Carbon monoxide (CO) levels have been decreasing across California due to the effectiveness of state and local control programs. In 1991, 11 areas were designated as nonattainment for the federal 8-hour CO standard of 9 parts per million (ppm). All the areas designated as nonattainment, including the Los Angeles urbanized area, now attain the federal 8-hour CO standard.
In April 1996, the California Air Resources Board (CARB) adopted California’s CO Maintenance Plan, and the U.S. Environmental Protection Agency (U.S. EPA) approved it in 1998. This 1996 CO Maintenance plan showed that California’s CO maintenance areas would continue to attain the CO standard through 2010. In July 2004, CARB adopted the 2004 CO Maintenance Plan, and the U.S. EPA approved it in November 2005. The 2004 CO State Implementation Plan (SIP) Update showed how 10 of the 11 California CO maintenance areas would continue to maintain the CO standard through 2018. The 10 original CO maintenance areas are: Bakersfield Metropolitan Area, Chico Urbanized Area, Fresno Urbanized Area, Lake Tahoe North Shore Area, Lake Tahoe South Shore Area, Modesto Urbanized Area, Sacramento Urbanized Area, San Diego Area, San Francisco-Oakland-San Jose Area, and Stockton Urbanized Area. To date, California has completed the 20-year maintenance requirement for the CO standard for the 10 areas.
The Proposed 2023 Revision to the California State Implementation Plan for Carbon Monoxide (2023 CO SIP revision) is updating the maintenance plan to remove the contingency measures and monitoring requirement for 3 of the 10 maintenance areas included in the 2004 CO Maintenance Plan:
Chico Urbanized Area
Modesto Urbanized Area
Stockton Urbanized Area
By 2022, these three maintenance areas were monitoring CO levels 81% to 86% below the federal 8-hour CO standard of 9 ppm. These levels, together with declining emissions due to an ever-cleaner vehicle fleet and local air district rules for stationary sources, provide assurance that the three maintenance areas will continue to attain the federal 8-hour CO standard.