Ambient Air Quality Standards (NAAQS), and Air Quality Related

Values (AQRVs). AQRV impacts include such effects as

visibility degradation and acidic deposition. The recommended

modeling technique is suitable for conducting single source

impact analyses, as well as, cumulative impact analyses. The

results from this technique will frequently need to be combined

with the results from techniques used to estimate

concentrations from sources closer than 50 kilometers to a

receptor area.

It is important to note that by restricting the models

considered for Phase 1 to "off-the-shelf" techniques, the IWAQM

recognizes certain limitations in the suggested techniques.

These include limits in considering 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 2 4


clouds, and an overestimation of particulate nitrate when a

limited number of sources is 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 over 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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