments.

Please note separate guidance exists in respect of nuisance from constructional activities. This is available to download from the Planning and Pollution pages of WRS website.


Policy Context:

  • Assessments should be undertaken with due regard for national policies including Air Quality Standards and Objectives, National Planning Policy Framework (NPPF) paragraphs 8, 102, 03, 170 and 181, and Local Authority Policies.

Proposed Development Details:

  • Include National Grid Reference (NGR) coordinates and a location plan of proposed
    development
  • A detailed description of the operational activities, including operating hours if appropriate, should be provided.
  • Detail of the location of the development and the surrounding area, including proximity to any AQMA, topography etc., together with consideration for sensitive receptors should be included.

Model:

  • Where an air quality assessment is required WRS recommend undertaking detailed dispersion modelling utilising a UK appropriate model, such as ADMS Roads. Please include rationale for model used in assessment within methodology detail.
  • The DMRB screening tool is not considered sufficient where an air quality assessment is required and WRS may reject any assessments utilising DMRB in place of a detailed dispersion model.

Monitoring:

  • In some cases it may be necessary for the applicant or consultant to conduct additional monitoring studies in order to allow appropriate model verification to be undertaken, for example where existing monitoring locations are some distance from the area being assessed. The applicant and/or consultant is encouraged to discuss these requirements with WRS prior to undertaking any assessment.
  • Where only diffusion tubes are available to verify a model WRS recommend as best practice that at least 3 separate monitoring locations are used (not triplicate location). Using only one or two diffusion tubes to verify a model is not acceptable due to the level of uncertainty introduced.
  • Any additional monitoring undertaken for the assessment should be compliant with paragraph 7.123 in LAQM.TG(16).
  • Measured data should be annualised as set out in paragraph 7.189 and Box 7.9 or Box 7.10 of LAQM.TG(16).
  • If monitoring locations are not representative of relative exposure locations use Defra tool to calculate necessary adjustments.
  • Include (NGR) coordinates of additional monitoring positions and show on a location plan.

Model Input Data:

  • Include (NGR) coordinates of receptor points and show on a location plan.
  • Provide detail (including NGR coordinates) of any background, local authority or other monitoring data utilised to establish baseline conditions.
  • Details of meteorological dataset used in assessment must be provided and include a diagrammatic representation e.g. a wind rose diagram. A >90% complete dataset should be utilised within the model.
  • Traffic data count data: include details of accuracy and how arrived at 24hr levels if from peak hour count data. Application of a Local Authority expansion factor may be required, WRS recommend discussing the suitability of the dataset with Worcestershire County Council Highways department.
  • Please provide details on source of speed data. Consider accuracy and suitability of data and whether requires adjustment within model.
  • Cumulative Development Impact: The cumulative impacts on air quality from individual sites in local areas should be taken into account (NPPF para. 181). WRS recommend that cumulative impact for significant committed developments is included in the future baseline assessment.
  • Include a table of traffic links and data utilised in model and demonstrate links on a plan.
  • Include basis for NOx:NO2 calculations

Model scenarios:

  • Include a Baseline Year, With and Without the Scheme scenarios for development operational year
  • Include With development and other committed developments (Cumulative Impact) scenarios for development operational year.
  • A further sensitivity test is required in the following situations:
    - where the development site is within or adjacent to a declared AQMA(s),
    - where the development site is in or adjacent to areas where measured NO2 concentrations are within 10% below the AQO (i.e. 36ug/m3 for NO2)
    This test requires the modelling of a ‘Without Emissions Reduction’ scenario. See attached Sensitivity Rationale Note for further information. WRS consider this provides a more appropriate conservative model of air quality impacts from developments that the Local Authority can be confident in. This is to address the issue identified by Defra (Carslaw, Beevers, Westmoreland, & Williams, 2011) that road traffic emissions have not been declining as expected (see attached Sensitivity Rationale Note). WRS acknowledge that real world emissions indicate Euro 6 compliant diesel vehicles have much lower NOx emissions than Euro 5, and that Euro VI HGVs are close to the emission limits. It is anticipated that vehicles sold in 2019 are very likely to meet the emission limit in the real world as this mode is part of the emission test. Real world emission test results that available circa 2015 were included in COPERT 5 however there has been no comprehensive comparison of how COPERT 5 compares to the current and growing extensive measurement database.
  • A summary table combining the various scenarios would be beneficial to clearly demonstrate the differences between the scenarios.


Model accuracy verification:

  • LAQM.TG(16) paragraph 7.513 states that: ‘…model adjustment is not the first step in improving the performance of a dispersion model.’ Please provide further information on the steps taken within the assessment to improve model uncertainty. WRS anticipate this will include a table of measured versus modelled data prior to adjustment and subsequent alteration as outlined in LAQM.TG(16) Box 7.15.
  • Consideration should be given to any further improvements that can be made to the model to improve the primary adjustment factor.
  • An explanation for primary adjustment factor greater than 2 following model improvements should be provided.
  • Include modelling error (e.g. Root Mean Square Error). NB – LAQM.TG(16) paragraph 7.541 recommends a RMSE value less than 10% of objective under consideration.

 

Assessment of Impacts:

  • Include a table of impact (as defined within IAQM/EPUK 2015 guidance) at each sensitive receptor point for each scenario.
  • Demonstrate results on concentration contour maps of the study area

 

Mitigation measures:

  • Please outline measures to be undertaken to avoid, reduce or (where appropriate and approved by LA) offset development impact on local air quality. NB Even where effect is judged to be insignificant, consideration should be given to application of good design and good practice measures to minimise development impact.

 

Files:

  • WRS may request copies of model Log file, Input file and Output files for run relating to the results submitted and use ADMS Roads to confirm outcomes of report.
  • Separate files for each pollutant contour: Export contoured pollutants GIS file as TIFF and add a word file so can be imported into WRS GIS as required.

 

V10 – 29.01.2019