Air Dispersion Modelling (ADM)
One way to establish airborne pollutant concentrations arising from industrial facilities would be to measure the concentration of pollutants within the surrounding environment, however measurement is not possible for projects that are not yet built, or where there is more than one source of pollution in the airshed. Therefore, we rely on predicted changes in air quality (using air quality computer models) to assess estimated changes in air pollution levels, and establish the key contributors.
A mathematical simulation or “model” attempts to replicate the effects of meteorology and topography on the transport and dispersion of air contaminants for a location or region. Air quality impact assessments are unique to each project and require case-by-case consideration by the specialist consultant and local regulator.
A classical tiered approach is used in the selection of an appropriate air dispersion model, in which simpler screening models (Level 1) are first considered before moving on to more advanced models if the situation requires (Level 2 or 3). A summary of the tiered approach is outlined below:
- Level 1 assessment provides an estimate of the worst-case air quality impacts using screening models. A user friendly model that is available is US EPA Screen 3. Lakes Software provide an excellent free interface available here.
- Level 2 assessment is used for air quality impact assessment in standard/generic licence or amendment processes, and usually requires the use of US EPA models such as AERMOD and CALPUFF. These models are also available for download.
- The aim of a Level 3 assessment is to provide reasonably accurate estimates and a detailed assessment of the likely air quality impacts associated with a project. Level 3 assessments require more sophisticated models and corresponding input data, resources and model operator expertise. These more powerful models require detailed meteorological, geophysical and source input. Usually Level 3 assessments are undertaken with US EPA CALPUFF or more advanced photochemical models.
For Environmental and Social Impact Assessment (ESIA) purposes, air dispersion modelling usually adopts a conservative approach through the modelling of worst-case emissions combined with a range of meteorological conditions. This ensures that the worst-case emissions are reasonably combined with the worst-case meteorological conditions to provide a conservate prediction of the impacts on ambient air quality.
At WKC, we commonly use air dispersion models advocated by the US EPA, as these are the most widely adopted by Regulatory bodies across the globe. Our team has collectively completed in excess of 150 air dispersion modelling studies since inception.
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