solutions for some particular set of reasons: it is either the least expensive solution or the least burdensome solution from an administrative point of view. If the objective is to choose an economically attractive solution rather than an administratively convenient one, then the control-strategy design team has certain tools available to it that can be used to help identify cost-efficient control schemes. The tools are discussed below.

The search for feasible solutions to a regional visibility control problem begins by constructing a mathematical model for the regional visibility problem. This model is on a larger conceptual scale than has been discussed earlier. At the core of the regional model is one of the emissions-to-air-quality models described in Chapter 5 for computing source contributions to ambient pollutant levels. Adjoined to this emissions-to-air-quality model is a model for translating air pollutant levels into effects on visibility. Then these two models are subjected to model verification tests over a base-case historical period when meteorological conditions, emissions, ambient pollutant concentrations, and atmospheric optical properties are known. The purpose is to demonstrate that the chosen models can produce accurate results in the presence of well-defined inputs and to demonstrate that cause and effect relationships in that particular airshed are understood. Following confirmation of the models' technical performance, the available emissions controls that could be applied to the problem are used to compute the emission rates from the sources that would prevail in the presence of each of the controls. Then, the effect of those controls on air quality is tested by application through the completed air quality and visibility models.

Because the number of controls that must be tested for their effect on air quality and visibility is potentially quite large, care must be taken to structure an efficient search for feasible solutions. It usually is not practical to re-run an elaborate environmental model hundreds of times to learn about the properties of each control technique separately. Instead, for those models that are linear in emissions (e.g., most of the models with simplified chemistry), it is possible to perform the control evaluation without rerunning the models in their entirety. Transfer coefficients can be calculated for the linear models that state the pollutant concentration or light extinction increment at each receptor site per ton of emissions per day from each source. The transfer coefficients depend on meteorological conditions, the spatial location of the sources

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