As noted in the March 23, 2010 PM2.5 guidance memo, although dry and/or wet

15 deposition may be important processes when estimating ambient concentrations of PM in

16 general, these factors are expected to be minor for PM2.5 due to the small particle size. In

17 addition, there may be additional uncertainty associated with deposition modeling for PM2.5 due

18 to the fact that deposition properties may vary depending on the constituent elements of PM2.5.

11 The best performing model is one that best predicts regulatory design values for a particular pollutant. The EPA’s

Protocol for Determining the Best Performing Model (EPA-454/R-92-025) defines appropriate methodologies and

statistical criteria for this evaluation. According to the document, “For a pollutant… for which short-term ambient

standards exist, the statistic of interest involves the network-wide highest concentration…the precise time, location,

and meteorological condition is of minor concern compared to the magnitude of the highest concentration actually


12 The final rule can be found at: Extensive

documentation is available describing the various components of AERMOD, including user guides, model

formulation, and evaluation papers. See EPA’s SCRAM website for AERMOD documentation:


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