Air Dispersion Modeling

Dispersion models are the primary tools used in the air quality analysis. These models estimate the ambient concentrations that will result from the PSD applicant's proposed emissions in combination with emissions from existing sources. The estimated total concentrations are used to demonstrate compliance with any applicable NAAQS or PSD increments. The applicant should consult with the permitting agency to determine the particular requirements for the modeling analysis to assure acceptability of any air quality modeling technique(s) used to perform the air quality analysis contained in the PSD application.

The dispersion modeling analysis usually begins with a preliminary or screening analysis, which models only the significant increase in potential emissions of a pollutant from a proposed new source, or the significant net emissions increase of a pollutant from a proposed modification.

Specifically, the preliminary analysis

• determines whether the applicant can forego further air quality analyses for a particular pollutant
• may allow the applicant to be exempted from ambient monitoring data requirements
• is used to define the impact area within which a full impact analysis must be carried out

The results of this preliminary analysis determine whether the applicant must perform a full impact analysis, involving the estimation of background pollutant concentrations resulting from existing sources and growth associated with the proposed source.

A full impact analysis is required for any pollutant for which the proposed source's estimated ambient pollutant concentrations exceed prescribed significant ambient impact levels.

This analysis expands the preliminary analysis in that it considers emissions from:

• the proposed source
• existing sources
• residential, commercial, and industrial growth that accompanies the new activity at the new source or modification (i.e., secondary emissions)

For SO2 , particulate matter, and NO2 , the full impact analysis actually consists of separate analyses for the NAAQS and PSD increments.   the selection of background sources (and accompanying emissions) to be modeled for the NAAQS and increment components of the overall analysis proceeds under somewhat different sets of criteria. In general, however, the full impact analysis is used to project ambient pollutant concentrations against which the applicable NAAQS and PSD increments are compared, and to assess the ambient impact of non-criteria pollutants.

Air dispersion modeling begins with the selection of an appropriate model, acquisition of appropriate input data, and following recommended procedures to complete the air quality analysis.