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

available for free (or for a “reasonable” cost). Furthermore, the user must be able to revise the

code93 to perform diagnostic analyses and/or to improve the model’s ability to describe

observations in a credible manner. Several additional prerequisites should be met for a model to

be used to support an attainment demonstration or uniform rate of progress assessment.

92Appendix W recommends specific dispersion models for primary pollutants, but does

not recommend specific photochemical models. Recommended dispersion models should be

used for primary PM2.5 analyses if dispersion models are used for a “local area analysis”(see

Section 5.3).

93Air quality models are generally identified by a version number. The version of the

model that is used in SIP applications should be identified. Code revisions to standard versions

of models should be noted and documented.

(1) It should have received and been revised in response to a scientific peer review.

(2) It should be appropriate for the specific application on a theoretical basis.

(3) It should be used with a data base which is adequate to support its application.

(4) It should be shown to have performed well in past ozone or PM modeling applications.

(If the application is the first for a particular model, then the State should note why it

believes the new model is expected to perform sufficiently.)

(5) It should be applied consistently with a protocol on methods and procedures.

An air quality model may be considered to have undergone “scientific peer review” if

each of the major components of the modeling system (i.e., air chemistry/deposition,

meteorological and emissions models) has been described and tested, and the results have been

documented and reviewed by one or more disinterested third parties. We believe that it should

be the responsibility of the model developer or group which is applying an air quality model on

behalf of a State/Tribe to document that a “scientific peer review” has occurred. States/Tribes

should then reference this documentation to gain acceptance of an air quality model for use in a

modeled attainment demonstration.


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