19 option to specify four seasonal background values to be combined with modeled concentrations

20 on a seasonal basis (appropriate for a Second Tier analysis). As explained above, the monitored

21 design value is the recommended input for the annual (First Tier) option, whereas the 98th

22 percentile of the values for each season, averaged across three years of monitoring, is the

23 recommended input for Second Tier analyses. The appropriate rank associated with the 98th

Public Review Draft 03/04/2013


percentile of the 24-hour values for a season will depend on t 1 he sampling frequency of the

2 particular monitor being used for background. For a monitor with a daily, 24-hour sampling

3 frequency, the 98th percentile recommended rank is the 3rd highest 24-hour value for each

4 season.17 The AERMOD model also allows the user to track the contribution from background

5 concentrations to the cumulative modeled design value.


7 IV.4 Determining Significant Contributions to Modeled Violations

8 If the cumulative impact assessment following these recommendations results in modeled

9 violations of the PM2.5 NAAQS, then the applicant will need to determine whether the project’s

10 emissions represent a significant contribution to those modeled violations. The EPA has

11 previously supported making a “significant contribution” determination based on a comparison

12 of the modeled impacts from the project emissions associated with the modeled violation, paired

13 in time and space, to the SIL for the relevant pollutant and averaging period contained in section

14 51.165(b) of the EPA’s regulations. The EPA has interpreted this regulation to support the

15 conclusion that a source with an impact below the relevant value in 51.165(b)(2) does not

16 significantly contribute to either an existing exceedances of the NAAQS in a nonattainment area

17 or exceedances predicted in an attainment area based on a cumulative analysis.18

18 The January 22, 2013, court decision did not vacate the PM2.5 SIL value in section

19 51.165(b) of the EPA’s regulations. However, the Court recognized that the language in Section

20 51.165(b)(2) operates in a manner different from the vacated Sections 51.166(k)(2) and

17 An


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