Classes of Air Quality Models

The air quality modeling procedures can be categorized into four generic classes: Gaussian, numerical, statistical or empirical, and physical. Gaussian models are the most widely used techniques for estimating the impact of nonreactive pollutants. Numerical models may be more appropriate than Gaussian models for area source urban applications that involve reactive pollutant, both require much more extensive input data bases and resources and therefore are not as widely applied. Statistical or empirical techniques are frequently employed in situations where incomplete scientific understanding of the physical and chemical processes or lack of required data bases make the use of a Gaussian or numerical model impractical.

Physical modeling, the fourth generic type, involves the use of wind tunnel or other fluid modeling facilities. This class of modeling is a complex process requiring a high level of technical expertise, as well as access to the necessary facilities. Nevertheless, physical modeling may be useful for complex flow situations, such as building, terrain or stack downwash conditions, plume impact on elevated terrain, diffusion in an urban environment, or diffusion in complex terrain. It is particularly applicable to such situations for a source or group of sources in a geographical area limited to a few square kilometers. The EPA publication "Guideline for Fluid Modeling of Atmospheric Diffusion," provides information on fluid modeling applications and the limitations of that method.

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