SO2 Modeling Guidance

***
Although this guidance offers a "first tier" approach of using the "overall highest hourly background SO2 concentration from a representative monitor" (emphasis added), it goes on to seemingly stress the appropriateness of applying professional judgment when characterizing background by modeling nearby sources and by using monitoring data. It even goes so far as to suggest that you don't necessarily have to follow the modeling guidance in the 1980 Draft NSR Workshop Manual.

EPA SO2 Modeling Policy Memo, August 2010

I noticed, too, that Appendix W seems to encourage efforts to avoid double counting through use of modeling and monitored data. This link accesses a copy of the current version of Appendix W:

EPA SCRAM site with link to Appendix W

***