On March 23, 2010, to assist sources and permitting authorities in carrying out the

15 required air quality analysis, the EPA issued, a guidance memorandum entitled “Model

16 Procedures for Demonstrating Compliance with PM2.5 NAAQS” (U.S. EPA, 2010b). The

17 guidance memorandum recommended certain interim procedures to address the fact that

18 compliance with the PM2.5 NAAQS is based on a statistical form, and that there are technical

19 complications associated with the ability of existing models to estimate the impacts of

20 secondarily formed PM2.5 in the atmosphere resulting from emissions of PM2.5 precursors. For

the latter issue, the EPA recommended that special attention 1 be given to the assessment of

2 monitored background air quality data since such data account for the contribution of both

3 primary and secondarily formed PM2.5 in the atmosphere associated with both nearby and

4 regional sources.

5 On January 7, 2011, the NACAA PM2.5 Modeling Implementation Workgroup (NACAA

6 Workgroup) delivered a final report (NACAA, 2011), including a set of specific

7 recommendations to the EPA. The NACAA Workgroup was formed in early 2010 with the

8 objective of providing technical recommendations to the Agency to aid in further development of

9 PM2.5 permit modeling guidance. The NACAA Workgroup’s final report addressed three

10 specific issues regarding PM2.5 modeling implementation: 1) Emissions Inventories; 2)

11 Secondary Formation from Project Source; and 3) Representative Background Concentrations.


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