18 Region 10 example:

19 In support of the 2011 Revised Draft Permits, Region 10 provided a detailed

20 explanation for why it believes that modeling secondary PM2.5 emissions is not

21 needed in order to determine that emissions of PM2.5 precursors from the Discoverer

22 and Associated Fleet would not, together with emissions of primary PM2.5, cause or

23 contribute to a violation of the 24-hour PM2.5 NAAQS. The factors Region 10 relied

24 on to reach this conclusion include:


26 1) The background PM2.5 monitoring data considered in the air quality analysis is

27 quality assured, quality controlled data from monitors operating for more than one

28 year that Region 10 believes will have accounted for much of the secondary

29 formation from existing regional emission sources that will occur in the Chukchi

30 Sea and Beaufort Sea regions. Monitoring data show low levels of daily PM2.5,

31 generally in the range of 2 μg/m3, with the higher PM2.5 values generally

32 occurring on days where windblown dust or fires are believed to be contributing

33 factors. Thus, there is no indication that secondary formation of PM2.5 from

34 existing sources in the North Slope is currently causing or contributing to

35 exceedances or a violation of the PM2.5 NAAQS in the onshore communities.


37 2) Modeled primary PM2.5 impacts from the Discoverer and Associated Fleet that,

38 when using a conservative “First Tier” approach to combining modeled primary

39 PM2.5 impacts with monitored background PM2.5 concentrations, are less than 67

40 percent of the PM2.5 NAAQS. Thus, although not expected, considerable

41 formation of secondary PM2.5 emissions could occur before the NAAQS would be

42 threatened.



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