The 98th percentile PM2.5 values may be highly variable from year to year. Therefore, it

is important to model a year or time periods when high PM2.5 episodes occur in the area. The

modeling should include a year and/or episodes that are representative of the 24-hour design


Calculation of RRFs- Regardless of the time periods modeled, component specific

RRFs should be calculated for all four quarters or for all quarters in which high 24-hour PM2.5

concentrations occur (following the recommendations in section 5.2). We do not have specific

analyses that have analyzed an appropriate minimum threshold concentration for 24-hour

average PM2.5. If there are very few days that exceed the 65 ug/m3 standard, then a secondary

recommendation is to include all days with concentrations > 50 ug/m3, or calculate RRFs based

on the highest modeled PM2.5 days from each quarter. A reasonable estimate of the high end of

the distribution might be between the top 10% of days to the top 25% of days. We recommend

that RRF calculations contain a similar number of days as for ozone. This means that each

quarterly RRF calculation should contain no less than 5 days99.

EPA will be re-examining these recommendations as they relate to the new 35 ug/m3 24-

hour NAAQS as more information becomes available regarding the typical features of areas

violating the revised standard. Additional guidance will be issued at a later date.


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