monitoring sites were screened to eliminate sites that had a limited number of “high” modeled
ozone days. All sites that had less than 10 days with an 8-hour average maximum modeled
concentration of 85 ppb were dropped from the analysis. This left 206 sites in the analysis32.
Table 4.1 shows that the mean RRF (averaged across all sites) is sensitive to the
minimum ozone threshold. As the threshold is raised from 70 ppb to 85 ppb, the RRFs become
smaller (larger percent decrease in ozone). On average, the model predicts a 0.2% greater ozone
reduction for every 5 ppb increase in the minimum threshold. Additionally, the variability of the
daily RRFs (as measured by the standard deviation of the daily RRFs) is reduced as the threshold
is increased. This is an important finding because lower variability in day to day RRFs indicates
lower uncertainty in the mean RRFs, and hence the attainment test results.
As a result of the sensitivity in model response at concentrations below the NAAQS,
combined with the increased variability of the RRFs at lower concentrations, we recommend
using a minimum concentration threshold of 85 ppb33. This will result in RRFs that adequately
represent high ozone days and also provide for more robust RRFs and future design values. For
a more detailed description of selecting modeling days for ozone, see Section 14.1.1.
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