USES AND LIMITATIONS OF MODELING IN ANALYZING AND DESIGNING O3 STRATEGIES

A sufficient historical record in grid-based O3 modeling now exists to permit assessment of its role and limitations in policy analysis. In this presentation, several topics are examined: past experience in mod el evaluation and use, the present status of modeling, the value of mo deling in policy analysis, and key issues and future needs. First, the role of grid-based O3 modeling in policy analysis was assessed throug h a case study of nearly 10 years of model applications in the South C oast (Los Angeles) Air Basin. Changes in quality of performance and in degree of acceptance of grid-based models (in policy analysis) with t ime were analyzed and compared. Degree of acceptance appears to depend on a variety of factors, including level of understanding and familia rity, perception of need, and relative degree of acceptability, as com pared with other available models. Improvements in quality of performa nce with time were limited (and, in any event, knowledge of such chang es were not available), and thus this factor seems to have had little or no role in influencing model acceptance. Second, the current state of modeling, in terms of both science and art, is appraised, consideri ng the state of knowledge, availability of data bases, adequacy of per formance evaluation procedures, and quality of predictive performance. Significant deficiencies have existed in knowledge and treatment of k ey governing processes in models. Until recently, data bases suitable for use in evaluating model performance were unavailable. Quality of p erformance has not been sufficiently good to confer confidence in mode ls' use in a regulatory environment. Evaluative testing did not provid e sufficiently for 'stressing' models. For many, but not all of these issues, attention is now being, or soon will be, given to alleviating concerns. Third, the role and value of modeling in policy analysis is discussed. Attention is given to the role of the modeling expert in th e policy forum, as compared with the model itself, and with the consid eration of modeling as a longer term process rather than as a direct o r short term means of generating 'answers'. Finally, key concerns and future needs are delineated, and some unsolved problems are discussed.