CONCENTRATIONS OF MONOCARBOXYLIC AND DICARBOXYLIC-ACIDS AND ALDEHYDESIN SOUTHERN CALIFORNIA WET PRECIPITATIONS - COMPARISON OF URBAN AND NONURBAN SAMPLES AND COMPOSITIONAL CHANGES DURING SCAVENGING

Rain and snow samples collected at nine southern California sites and time series rain samples obtained during 13 rain events were studied f or their content of polar organic compounds, including C-1-C-9 monocar boxylic acids, C-2-C-10 alpha,omega-dicarboxylic acids, C-1-C-2 aldehy des and C-2-C-3 alpha-dicarbonyls. Formic (0.1-33 mu M), acetic (0.3-2 4 mu M), oxalic (0.2-28 mu M), succinic (0.03-7.3 mu M) and malonic (0 .01-5.5 mu M) acids, in addition to formaldehyde (0.3-37 mu M), are th e dominant species. The concentrations of the monocarboxylic and dicar boxylic acids in bulk rain were inversely proportional to the amount o f rainfall, however, those of aldehydes remained almost unchanged. The relative abundances of the major compounds are similar among the samp les and no significant trend was found to differentiate urban and nonu rban samples. However, some minor dissolved components showed differen t distributions; e.g. phthalic acid/benzoic acid ratios and alpha-dica rbonyl/aldehyde ratios for urban samples are higher than those of nonu rban samples. The monocarboxylic and dicarboxylic acids in the bulk ra inwaters were found to constitute 11-44% of the measured organic and i norganic anions and are important contributors to the lowering of pH i n Los Angeles rain samples. During wet precipitation events, the rain fluxes of the polar organic compound classes, generally decreased as a function of time, suggesting a rapid removal from the air. The concen tration ratios of monoacids to aldehydes, diacids to aldehydes and for mic acid to acetic acid showed a decrease during early stages of preci pitation, suggesting a preferential scavenging of carboxylic acids ove r aldehydes and C-1 acid over C-2 acid.