Impact of California's Air Pollution Laws on Black Carbon and their Implications for Direct Radiative Forcing

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 μgm-3 in 2008 compared to a 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 deployment of diesel particulate filters. Our conclusion that the reduction in diesel emissions is the 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 Wm-2 (±60%) over California. Figure 1 (a) Annual means of measured Black Carbon (left axis) and BC fossil fuel emissions (right axis) in California from 1985 to 2008. Error bars correspond to standard deviation between measurements at each station. Dashed lines indicate a linear fit. Aerosol measurements from the IMPROVE network, emission inventories from (1) CARB, (2) [Ito and Penner, 2005] (b) Annual means of BC measured in Southern (South of 35 N), Northern (North of 38 N), and Central California (c) Annual means of measured Sulfate, Nitrate, and OC from IMPROVE network.