Based on these results, we recommend a minimum number of 10 days to be included in
the mean RRF calculation for each monitoring site. This will ensure a relatively robust mean
RRF value that is within ±1% (on average) of the large dataset mean. If relatively few ozone
days are being modeled or certain monitors have relatively few exceedances (above 85 ppb),
then we recommend using an absolute minimum number of 5 days in the calculation.
The minimum number of days recommendations can be combined with the minimum
threshold recommendation to create a hierarchy of number of days/threshold combinations that
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).
can address any situation. The recommended minimum concentration threshold identified in
section 4.1 is 85 ppb. But similar to the minimum number of days, there may be situations
where there are relatively few “high” modeled ozone days at certain monitors. Therefore, when
possible, we recommend using the 85 ppb threshold97, but it is acceptable to use a threshold as
low as 70 ppb.
Therefore, the following criteria should be applied to determine the number of days and
the minimum threshold at each ozone monitor:
!If there are 10 or more days with daily maximum 8-hour average baseline modeled ozone
> 85 ppb then use an 85 ppb threshold.
!If there are less than 10 days with daily maximum 8-hour average baseline modeled
ozone > 85 ppb then reduce the threshold down to as low as 70 ppb until there are 10
days in the mean RRF calculation.
!If there are less than 10 days with daily maximum 8-hour average modeled ozone > 70
ppb then use all days > 70 ppb.
!Don’t calculate an RRF for sites with less than 5 days > 70 ppb98.
The following table illustrates several examples of the recommended hierarchy of choosing the
number of days vs. the minimum threshold.
n751 - n752 - n753 - n754 - n755 - n756 - n757 - n758 - n759 - n760 - n761 - n762 - n763 - n764 - n765 - n766 - n767 - n768 - n769 - n770 - n771 - n772 - n773 - n774 - n775 - n776 - n777 - n778 - n779 - n780 - n781 - n782 - n783 - n784 - n785 - n786 - n787 - n788 - n789 - n790 - n791 - n792 - n793 - n794 - n795 - n796 - n797 - n798 - n799 - n800
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