STABLE CARBON-ISOTOPE MEASUREMENTS OF ATMOSPHERIC ORGANIC-ACIDS IN LOS-ANGELES, CALIFORNIA

Atmospheric organic acids are ubiquitous constituents of urban smog an d haze and are also present in the atmospheres of rural and largely un inhabited areas (e.g., the Amazon Rain Forest Basin). We report here s ource characterization of atmospheric organic acids in Los Angeles by measurements of their stable carbon isotopic ratios, e.g., C-13/C-12(d elta(13)C). The study was performed by separating formic and acetic ac ids using ion chromatography exclusion (ICE) and isolating milligram q uantities of individual organic acids from atmospheric rain samples. A uthentic reference samples of formic and acetic acids were used to det ermine that only a negligible isotope fractionation of delta(13)C valu e occurs after the ICE separation. During 1991-1992, rainwaters were c ollected in Los Angeles to isolate formic and acetic acids for the iso tope measurements. Results presented in this paper demonstrate a signi ficant isotopic difference between the mean delta(13)C value for aceti c acid (-20.5 parts per thousand) and formic acid (-30.1 parts per tho usand). We conclude from these results that the formic acid is derived from mixing of formic acid from direct emissions with the acid formed by secondary oxidation processes in the atmosphere, most probably fro m aldehydes, whereas acetic acid originates only from direct source em issions.