California Institute for Energy and Environment (CIEE)

Soot produced from the combustion of fossil fuel is a public health concern. The strongly light-absorbing black carbon (BC) core of soot contributes to global warming and regional climate change. This research evaluates the accuracy of BC measurements and estimates trends in ambient BC concentrations and BC emission factors from diesel vehicles in California. The aethalometer is a widely used instrument for measuring BC concentrations. Laboratory and field measurements show that temporally resolved BC measurements are subject to error because the response of the aethalometer diminishes as its sampling filter becomes increasingly darkened with black particles. The magnitude of the error depends on the relative amounts of light-absorbing BC and light-scattering particles sampled. A retrospective analysis of ambient BC concentrations is performed, based on archived measurements of coefficient of haze (COH). It is shown that COH levels can be converted to BC concentrations. Estimated annual BC concentrations for the San Francisco Bay Area, where diesel vehicle are the dominant BC source, decreased by a factor of ~3 from the late 1960s to the early 2000s despite a factor of ~6 increase in diesel fuel consumption. Derived diesel BC emission factors (expressed as mass of BC emitted per mass of diesel fuel consumed) decreased from ~13 to <1 g kg-1 over this period. The decreases in ambient concentrations and emission factors are attributed to pollutant abatement policies in California. Further evaluation of BC measurement methods and analysis of BC trends at other locations in California is recommended.