NAAQS. But at the same time, it is necessary to have a sufficient number of days in the mean
RRF calculation to develop a robust estimate (this is addressed in more detail in section 14.1.1).
32The same set of sites was used for each of the threshold concentrations (70-85 ppb).
33The analysis suggests a threshold of greater than 85 ppb may be appropriate, but 85 ppb
was chosen as the upper end of the threshold because it is equal to the NAAQS. For areas with
very high baseline design values (>110 ppb) it may be necessary to use a higher threshold.
Example 4.1 illustrates how to apply the minimum concentration threshold.
Given: The same simulations as performed in Example 3.2 yield low predictions near site I with
baseline emissions on day 3, such that the 8-hour daily maximum ozone concentration predicted
for that day is 65.0 ppb (rather than the 91.6 ppb shown in Example 3.2).
Find: The relative response factor near site I ((RRF)I).
Solution: (1) Calculate the mean 8-hour daily maximum ozone concentration obtained near site
I for baseline and future emissions. Exclude results for day 3 from the calculations. From
(a) (mean 8-hr daily max)future = (87.2 + 82.4 + 81.1)/3 = 83.6 ppb
(b) (mean 8-hr daily max)baseline = (98.3 + 100.2 + 90.7)/3 = 96.4 ppb.
(2) Compute the relative response factor by taking the ratio of future/baseline.
(RRF)I = 83.6/96.4 = 0.867
4.2 How Do I Apply The Recommended 8-Hour Ozone Modeled Attainment
States/Tribes should apply the modeled attainment test at all monitors within the
nonattainment area plus other nearby counties within the State34. Inputs described in Section 3.1
are applied in Equation (3.1) to estimate a future design value at all monitor sites and grid cells
for which the modeled attainment test is applicable. When determining compliance with the 8-
hour ozone NAAQS, the standard is met if, over three consecutive years, the average 4th highest
8-hour daily maximum ozone concentration observed at each monitor is < 0.08 ppm (i.e.,# 84
ppb using rounding conventions)35. Thus, if all resulting predicted future design values (DVF)
are# 84 ppb, the test is passed. The modeled attainment test is applied using 3 steps.
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