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strategies, models are essential for examining of the complex interactions among emissions, meteorology, and atmospheric chemistry.

A practical model consists of four structural levels:

• The conceptual formulation; that is, a set of assumptions and approximations that reduce the actual physical problem to an idealized one, which, within the limits of present understanding, retains the most important features of the actual problem.

• The basic mathematical relations and auxiliary conditions that describe the idealized physical system.

• The computational schemes (numerical procedures) used to solve the basic equations.

• The computer program or code that actually performs the calculations.

The term ''model'' has been used to apply collectively or separately to all four levels. Models of a particular process, or a group of interacting processes, are called component models or modules. The basis for air-quality models is the atmospheric diffusion equation, which expresses the conservation of mass of each pollutant in a turbulent fluid in which chemical reactions occur (Seinfeld, 1986).

For at least a decade, the Environmental Protection Agency (EPA) has offered guidelines on the selection of air-quality modeling techniques for use in State Implementation Plan (SIP) revisions, new source reviews, and studies -aimed at the prevention of significant deterioration of air quality. EPA guidelines (EPA, 1986b) identify two kinds of photochemical model: The urban airshed model (UAM) is the recommended model for modeling ozone over urban areas and EKMA (empirical kinetic modeling approach) is identified as an acceptable approach.

As noted in Chapter 3, the 1990 Clean Air Act Amendments specify that three-dimensional, or grid-based, air-quality models, such as UAM, be used in SIPs for ozone nonattainment areas designated as extreme, severe, serious, or multistate moderate (EPA, 1991b). Grid-based models use a fixed Cartesian reference system within which to describe atmospheric dynamics (Seinfeld, 1988). The region to be modeled is bounded on the bottom by the ground, on the top by the inversion base or some other height that characterizes the maximum extent of vertical mixing, and on the sides by east-west and north-south boundaries, unless the coordinates are rotated. This space is then subdivided into a three-dimensional array of grid cells. The horizontal dimensions of each cell are usually a few kilometers for urban applications. Some older grid-based models assumed only a single, well-mixed vertical cell extending from the ground to the inversion base; current models subdivide the re-



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