As we note in Section 3, fewer emission estimates and fewer air quality
Choose time periods which are drawn from the period upon which the baseline
design value is based.
model simulations may be needed if the base case period used to evaluate model performance,
and the baseline period used in the recommended modeled attainment test are the same.
Following this criterion could also make the second primary criterion more straightforward.
Choose episodes having observed concentrations “close to” the NAAQS on as many
days and at as many sites as possible. This criterion mainly applies to the standards with short
term averaging times (8-hour ozone and 24-hour PM2.5). It is related to the modeled attainment
test and to the fourth primary criterion for episode selection. The more days and sites for which
it is reasonable to apply the test, the greater the confidence in the outcome of the modeled
It is desirable to include weekend days among those chosen, especially if
concentrations greater than the NAAQS are observed on weekends. Weekend days often
reflect a different mix of emissions than occurs on weekdays101. This could also lead to different
spatial patterns of 8-hour ozone and/or 24-hour PM2.5 concentrations. Thus, for increased
confidence that a control strategy is effective it needs to be tested on weekends as well as on
weekdays. If emissions and spatial patterns of high ozone or PM2.5 do differ on weekends versus
weekdays, including weekend days in the choice of episodes will provide a mechanism for
evaluating the accuracy of a model’s response to changes in emissions.
If it has been determined that there is a need to model several nonattainment areas
simultaneously, choose time periods which meet the primary and secondary criteria in as
many of these nonattainment areas as possible. As discussed in Section 15, a State/Tribe or
group of States/Tribes may decide to apply a model on a regional or a nested regional scale to
demonstrate attainment or assess uniform rate of progress in several nonattainment areas or
Class I areas at the same time. Time and resources needed for this effort could be reduced by
choosing time periods which meet the primary and secondary criteria in several nonattainment
areas (or Class I areas) which are modeled. Several organizations are modeling much of the
Eastern U.S. for an entire year. This type of application should allow for application of the
attainment tests at many locations using the appropriate set of days for each area.
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