Emissions Modeling in Action: Estimating Emissions for Use in the San Francisco Bay Area Air Quality Modeling Study

It is well understood that emissions estimates are a critical component to the air quality modeling process. However, even after nearly four decades, developing adequate emissions estimates for use in air quality modeling studies continues to be a difficult task.

Many of the shortcomings that exist in the efforts to develop emissions estimates were highlighted in an ongoing study of regional air quality in central California for ozone episodes occurring in July 1999 and July-August 2000. Though these problem areas are not necessarily new or unique to the California modeling study, they continue to be a burden on efforts to efficiently perform the activities necessary to complete such a study on tight regulatory schedules. For example, estimating on-road mobile source emissions proved to difficult given that the process appeared to be driven as much by policy factors (e.g., conformity budgets) as it was driven by scientific and technical factors. Further, as emissions estimates were compared across episodes, unexplained discrepancies appeared. In addition, difficulties were encountered in simply knowing the specific version of the emissions data or estimates that were used.

A brief overview of the emissions modeling process is given for each of the major emissions source categories. Problems that occurred in this effort are discussed, and specific recommendations to mitigate the issues are described.