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Air Quality Standard (NAAQS). The critical question is "What is the effect of uncertainties in base-year inputs on projections of control levels for future years?" Once this question is answered, these uncertainties need to be incorporated in the State Implementation Plan. For example, it is important to know whether such uncertainties affect the choice of control strategies—for example, control of VOCs versus control of oxides of nitrogen (NOx) versus both. Model performance evaluation procedures must be designed to reveal flaws in a base-case simulation to ensure that a model gives the right answer for the right reason.

Computational constraints historically have limited the use of advanced three-dimensional, photochemical air-quality models. Instead, less-comprehensive, less computationally intensive, and more limited models, such as EKMA have been used. Such models are not capable of fully characterizing ozone dynamics in urban and regional areas over multiple days, nor the response of ozone to emission changes. Rapid increases in computational power and algorithmic efficiencies now allow for more widespread use of advanced models, and are playing a significant role in the ability to understand atmospheric pollutant dynamics. The continued evolution of computational capability will allow for in-depth studies using more chemically and physically comprehensive models.