In an attempt to address some of the concerns unique to Class I area analyses, it was decided

that a case study would be conducted to apply the MESOPUFF II air quality modeling

system following the IWAQM interim recommendations (EPA, 1993). This study would

identify and summarize the decisions made, would record and summarize the resolution

process for these decisions, and would provide a written record of the resources used to

complete the effort. The objective was to learn by experience where the difficulties are in

the process of conducting such an analysis, and when possible, to provide a means for

resolving these difficulties. It was not an objective to provide a meaningful assessment of

PSD, NAAQS or AQRV impacts for the Class I areas considered in the study. As will be

seen in the following discussion, on several occasions significant departures were made in

conducting this study from that which would be expected if a realistic assessment were to be

developed. For instance, the source inventory considered only some of the states

surrounding the Shenandoah National Park and the James River Face Wilderness, and thus is

incomplete. And in order to conserve resources, the sources were consolidated into ten

surrogate sources for the purposes of this study. These departures allowed the emphasis of

the project to be focused on a critique of the process and resource needs, which were the

primary study objectives.

A realistic assessment, following the interim IWAQM recommendations, would require that

all important sources be modeled (without consolidation). If the modeling objective is to

determine PSD impacts, then all relevant sources that consume PSD increment must be

considered. If one desires to determine the impact of a single new (or modified) source, then

the PSD increment from the new source must be added to all pre-existing PSD sources. It

would be possible to model the impacts from a single source and then add those impacts to

prior MESOPUFF II results, assuming the prior results were available. If not, it would be

necessary to model all relevant PSD sources to assess the total PSD increment consumed.

The demonstration modeling assessment described in this report did not completely follow

IWAQM's interim recommendations. One of the goals of the study was to perform the

demonstration assessment using the MESOPUFF II modeling system as developed for a

personal computer (PC) system. This presented some significant limitations regarding the

ability to follow the interim IWAQM recommendations. As an example, for a realistic

assessment of multiple sources, the interim IWAQM recommendations require that two fiveyear

MESOPUFF II modeling exercises be conducted; one run using all relevant sources to

determine impacts to secondary NAAQS pollutants (secondary particulate matter) and

AQRVs (visibility and deposition), and a second model run using only sources beyond 50

km from a receptor for SO and NO (and primary PM ). The MESOPUFF II results for 2 x 10

SO and NO are then to be added to results from a Gaussian model (such as ISCST2) for 2 x

sources within 50 km of the receptor. In the MESOPUFF II modeling demonstration

described in this report, only one run was performed (for three years) using sources beyond

50 km of Shenandoah National Park. The MESOPUFF II results (for one month) were

added to ISCST2 results to demonstrate the integration process.



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