California Air Resources Board

The California Clean Air Act of 1988 requires that the California Air Resources Board assess the relative contributions of upwind emissions to downwind State ozone standard exceedances and to update this assessment at least every three years. Also, the Act directs CARB to establish mitigation requirements for upwind districts that contribute to ozone exceedances in downwind districts. The initial identification of districts affected by transported air pollutants was approved by CARB in December 1989. This identification was followed by a report much broader in scope one year later. The report included an update and expansion of districts affected by transport, along with the addition of transport mitigation requirements. CARB approved this report in August 1990. The first triennial update to the 1990 ozone transport report as approved by CARB in August 1993: the second update was completed in November 1996; and the third update was completed in March 2001.

CARB staff presented the third triennial update and the summary report to the Board at its April 26, 2001, hearing. The Board adopted changes to the current list of identified transport couples.

Based on the transport assessments performed over the last decade, we now have a good understanding of the fundamental transport relationships between the various regions of California. These transport relationships and planning implications are presented in a summary report that was completed in 2001.

Beginning in the late 1990's with the Southern California Ozone Study (SCOS) and followed in the early 2000's by the Central California Ozone Study (CCOS) programs, the State was split into two modeling domains, one for Southern California and one for Northern and Central California, respectively. Transport relationships between air districts within these large domains are implicitly captured within photochemical models. This is the mechanism ARB uses in its oversight role to ensure transport impacts on downwind areas are addressed for purposes of both State and federal air quality standards. Modeling attainment demonstrations take into account the shared responsibility for reducing emissions in regions where air pollution transport can at times be significant.

For additional information, contact Jin Xu at (916) 327-1511.