a b s t r a c t We examine the temporal and the spatial trends in the concentrations of black carbon (BC) e recorded by the IMPROVE monitoring network for the past 20 years e in California. Annual average BC concen-trations in California have decreased by about 50% from 0.46 mg m À3 in 1989 to 0.24 m gm À3 in 2008 compared to the corresponding reductions in diesel BC emissions (also about 50%) from a peak of 0.013 Tg Yr À1 in 1990 to 0.006 Tg Yr À1 by 2008. We attribute the observed negative trends to the reduction in vehicular emissions due to stringent statewide regulations. Our conclusion that the reduction in diesel emissions is a primary cause of the observed BC reduction is also substantiated by a significant decrease in the ratio of BC to non-BC aerosols. The absorption efficiency of aerosols at visible wavelengths e determined from the observed scattering coefficient and the observed BC e also decreased by about 50% leading to a model-inferred negative direct radiative forcing (a cooling effect) of À1.4 W m À2 (AE60%) over California.