There were numerous comments received with respect to issues and concerns of

13 demonstrating compliance with the NAAQS for PM2.5 during the formal public comment period

14 for the 10th Conference on Air Quality Modeling. Several of these commenters and other

15 communications from various stakeholders have provided the EPA with additional information

16 and recommendations that have been particularly useful in the consideration of a range of

17 acceptable options for PM2.5 NAAQS compliance demonstrations.

18 We would also like to acknowledge the contributions of the staff of the EPA Office of

19 Transportation and Air Quality (OTAQ) for their input and assistance in the development of the

20 Draft Guidance for PM2.5 Permit Modeling. The OTAQ’s “Transportation Conformity Guidance

21 for Quantitative Hot-spot Analyses in PM2.5 and PM10 Nonattainment and Maintenance Areas”

22 (U.S.EPA, 2010a) guidance document served as a foundation for many aspects of the modeling

23 guidance contained within this document.


I. Background

2 On July 18, 1997, the EPA revised the NAAQS for particulate matter (PM) to add new

3 annual and 24-hour standards for fine particles using particulate matter less than 2.5 micrometers

4 (PM2.5) as the indicator. (62 FR 58652). The EPA revised the 24-hour NAAQS for PM2.5 on

5 September 21, 2006, by lowering the level of the standard from 65 μg/m3 to 35 μg/m3. (71 FR

6 61144). In the September 21, 2006, action, the EPA also retained the previous 1997 annual

7 standard for PM2.5 and the 24-hour standard for PM10, and revoked the previous annual standard

8 for PM10. Subsequently, the Agency revised the PM2.5 standard again on December 14, 2012, by

9 lowering the level of the annual PM2.5 NAAQS from 15 μg/m3 to 12 μg/m3 and retaining the 24-

10 hour standards for PM2.5 and PM10. (78 FR 3086). The annual PM2.5 standard is met when the

11 3-year average of annual arithmetic mean concentrations is less than or equal to 12.0 μg/m3. The

12 24-hour PM2.5 standard is met when the 3-year average of the 98th percentile 24-hour

13 concentrations is less than or equal to 35 μg/m3.

14 On October 23, 1997, citing significant technical difficulties


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