determining whether there are particular component species (e.g. sulfates, nitrates, and elemental

15 or organic carbons) that dominate the makeup of high, low, and average PM2.5 concentrations

16 will help guide the degree of analysis and ultimately justification that will be required in the

17 qualitative assessment based on the magnitude and characteristics of any significant precursor

18 emissions from the source. It may also be important to describe the typical background

19 concentrations of certain chemical species that participate in the photochemical reactions that

20 form secondary PM2.5, such as NH3, VOC, and ozone. It is possible that there are mitigating

13 For more detailed information on the development of such conceptual descriptions for an area, please refer to the

following: Chapter 10 of “Particulate Matter Assessment for Policy Makers: A NARSTO Assessment.” P.

McMurry, M. Shepherd, and J. Vickery, eds. Cambridge University Press, Cambridge, England (NARSTO, 2004).

Section 11, “How Do I Get Started? 'A Conceptual Description'” of “Guidance on the Use of Models and Other

Analyses for Demonstrating Attainment of Air Quality Goals for Ozone, PM2.5, and Regional Haze.” U.S.

Environmental Protection Agency, Research Triangle Park, North Carolina (U.S. EPA, 2007a)


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