This paper examines whether the 1990 Clean Air Act Amendments caused the decline in ambient concentrations of particulate matter (PM10) in California over the 1990–2000 period. Consistent with previous studies, we find that PM10 nonattainment status at the county level is not a significant factor in explaining the reductions in PM10. However, when we allow for spatially heterogeneous treatment effects within nonattainment counties, and incorporate measures of community characteristics, climate, geography, transboundary pollution, and industrial composition and scale, we find that nonattainment designations at the city level account for a 6% reduction in ambient PM10 concentrations.