Dispersion Modeling

Dispersion modeling is a method for estimating the ground-level concentration of pollutants at various distances from a source. Modeling refers to a general technique that uses mathematical representations of the factors affecting pollutant dispersion. Computers are used extensively to help scientists model the complex systems responsible for transport and dispersion of air pollutants.

Figure:Dispersion modeling

In modeling air pollution transport and dispersion, specific information is gathered for an emission point. This information includes the location of the emission point (latitude and longitude), the quantity and type of pollutants emitted, stack gas conditions, the height of the stack, and many meteorological factors that include wind speed, ambient temperature profiles, and atmospheric pressure. Using this data as input for a computer model, scientists can predict how pollutants will be dispersed into the atmosphere. Concentration levels can be estimated for various distances and directions from the site of the stack.

Selection of an air quality model for a particular air quality analysis is dependent on the type of pollutants being emitted, the complexity of the source, and the type of topography surrounding the facility. Two common modeling systems used by EPA are the Industrial Source Complex model (ISC) and the Assessment System for Population Exposure Nationwide (ASPEN). The ISC model can be used to assess pollutant concentrations from a wide variety of sources associated with an industrial complex up to a distance of 50 kilometers. In addition to concentrations, it can estimate deposition rates and is appropriate for simple and complex terrain. The ASPEN model is used to estimate toxic air pollutant concentrations over a large-scale domain such as the entire continental U.S. These models are tools that help scientists make evaluations of air pollution dispersion. The accuracy of the models is limited by the inherent problems of trying to simplify complex and interrelated factors affecting air pollution transport and dispersion.

Dispersion modeling is used for two major reasons: modeling can predict pollutant concentration estimates at most locations where there are no air monitors and models can predict the potential impact of new sources before they are built as well as how new pollution control devices will affect the outcome of the pollutant.

As we mentioned earlier, Air Quality Management is built upon air quality monitoring or sampling of air pollutants and emission inventories estimates. Monitoring data are a critical part of the nation's air program infrastructure. Ambient air data are used to determine progress in meeting the National Ambient Air Quality Standards (NAAQS). They are also used to establish baseline conditions for a given area before new sources are constructed and to develop air dispersion models. Other uses of ambient air data include scientific research and assessment of human exposure to pollutants. In general, the nation's ambient air monitoring networks:

• inform the public of air quality levels and exposure
• establish the compliance status of cities and other areas
• track air quality trends and evaluate progress of emission control programs
• support development of emission control and air quality research programs

More information on air quality modeling can be found at http://www.epa.gov/oar/oaqps/modeling.html