Modern photochemical modeling applications typically are designed to account for long-range transport of ozone and its precursor pollutants. Because these airborne chemicals can be carried by the wind over distances of several hundred kilometers, accurate simulation of transport requires modeling domains covering very large multi-state regions. To reduce the computational requirements associated with such large-scale applications, regional air quality models typically use relatively large grid cells covering perhaps 1,000 km~2 each. However, modeling to develop detailed control strategies for urban ozone nonattainment areas requires much higher resolution, typically utilizing grid cells covering approximately 20 km~2 each. The integration of regional and urban-scale modeling is most easily accomplished through application of a nested grid model, such as the variable-grid version of the Urban Airshed Model (UAM-V) or the Comprehensive Air Quality Model with Extensions (CAMx).
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