Modeling for the 24-hour PM2.5 NAAQS is further complicated by the fact that violations of the

24-hour standard may occur during different times of the year and may be caused by a variety of

different PM2.5 species and precursors.

Premise PM7. Causes of PM2.5 concentrations which violate NAAQS may be diverse.

Modeling approaches needed to address primary vs. secondary particulate matter differ in their

requirements. Earlier, we noted differing requirements for size and resolution of the modeling

domain. Another difference is the need to consider atmospheric chemistry in the modeling. It is

essential to have some understanding of the nature of an area’s PM2.5 or visibility problem before

modeling begins. Otherwise, a State runs the risk of selecting inappropriate analysis tools as

well as selecting a strategy which will prove to be ineffective at reducing its problem. Thus, a

State needs to perform data analysis before using air quality models. This analysis should be

used to develop a conceptual description of the problem at hand. The conceptual description

may then be used to select a broad strategy (e.g., do I focus on reducing primary or secondary

particulate matter or both?) as well as to help implement a modeling protocol to best address the

nature of the problem and the qualitative strategy which has been tentatively selected to address

it. The guidance needs to provide States with flexibility in choosing model(s) to address specific




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