transport models and recommend a specific model, which meets

the Guideline criteria, for long range transport analysis, in

the interim, until a more comprehensive solution can be

formulated and tested. In addition to the recommendations of a

specific long range transport model the Phase 1 recommendation

also specifies how this model, in conjunction with existing

regulatory models, should be used to provide those analyses

necessary for Class I PSD permitting.

By restricting the models considered for Phase 1 to "offthe-

shelf" techniques, the IWAQM recognizes certain limitations

in the suggested techniques. These include a lack of

consideration of the effects of terrain on the long range

transport and dispersion, an underestimation of the conversion

of SO to SO when polluted air interacts with clouds, and an 2 4

=

overestimation of particulate nitrate when a limited number of

sources are considered. Furthermore, the estimations of the

impacts of sources on regional visibility are simple and do not

account for all of the processes important to regional

visibility. Nonetheless, the IWAQM considers the techniques,

suggested herein, to be a significant improvement to those

previously used, in that previous techniques ignored many of

the processes important to the assessment of air quality

impacts in Class I areas. Under some circumstances, the

concentrations of sulfates in the atmosphere may be

underestimated, and hence the impacts on regional visibility,

due to the inability of the model to treat in-cloud processes.

The IWAQM, including the representatives of the land management

agencies, recognize these limitations and consider the

suggested techniques to be technically superior to simply

assuming that there are no impacts on regional visibility. As

the IWAQM work continues, these limitations will be addressed,

to the extent possible.

 

 

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