BOX 5-1

Example of a Generic Model for Application to Specific Settings

A description of one example of model application information is found at EPA’s Support Center for Regulatory Atmospheric Modeling web site (EPA 2006l). One modeling system described on the site, AERMOD, was developed by the American Meteorological Society and the EPA Regulatory Model Improvement Committee. The AERMOD system is a steady-state plume model that simulates dispersion of air pollutants from point or area sources. It is a good example of an extensively documented model targeted at a broad range of users for regulatory purposes. The AERMOD modeling system includes extensive documentation, including model code, a user’s guide, supporting documents, and evaluating databases, all of which are available on the web site of the EPA Support Center for Regulatory Atmospheric Modeling. The supporting documents include details of the model formulation and evaluation, comparison of regulatory design concentrations, an implementation guide (information on recommended use of the model for particular applications), evaluation of model bias, sensitivity analysis, a parameterizations document and peer review document. The evaluation databases include input and output data for model evaluation. User’s manuals include instructions for novice and experienced users, decision makers, and programmers. The model code and supporting documents are not static but evolve to accommodate the best available science.

ISSUES IN MODEL SELECTION AND APPLICATION

Model developers and regulators must evaluate how appropriate an existing model is for a specific setting and whether the assumptions and input data are relevant under the conditions of the application. Optimally, a model is applied to a problem within the model-specific application domain near the time of model development. However, frequently, this is not feasible. Thus, models need to be evaluated in context with each application, the degree of evaluation being commensurate with the case. A number of issues arise when selecting and applying a model or a set of models for environmental regulatory activities. These issues are discussed below and include the following: the selection of a model from multiple possibilities, the level of expertise, the assumptions and range of applicability, the cost and availability, the adaptability of the model; and the data availability.

Model Selection

The committee recognizes the wide variability in the availability of alternative modeling approaches for specific regulatory applications.



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