Air dispersion modelling is used to predict ground level pollutant concentrations in an area around the source(s).  The results are presented as concentration contour plots, which can be used to identify areas of unacceptable impact.

Modelling is more expensive than simple stack height calculations, but becomes a very cost-effective approach in the following circumstances:  

• where simple calculations result in excessively high stack heights that would be expensive to install; 
• where multiple discharges from stacks and vents are present; 
• where a better understanding of the impact of discharges is required, e.g. if the company is located near a Site of Special Scientific Interest; 
• where management controls are needed to minimise odour impact.  The model determines the conditions under which odour complaints are likely to occur, which can be used to plan a mitigation strategy; 
• where the impact of changes in emissions, emission controls or stack heights are being assessed. Potential changes can be input into the model to see the effect on the dispersion pattern. 
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Modelling of the dispersion of emissions can be done for:

• Toxic and odorous substances 
• Single or multiple emission points 
• Point source releases, e.g. from stacks 
• Line source releases, e.g. from roof vents 

The model is site specific.  Input data includes stack measurement details from the site, current stack and building heights, local meteorological data and corrections for local topography and adjacent buildings.

http://www.castingsdev.com/public/services/environmenthealthandsafety/EHSAirDispersionModelling.asp