1) Model every day for a full year (or multiple years). This is recommended for both dispersion

modeling of primary PM2.5 components and photochemical modeling of secondary and primary

components. Many areas that violate the 24-hour PM2.5 NAAQS will also violate the annual

PM2.5 NAAQS. Therefore, full year modeling may already exist or is being planned for the

annual NAAQS attainment test. States that have nonattainment areas that violate both PM2.5

standards should take advantage of planning opportunities to address both standards at the same

time. Modeling at least a full year will also help ensure that a sufficient number of days are

included in the RRF calculations.

2) Model episodes when high PM2.5 concentrations occur. Similar to ozone, episodes should be

selected where PM2.5 concentrations are greater than the NAAQS (> 65 ug/m3) and are close to

the baseline design value. Similar to ozone, data analyses can be completed to help select a

variety of meteorological episodes which lead to high PM2.5 concentrations. In some cases, there

may be very limited conditions which lead to high 24-hour average PM2.5 concentrations, and in

other cases there may be a wide variety of cases. The specific situation in each nonattainment

area will determine the number of episodes and the time periods which need to be modeled. For

example, if exceedance level PM2.5 concentrations in an area only occur in the winter, then a

limited number of winter days can be modeled. In other areas, exceedance days may occur in all



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