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).

146

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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