In this section, we first discuss each of the four identified primary criteria for choosing
meteorological episodes to model. We then discuss the secondary criteria, which may be
important in specific applications.
14.1 What Are The Most Important Criteria For Choosing Episodes?
In the following section, we identify how the four selection criteria apply to ozone. We
then discuss issues related to time period selection for PM2.5 and regional haze. We also
recommend minimum time periods to model for 8-hour ozone, 24-hour PM2.5, annual PM2.5, and
regional haze. As a matter of convenience, section 14.1 is organized by pollutant, but it could
also be organized by the nature of the ambient data. For example, 8-hour ozone and 24-hour
PM2.5 episode selection criteria are similar because both standards are based on short term peak
concentration periods. Regional haze calculations are based on an average of 20 or more days
per year (20% best or worst days). Therefore regional haze episode selection will likely include
more days throughout the year. The annual PM2.5 NAAQS is unique because each and every
ambient observation is included in the average. It is likely that a full year (or more) will be
modeled to represent the annual PM2.5 standard.
14.1.1 8-Hour Ozone NAAQS
Simulate a variety of meteorological conditions This criterion is important, because we want
to be assured that a control strategy will be effective under a variety of conditions leading to
elevated ozone concentrations. The time periods chosen should reflect the variety of conditions
that lead to 8-hour ozone concentrations > 85 ppb.
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