the horizontal/vertical resolution considered. For PM2.5 nonattainment applications where large

spatial gradients of primary PM2.5 exist, a dispersion model may be needed in addition to a

photochemical model.

4. Select appropriate meteorological time periods to model. Like the preceding step, this

step requires review of available air quality and meteorological data. It also requires a thorough

understanding of the form of the ozone and PM2.5 NAAQS, the regional haze goals, and the

modeled attainment tests described in Sections 3-6. Finally, it requires a review of

meteorological conditions which represent annual PM2.5 concentrations, both good and bad

visibility days, and time periods which have been observed to accompany monitored

exceedances of the 8-hour ozone or 24-hour PM2.5 NAAQS. The object of these reviews is to

select periods which a) include days with observed concentrations exceeding site-specific design

values (8-hour ozone or 24-hour PM2.5 NAAQS), b) select a representative mix of days for each

quarter in applications dealing with the annual NAAQS and/or c) select days which are

representative of those corresponding with good and poor visibility.

Due to increased computer speeds, it is now prudent to recommend modeling relatively

long time periods. For 8-hour ozone and 24-hour PM2.5, at a minimum, modeling episodes which

 

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