| Article ID | Journal | Published Year | Pages | File Type |
|---|---|---|---|---|
| 4440101 | Atmospheric Environment | 2011 | 6 Pages |
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.
Research highlights► Annual average BC concentrations decreased by about 50% from 0.46 μg m−3 in 1989 to 0.24 μg m−3 in 2008 in California. ► A reduction in diesel BC emissions from a peak of 0.013 Tg Yr−1 in 1990, to 0.006 Tg Yr−1 by 2008 is also observed. ► No long-term trend in the concentrations of scattering aerosols (nitrate, sulfate, and organic carbon) in this period. ► The negative trend in BC is attributed to statewide regulations on vehicular and non-vehicular diesel exhaust. ► That the reduction in absorption efficiency of aerosols at visible wavelengths has led a cooling effect of −1.4 W m−2 over California.
