Biomass and Fossil Fuel Combustion Contributions to Elemental Carbon Across the San Francisco Bay Area

Ambient black carbon (BC) has detrimental effects on both human health and near-term global warming. To mitigate these negative effects, there have been significant efforts to reduce emissions of BC from anthropogenic and biomass burning sources in California's Bay Area since the 1960s. Recent reductions in BC have mainly been from fossil fuel combustion sources such as diesel but additional reductions may be needed for contemporary carbon sources like biomass burning and meat cooking. In this study, PM10 (particulate matter with aerodynamic diameter of less than or equal to 10 mu m) samples were collected at seven sites across the San Francisco Bay Area from November 2011 to October 2012 to create winter and non-winter composites for each site. Radiocarbon (C-14) abundance and chemical mass balance (CMB) modeling were used for source apportionment of ambient elemental carbon (EC, a proxy for BC). The C-14 abundance in the EC fraction was used to quantify the relative contributions of fossil carbon and contemporary carbon sources. The average biomass burning contributions are 48 +/- 8% and 41 +/- 5% for winter and non-winter seasons, respectively, across the Bay Area. Ambient concentrations of EC are approximately two to three times higher during the winter compared to the non-winter season, except for Cupertino. A CMB model, using bulk aerosol composition and select inorganic compounds, was used to understand the contributions of natural gas combustion, gasoline exhaust, and diesel exhaust to fossil carbon as well as the contribution of biomass burning and meat cooking to contemporary carbon. The different apportionment methods for EC (C-14 and CMB analysis) agree within 16 +/- 12% for fraction fossil and biomass burning for both winter and non-winter seasons. The contemporary contribution to EC is much higher than estimated by current emission inventories, suggesting that the inventories may overestimate the contribution from fossil fuels, particularly diesel exhaust. The results from this study are compared to literature values from other C-14-EC or BC studies from across the world.