atmospheric transport, dispersion, chemical conversion, and deposition. Mechanistic models are source oriented; they take emissions as given and ambient concentrations as quantities to be estimated. Because these models require pollutant concentrations only as initial and boundary conditions for a simulation they can therefore be used to predict the effects of sources before they are built.

The members of the committee do not aim to give advice on how to choose a single best source apportionment technique for analyzing a given visibility problem. Instead, the committee offers guidance on how to view the air quality modeling process. The way to view air quality models is that they provide a framework within which information about the basics of the problem can be effectively organized. This basic information includes data on the air pollutant emission sources, observations on meteorological conditions, data on the ambient air pollutants that govern visibility, and information on emission control possibilities. The quality of the outcome of the modeling process usually depends as much or more on the quality of the data used as inputs to the model than it does on the modeling method chosen, thus placing a premium on the accuracy with which the basic facts of the problem are known. The objective of the analyst is to capture the scientific relationships between emissions and air quality such that important decisions about the effect of emission controls or about the siting of new sources can be answered. Depending on the decisions to be made, there may be either a strict or a more relaxed requirement for technical accuracy or detail. Federal regulatory programs are permitted to make regulatory decisions in the face of continuing scientific uncertainty. Within many likely regulatory structures, attribution of contributions due to individual sources may be unnecessary—attribution to classes of like sources or upwind geographic regions would suffice. In those cases where approximate answers are satisfactory, there are many possible ways to approach answering questions about the relationship between emissions, air quality, and visibility.

When approaching the analysis of the causes of a particular visibility problem, the best strategy is generally to use a nested progression of techniques from simple screening through more complex methods. Simple methods can be used to screen the available data and find an approximate solution. Next, more complex methods can be applied to determine source contributions with greater resolution. Advanced methods are appropriate when a problem is scientifically complex or when



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