Impact of California’s air pollution laws on black carbon and their implications for direct radiative forcing

Abstract: We examine the temporal and the spatial trends in the concentrations of black carbon (BC) – recorded by the IMPROVE monitoring network for the past 20 years – in California. Annual average BC concentrations in California have decreased by about 50% from 0.46 μg m−3 in 1989 to 0.24 μ gm−3 in 2008 compared to the corresponding reductions in diesel BC emissions (also about 50%) from a peak of 0.013 Tg Yr−1 in 1990 to 0.006 Tg Yr−1 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 – determined from the observed scattering coefficient and the observed BC – also decreased by about 50% leading to a model-inferred negative direct radiative forcing (a cooling effect) of −1.4 W m−2 (±60%) over California. [Copyright &y& Elsevier]