Alternative base cases in photochemical modeling: their construction, role, and value

This study assesses the potential influence compensating errors in photochemical model inputs may have on estimates of the effects of emission control scenarios. Motivation stems from the apparent ability to achieve satisfactory model performance despite evidence suggesting the existence of significant biases in emissions estimates. Urban Airshed Model (UAM) sensitivity studies were carried out using simulations of two summer 1987 O3 episodes in the South Coast Air Basin of California. Since existing simulations exhibited inadequate performance, efforts were made to rectify these problems. Plausible conditions that might define acceptable base cases were established, and model runs were made to determine which alternative base cases provided a level of UAM performance comparable to the best achieved for the two summer episodes. The alternative base cases produced different estimates of the air quality benefits associated with hypothetical emission control scenarios. For example, one set of base cases indicated that NOx, controls would be counterproductive in helping to reduce the estimated peak O3 concentration in the eastern portion of the modeling domain. Another base case suggested that such controls would yield almost no change in the peak value. The results from alternative base case simulations provide a lower bound estimate of the uncertainty that attends future year modeling results. Such analyses should be incorporated into current photochemical modeling practice.