Concentrations of monocarboxylic and dicarboxylic acids and aldehydes in 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 for their content of polar organic compounds, including C-1-C-9 monocarboxylic acids, C-2-C-10 alpha,omega-dicarboxylic acids, C-1-C-2 aldehydes and C-2-C-3 alpha-dicarbonyls. Formic (0.1-33 mu M), acetic (0.3-24 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 the dominant species. The concentrations of the monocarboxylic and dicarboxylic acids in bulk rain were inversely proportional to the amount of rainfall, however, those of aldehydes remained almost unchanged. The relative abundances of the major compounds are similar among the samples and no significant trend was found to differentiate urban and nonurban samples. However, some minor dissolved components showed different distributions; e.g. phthalic acid/benzoic acid ratios and alpha-dicarbonyl/aldehyde ratios for urban samples are higher than those of nonurban samples. The monocarboxylic and dicarboxylic acids in the bulk rainwaters were found to constitute 11-44% of the measured organic and inorganic anions and are important contributors to the lowering of pH in 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 concentration ratios of monoacids to aldehydes, diacids to aldehydes and formic acid to acetic acid showed a decrease during early stages of precipitation, suggesting a preferential scavenging of carboxylic acids over aldehydes and C-1 acid over C-2 acid.