Choose time periods which are drawn from the period upon which the baseline

design value is based. As we note in Section 3, fewer emission estimates and fewer air quality

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

attainment test.

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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