in the region. The qualitative assessment should expand upon the characterization of

16 meteorology described in the conceptual description to explain any meteorological factors that

17 could limit or enhance the formation of secondary PM2.5 from any significant precursor

18 emissions.

19 Analysis of existing photochemical grid modeling developed for regional haze, ozone,

20 and PM2.5 SIPs or other photochemical grid modeling used in related sensitivity projects or

21 analysis to support prior air quality rules may also be considered to help understand the general

22 response of secondary PM2.5 formation to certain magnitudes of a precursor pollutant in that

23 region. While the new or modifying emissions source may emit a significant level of a precursor

pollutant under PSD regulations, that level of emission may be extremely 1 small when compared

2 against the total emissions of that precursor pollutant throughout the region. The qualitative

3 assessment of the impact from secondary PM2.5 formation can be strengthened if substantial

4 regional reductions or increases of that precursor pollutant have been demonstrated through

5 photochemical grid modeling exercises to not cause significant decreases or increases of

6 secondary PM2.5.

7 An example of a thoroughly developed qualitative assessment of the potential for

8 secondary PM2.5 formation to cause or contribute to a violations of the NAAQS was provided by

9 the EPA Region 10 Office through a response to public comments document regarding a CAA

10 permit issued for Shell’s Discoverer drill ship and support fleet to explore for oil and gas in the

11 Chukchi Sea off Alaska. While the environment in and around the Chukchi Sea and North Slope

12 of Alaska is unique when compared to the rest of the United States, the various components


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