Impact of California’s air pollution laws on black carbon and their implications for direct radiative forcing
Authors
Bahadur, Ranjit; Feng, Yan; Russell, Lynn; Ramanathan, V
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 11g m-3 in 1989 to 0.24 11 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 Tr -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.