There are several modeling tools that can be used to evaluate contributions of local PM2.5

sources. A grid model can be run at very high resolution (down to 1 or 2 km) or a Gaussian

dispersion can be used. Grid based models usually simulate chemical transformation and

complex meteorological conditions, while dispersion models are generally more simplistic;

being limited to a local-scale and using Gaussian approximations with little or no chemistry.

Therefore, while dispersion models may not be an appropriate tool for determining secondary

PM2.5 concentrations, they work well for use in determining local primary PM2.5 impacts in a

small area. The model(s) and model setup should be evaluated to determine the most appropriate

tools for a specific situation.

Regardless of which type of models are used to evaluate changes in primary PM2.5 at

monitors, we recommend that the model results be used in a relative manner. This is consistent

attainment test as described in Section 5.1 should be followed. If a Gaussian dispersion model is

used then the application of the attainment test will vary slightly. The test will need to combine

results from the photochemical grid based modeled attainment test and the results from localscale

dispersion modeling. If such an approach is taken, the suggested variations on the

attainment test are discussed in the following section.


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