This guidance identifies several modeling systems8 with nesting capabilities to resolve

meteorological parameters, emissions, chemistry, and transport. We believe it is not beneficial

to identify any modeling system as the preferred, or “guideline model” for ozone, PM2.5

9 or

regional haze modeling. States/Tribes may use any appropriate modeling system provided that

the requirements of 40 CFR 51.112 are met. In this guidance, we provide certain criteria to

assist States/Tribes in justifying the use of such modeling systems. These criteria apply equally

to U.S.EPA models and alternative air quality model(s). The guidance also provides

recommendations for developing meteorological, air quality and emissions inputs used in nested

regional model applications, and makes suggestions for quality assuring inputs and evaluating

performance of emissions, meteorological and air quality models.

Premise 3. Resource intensive approaches may often be needed to support an adequate

attainment demonstration. This follows from the regional nature of ozone and PM2.5

concentrations in the Eastern U.S. While we believe that existing and future regional reductions

in NOx and SO2 emissions will reduce ozone and PM2.5 over much of the eastern U.S., elevated

regional ozone and PM2.5 concentrations will continue to affect local strategies needed to attain

the NAAQS in the remaining nonattainment areas.

8A modeling system includes a chemical model, an emissions model and a meteorological

model. Terms, such as this one, which are introduced using italics are defined more fully in a

glossary at the back of this guidance. “Modeling system” and “air quality model” are used

interchangeably. “Air quality model” means “modeling system” in this guidance.

9In this context, we are not referring to dispersion modeling of primary PM2.5 sources

subject to New Source Review (NSR) or Prevention of Significant Deterioration (PSD) analyses.

We are only referring to PM2.5 analyses for SIP attainment demonstrations.


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