primary particulate matter, these estimates should be made at the monitoring location (see

Section 5.3). Thus, a State may use a regional photochemical grid model to address both

primary and secondary components of particulate matter or they may use a photochemical model

to address secondary PM and an inert model applied over a more limited domain to address

primary components.

In regional haze-related applications it may suffice when modeling primary particulate

matter to use grid cells which are the same size as those used to model secondary components of

particulate matter. If there is no reason to believe that there are major individual sources of

primary PM within about 50 km which affect the monitor site in a Class I area, primary

components can be considered using the same coarse grid used for the photochemical grid

model. This is generally a good default assumption.

13.2 What Prerequisites Should An Air Quality Model Meet To Qualify For Use

In An Attainment Demonstration?

A model should meet several general criteria for it to be a candidate for consideration in

an attainment demonstration or uniform rate of progress assessment. These general criteria are

consistent with requirements in 40 CFR 51.112 and 40 CFR part 51, Appendix W (U.S. EPA,

2005d). Note that, unlike in previous guidance (U.S. EPA, 1991a), we are not recommending a

specific model for use in attainment demonstration or uniform rate of progress assessment. At

present, there is no single model which has been extensively tested and shown to be clearly

superior than its alternatives. Thus, 40 CFR Part 51 Appendix W does not identify a “preferred

model” for use in attainment demonstrations of the NAAQS for ozone or PM2.5 or uniform rate

of progress assessments for regional haze92. Based on the language in 40CFR Part 51 Appendix

W, models used for these purposes should meet requirements for “alternative models”.

States/Tribes should use a non-proprietary model, which is a model whose source code


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