Recognizing the immediate need within the permitting community for evaluating the
impacts of sources of air pollution located more than 50 kilometers from Class I
wilderness areas and national parks, the Interagency Workgroup on Air Quality Modeling
(IWAQM) drafted an interim Phase 1 recommendation from existing "off-the-shelftechniques"
(EPA, 1993). The interim approach recommended the use of the Lagrangian
puff model, MESOPUFF-II (Scire et al., 1984), to evaluate the impacts of pollutants from
sources located more than 50 kilometers from Class I areas, and up to several hundred
kilometers from Class I areas. The impacts of concern are the allowable Prevention of
Significant Deterioration (PSD) Class I increases in pollutants (increments), the National
Ambient Air Quality Standards (NAAQS), and the Air Quality Related Values (AQRVs).
AQRV impacts include such effects as visibility degradation and acidic deposition. The
interim recommendation was envisioned as suitable for single source impact analyses, as
well as for multiple source (cumulative) impact analyses.
It is important to note that by restricting the modeling techniques to "off-the-shelf,"
certain limitations were incurred. These include limits in considering the effects of
terrain on long range transport and dispersion, an underestimation of the conversion of
sulfur dioxide to sulfate when polluted air interacts with clouds, and an overestimation of
particulate nitrate when a limited number of sources are considered. Furthermore, the
estimations of the impacts of sources on regional visibility are simplistic and do not
account for all of the processes important to regional visibility. Nonetheless, the
IWAQM considers the techniques to provide a useful assessment of air quality impacts in
Class I areas.
In the course of developing the Phase 1 interim recommendations, it was recognized that
even if the modeling techniques could be agreed upon between the various federal
agencies, there would still remain both technical and policy issues in implementing an
assessment of pollutant impacts on a Class I area. In general, the fact that Class I area
analyses focus on a fixed piece of real estate sets them apart from Class II analyses. In
Class II analyses, the area of concern is a circular area (typically of radius 50 kilometers
or less) centered on the source in question; whereas in Class I analyses the source and the
Class I area are usually separated by a significant distance. This occasions a unique set of
issues which need consideration from and cooperation among a variety of organizations,
given the need to assess the incremental increase in concentration values, the need to
evaluate AQRVs, and the added role of the Federal Land Mangers (FLMs).
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