Sources of Urban Contemporary Carbon Aerosol Lynn M. Hildemann' Civil Engineering Department, Stanford University, Stanford, California 94305-4020
Donna B. Klinedinst, George A. Klouda, and Lloyd A. Currle
Chemical Science and Technology Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 Glen R. Cass
Environmental Engineering and Science, California Institute of Technology, Pasadena, California 9 1125 Emissions from the major sources of fine carbonaceous aerosol in the Los Angeles basin atmosphere have been analyzed to determine the amounts of the 12C and 14C isotopes present. From these measurements, an inventory of the fossil carbon and contemporary carbon particle emissions to the Los Angeles atmosphere has been created. In the winter, more than half of the fine primary carbonaceousaerosol emissionsare from sources containing contemporary carbon, including fireplaces, charbroilers, paved road dust, cigarette smoke, and brake lining dust, while in the summer at least one-third of the carbonaceous particle emissions are contemporary. Using a mathematical model for atmospheric transport, predictions are made of the atmospheric fine particulate fossil carbon and contemporary carbon concentrations expected due to primary source emissions. Model predictions are in reasonable agreement with the measured radiocarbon content of the fine ambient aerosol samples. It is concluded that the high fraction of contemporary carbon measured historically in Los Angeles is not due to local emission sources of biogenic material; rather, it is due to a combination of local anthropogenic pollution sources and background marine aerosol advected into the city.
Introduction A significant fraction (typically 30-50 % by mass) of the fine particulate matter in urban atmospheres consists of carbonaceous material (1). To verify the accuracy of models used to establish urban area source-receptor relationships for fine carbonaceous aerosol, stable tracers for the carbon-containing emissions are needed that distinguish one source from another in ambient samples. Carbon isotope analysis can help to provide such source separation, because 12C and 14C concentrations in fine aerosol source emissions act as conserved tracers over the time scale for transport through an air basin, and the 14C content of emissions depends on the age of the source
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1994
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material. Emissions from fossil fuel combustion contain essentially no I4C because the geologic age of the fuel is much greater than the half-life (5730 yr) of that carbon isotope. In contrast, contemporary sources of carbon contain a proportion of 14C:12Ccomparable to current atmospheric COz radiocarbon ratios. Hence, measurement of ambient aerosol 12Cand 14Cconcentrations allows the contributions of fossil sources of particulate carbon to be distinguished from nonfossil sources (2). Past studies of the fine particulate matter present in urban atmospheres that have used carbon isotope dating to distinguish between material emitted from fossil fuel sourcesand that originating from contemporary (nonfossil) sources have produced what seemed to be surprising findings. Even in a heavily urbanized area like Los Angeles, where enormous amounts of fossil fuel are burned, substantial amounts of fine contemporary carbon aerosol have been observed. Two single-sample measurements of airborne particles in sizes
