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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