1 cumulative impact levels.

2 The representative monitored PM2.5 design value, rather than the overall maximum

3 monitored background concentration, should generally be used as the monitored component of

4 the cumulative analysis. The PM2.5 design value for the annual averaging period is based on the

5 3-year average of the annual average PM2.5 concentrations; for the 24-hour averaging period, the

6 design value is based on the 3-year average of the 98th percentile 24-hour average PM2.5

7 concentrations. Details regarding the determination of the 98th percentile monitored 24-hour

8 value based on the number of days sampled during the year are provided in the data

9 interpretation for the PM2.5 NAAQS, Appendix N to 40 CFR Part 50.

10 It should be noted here that although the monitored design values for the PM2.5 standards

11 are defined in terms of 3-year averages, this definition does not preempt or alter the Appendix W

12 requirement for use of 5 years of representative NWS meteorological data or at least 1 year of

13 site specific data for air quality modeling purposes. The 5-year average based on use of

14 representative NWS meteorological data, or an average across one or more (up to 5) complete

15 years of available site specific data, serves as an unbiased estimate of the 3-year average for

16 purposes of modeling demonstrations of compliance with the NAAQS. Modeling of ‘rolling 3-

17 year averages,’ using years 1 through 3, years 2 through 4, and years 3 through 5, is not required.

18 The March 23, 2010, clarification memo recommended as a First Tier that the modeled

19 annual (or 24-hour) concentrations of primary PM2.5 to be added to the monitored annual (or 24-

20 hour) design value for comparison to the NAAQS should be based on the highest average of the

21 modeled annual (or 24-hour) averages across 5 years for representative NWS meteorological

22 data or the highest modeled annual (or 24-hour) average for one year (or multi-year average of 2

23 up


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