7) With respect to the Chukchi Sea impacts, the predominant easterly wind

38 directions in the Chukchi Sea along with the distance between the project location

39 and the existing sources in the North Slope oil and gas fields are such that

40 emissions from the Discoverer and Associated Fleet are not likely to significantly

41 contribute to the maximum ambient concentrations resulting from the existing

42 source emissions.

43 8) Region 10 required post-construction monitoring in the previous permits

44 because the conservative screening modeling resulted in predicted levels that were

45 just below the 24-hour PM2.5 NAAQS. With the additional emission reductions in

Public Review Draft 03/04/2013


direct PM2.5 emissions and the use of a 1 refined model, predicted PM2.5

2 concentrations are now well below the NAAQS. However, Region 10 has decided

3 to retain the post-construction monitoring requirement in order to obtain better

4 information on the quantity of secondary particles in the North Slope

5 communities.

Based on these factors, and consistent with current guidance, Region 10 believes that

8 an adequate assessment has been made to demonstrate that the PM2.5 NAAQS will be

9 protected, accounting for primary PM2.5 impacts and potential contributions due to

10 PM2.5 precursors from the Discoverer and the Associated Fleet, and that it is not

11 necessary to use a photochemical model to further evaluate secondary PM2.5 formation

12 in these permitting actions.



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