Dispersion models have two levels of sophistication. The first, referred to as screening modeling, is a preliminary approach designed to simplify a source's emissions and provide conservative plume concentration impact estimates. The model user compares the results of screening modeling to the NAAQS, prevention of significant deterioration (PSD) increments, and/or ambient significance levels to determine if a second level of analysis, refined modeling, is required for a better estimate of the predicted concentrations. The purpose of screening is to identify if the potential for exceeding applicable air quality threshold levels exists, and thus the need for refined analysis. Screening eliminates the time and expense of refined modeling if the predicted concentrations do not approach the applicable levels.
A refined model provides a more detailed analysis of the parameters and thus gives a more accurate estimate of the pollutant concentration at receptors. However, a refined model demands more specific input data. The specific data can include topography, better receptor grid resolution, downwash or other plume adjustments, and pollutant decay or deposition.
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