Dispersion modeling is a mathematical simulation of emissions as they are transported throughout the atmosphere.
Dispersion models replicate atmospheric conditions, (which includes wind speed and direction, air temperature and mixing height), and provide an estimate of the concentration of pollutants as they travel away from an emission source. These models can also generate estimates of secondary formation of pollution by incorporating atmospheric chemistry into the model. Dispersion models can be used to determine whether a new source will adversely impact an area or to predict whether the control of an individual source will have a beneficial effect. Dispersion models are used when a prediction of ambient concentrations is necessary, such as in a new source review or evaluating emissions reduction plans.
The available dispersion models vary in their complexity. At a minimum, most of the models require meteorological data, emissions data, and details about the facilities in question (such as stack height, gas exit velocity, etc). Some of the more complex models require topography information, individual chemical characteristics and land use data. The output from this type of model is a prediction of the concentration of the pollutant in question throughout the appropriate region (which depends on the model chosen).
EPA has detailed recommendations (PDF, 352 KB) regarding which models to use under what circumstances. Briefly, the recommended models include BLP, CALINE3, CALPUFF, CTDMPLUS, ISC3, and OCD. All of these models (and many more) are available for download on the EPA website.
- BLP is recommended for aluminum reduction plants, rural areas, transport distances less than 30 km, simple terrain, and averaging times less than one year.
- CALINE3 is recommended for line sources (highways), urban or rural areas, simple terrain, transport distance less than 50 km, and less than 24-hour averaging times.
- CALPUFF is recommended for long-range transport (> 50 km); point, line, and area sources; and with fully characterized meteorological data (hourly data).
- CTDMPLUS is recommended for complex terrain, elevated point sources, transport less than 50 km, rural or urban areas, and averaging times up to one year.
- ISC3 is useful for modeling industrial source complexes, flat or rolling terrain, transport distances less than 50 km, and continuous air toxics emissions.
- OCD is recommended for over water sources.