Clearing theAir in the San Joaquin Valley

The San Joaquin Valley shares the distinction of having the worst air quality in the United States, driven by a combination of geographic trapping, rapid population growth, and a history of regulatory neglect. The region struggles with chronic ozone and particulate matter (PM) pollution, which disproportionately affects vulnerable populations and damages the agricultural economy. While recent legislation has begun to regulate previously exempt agricultural sources and diesel emissions, the report argues that coordinated action across jurisdictional levels is required to address emissions from sources outside local control, such as heavy-duty trucks and locomotives.

• The San Joaquin Valley is currently tied with the Los Angeles region for the worst air quality in the United States, with the valley exceeding the 8-hour national ozone standard on more days than the South Coast air basin between 1999 and 2003.
• The region's geography creates a 'bowl' effect that traps pollution, specifically through a counterclockwise wind pattern known as the 'Fresno eddy' that concentrates pollutants over the Fresno-Clovis metropolitan area.
• Rapid population growth and increased vehicle miles traveled (VMT) are offsetting emission reductions; from 1990 to 2000, the population grew by nearly 20% and daily VMT increased by more than 25%.
• Air pollution in the valley has significant public health impacts, particularly for children; Fresno County has the highest rate of childhood asthma in California at 16.4%.
• Poor air quality negatively impacts the regional economy by reducing crop yields for grapes, cotton, tomatoes, oranges, and alfalfa by an estimated 10% to 20%.
• The primary precursors to ozone in the valley are reactive organic gases (ROG) and oxides of nitrogen (NOx), with area sources (including farming and consumer products) being the largest ROG contributors and heavy-duty diesel trucks and farm equipment being the primary NOx sources.
• Particulate matter (PM) pollution is dominated by area sources such as road dust and agricultural burning, though diesel engines contribute approximately 10% of PM-2.5 emissions and are noted for their high toxicity.
• There is a jurisdictional gap in air quality management; as local stationary sources have been controlled, a larger share of the emission inventory now comes from sources under state and federal jurisdiction, such as passenger and heavy-duty vehicles.
• Recent legislative efforts in the 2003–2004 session have targeted previously unregulated sources, including SB 700, which repealed the agricultural exemption from air quality regulations, and SB 705, which phases out agricultural burning.
• The California Air Resources Board's Diesel Risk Reduction Plan aims to reduce diesel PM emissions and cancer risk by 75% by 2010 and 85% by 2020, though implementation delays mean only a 30% reduction is likely by 2010.

APA

Bedsworth, L. W. (2004). Clearing theAir in the San Joaquin Valley. Union of Concerned Scientists. https://www.ucs.org/sites/default/files/2019-09/central_valley_final-new.pdf

Chicago

Bedsworth, Louise Wells. Clearing theAir in the San Joaquin Valley. Union of Concerned Scientists, 2004. https://www.ucs.org/sites/default/files/2019-09/central_valley_final-new.pdf.

Wikipedia

{{cite report |last1=Bedsworth |first1=Louise Wells |title=Clearing theAir in the San Joaquin Valley |publisher=Union of Concerned Scientists |date=October 2004 |url=https://www.ucs.org/sites/default/files/2019-09/central_valley_final-new.pdf |access-date=23 October 2026 |via=Climate Insights Directory}}

BibTeX

@techreport{bedsworth2004clearing, author = {Bedsworth, Louise Wells}, title = {{Clearing theAir in the San Joaquin Valley}}, institution = {Union of Concerned Scientists}, year = {2004}, month = oct, url = {https://www.ucs.org/sites/default/files/2019-09/central_valley_final-new.pdf}, urldate = {2026-10-23}, note = {Indexed by Climate Insights Directory} }