Evaluating Uncertainties in Regional Photochemical Air Quality Modeling*

This review evaluates analyses that are or may be performed to estimate uncertainties associated with air quality modeling used in regulatory planning to meet National Ambient Air Quality Standards for ozone. The sources of uncertainties in photochemical air quality simulation models (PAQSMs) are described. Regulatory requirements for evaluating PAQSM performance and uncertainty concerns not addressed through standard performance evaluations are discussed. Available techniques for evaluating uncertainties are presented. Experiences with analyses conducted most commonly are reviewed, as are those that might be used in a cohesive model uncertainty evaluation. The review concludes with a call for renewed emphasis on applying current techniques complemented by heretofore sparsely used diagnostic, corroborative, and alternative approaches and enhanced observational databases.