97As noted in section 4.1, for areas with a very high design value (>110 ppb), a higher
threshold might be appropriate.
98Any situation where there are less than 5 days available for RRF calculations at
monitoring sites with relatively high concentrations, (above the NAAQS and/or close to the areawide
design value) should be discussed with the appropriate U.S. EPA Regional office(s).
In summary, States should try to model enough episode days so that the mean RRF
calculation at each monitor contains a minimum of 10 days with a modeled concentration > 85
ppb. If there are less than 10 days > 85 ppb, then the threshold should be lowered until 10 days
are included in the calculation. The threshold should not go below 70 ppb and the number of
days should always be at least 5. In trying to meet these recommendations, the greatest priority
should be given to identifying episode days with appropriate ozone concentrations at the
monitoring sites with the highest design values. Sites with design values below the NAAQS
should be given a low priority.
14.1.2 Recommendations for the 24-Hour PM2.5 NAAQS
The episode selection criteria for the 24-hour PM2.5 NAAQS are similar to the ozone
criteria. The complicating factor for the 24-hour NAAQS is that exceedances of the standard can
occur under a variety of meteorological conditions at different times of the year and can be due
to a variety of different PM components. Therefore the universe of episodes could be large and
much more varied. The form of the standard is a 3 year average of the 98th percentile
concentration from each year. Therefore, modeling 3 full years of meteorological data would
best capture the full range of conditions that lead to 24-hour design values. But the standard is
only influenced by the very high end of the distribution of PM2.5 days (top 2% from each year).
This argues for modeling a smaller set of days on an episodic basis. We recommend one of two
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