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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