Development of a comprehensive photochemical air pollution modeling system for the San Francisco Bay Area

The San Francisco Bay Area has historically exceeded U.S. federal and California state standards for 1-hour ozone. The Bay Area Air Quality Management District (BAAQMD) is responsible for monitoring ambient air quality, and for developing and enforcing local emission control plans. Over the years, the BAAQMD has submitted several implementation plans that have been effective in reducing ambient ozone levels within its jurisdiction; the U.S. EPA has recently determined that the Bay Area is now attaining the federal 1-hour ozone standard. However, this standard has been effectively replaced by a new and more stringent 8-hour federal ozone standard, and based upon air quality measurements in 2001-2003, the Bay Area has been re-designated as a “marginal” non-attainment area. Given the complexities surrounding the formation and fate of ozone, the development of control strategies to mitigate precursor emissions is always a technically challenging endeavor. Nonattainment areas must undertake complex photochemical computer modeling to understand the idiosyncrasies of their area's ozone problem, as well as to develop and evaluate ozone response to the various control scenarios under consideration.

ENVIRON International Corporation (ENVIRON) is assisting the BAAQMD to build a technically rigorous photochemical modeling and analysis system that will support on-going planning for the 8-hour ozone standard within the Bay Area. The core of this system comprises the Comprehensive Air quality Model with extensions (CAMx), a publicly available state-of-the-science photochemical grid model that contains all of the technical features needed to simulate ozone, while providing some superior apportionment and process analysis capabilities over other models of its kind. CAMx also provides capabilities to support analysis and management of particulate matter (PM), toxics, and mercury. The modeling system is supported by an emissions model (EMS-95), and two different prognostic meteorological models (MM5 and RAMS). Based upon extensive review of several past ozone episodes, and the criteria for data availability, the team selected two historical exceedance episodes for the initial modeling analyses: July 31, 2000; and July 11-12, 1999.

Furthermore, the BAAQMD, the California Air Resources Board (CARB) and the San Joaquin Valley and Sacramento Air Districts downwind of the Bay Area have continued interest in analyzing the role of regional transport of ozone and precursors as part of the Central California Ozone Study (CCOS). Integral contributions have been made to this study by all of these entities; given the plethora of CAMx modeling conducted by each of these groups under CCOS, the ENVIRON/ BAAQMD effort has attempted to bring together the best information and modeling approaches possible. As a result, the research, modeling, testing, and evaluation conducted under this effort was a rather complex and highly interactive endeavor.