contrived, it is the only workable approach under the

regulatory framework.

5.2.2 Visibility Analyses

The MESOPUFF-II model will be run for all sources, whether

nearer or farther than 50 km from a receptor, to provide

concentrations of SO and NO for visibility calculations. A = 

4 3

complete description of the visibility calculations is

described in Appendix B.

SO has been identified as the primary constituent of =


visibility degradation in the eastern U.S.; organic aerosols

and NO are less important contributors, but a significant 


species, nonetheless. Therefore, it is critical to have

estimates of SO in order to estimate visibility impacts. =


Estimates of nitrates are desirable and can be obtained from

the long range transport modeling system; estimates of organic

aerosols, while more important to total scattering than

nitrates in most cases, are not readily estimated by the

modeling system. Therefore, organic aerosol concentrations

will only be considered as a background concentration for

determining regional visibility impacts. The MESOPUFF-II model

is capable of producing concentrations of SO and NO , whereas = 

4 3

the preferred models, listed in the Guideline are not.

Therefore, the IWAQM is recommending that when a cumulative

visibility impact analysis is desired, all sources be modeled

with MESOPUFF-II in order to estimate regional haze impacts.

It is also important to note that although the application of

MESOPUFF-II for short transport distances is questionable the

contribution for such sources is relatively unimportant. In

many permitting analyses, it is likely that only the source

being permitted will be analyzed. Therefore, the MESOPUFF-II

analysis will be greatly simplified.




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