Trinity Consultants

Sensitivity of the AERMOD air quality model to the selection of land use parameters

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AERMOD is a new, advanced plume dispersion model that the U.S. Environmental Protection Agency (EPA) is expected to propose for regulatory use. It is intended to replace ISCST3 for most modeling applications. AERMOD is the result of an effort to incorporate scientific knowledge gained over the last three decades into regulatory plume models. AERMOD requires, as input, three site-specific land use parameters. These are the Bowen ratio (a measure of moisture available for evaporation), the albedo (portion of sunlight that is reflected), and surface roughness length. These parameters are functions of ground cover (land use), and affect the concentration calculations. It is important to know how sensitive the model results are to these parameters, so that their input values are characterized with sufficient accuracy for modeling purposes. This study evaluates the effect on design concentration predictions from AERMOD, for a range of sources, of variations of the albedo, Bowen ratio, and surface roughness length individually and in combination over the ranges of values one would expect to encounter in realistic modeling scenarios. The sources include a ground level source, and stacks ranging from 35 meters to 200 meters in height. The effects of variations of combinations of these parameters on design concentration predictions is further evaluated by selecting the land use parameters that are characteristic of each of four types of ground cover. The sensitivity of the design concentration predictions by the AERMOD model to these input parameters is discussed. Recommendations are provided as to the accuracy needed for values of the Bowen ratio, albedo, and surface roughness length that are used in the AERMOD model.

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