2) Model time periods in which observed concentrations are close to the appropriate

baseline design value or visibility impairment.

3) Model periods for which extensive air quality/meteorological data bases exist.

4) Model a sufficient number of days so that the modeled attainment test applied at each

monitor violating the NAAQS is based on multiple days (see section 14.1.1).

These four criteria may sometimes conflict with one another. For example, there may

only be a limited number of days with intensive data bases, and these may not cover all of the

meteorological conditions which correspond with monitored concentrations close to site-specific

design values during the base period. Thus, tradeoffs among the four primary criteria may be

necessary in specific applications.

Those implementing the modeling/analysis protocol may use secondary episode selection

criteria on a case by case basis. For example, prior experience modeling an episode or year, may

result in its being chosen over an alternative. Another consideration should be to choose time

periods occurring during the 5-year period which serves as the basis for the baseline design value

(DVB). If observed ozone or 24-hour PM2.5 exceedances occur on weekends, weekend days

should be included within some of the selected time periods. If it has been determined that there

is a need to model several nonattainment areas simultaneously (e.g., with a nested regional scale

model application), a fourth secondary criterion is to choose time periods containing days of

common interest to different nonattainment areas.


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