I started a study of polar organic compounds in the atmosphere of Los Angeles and its vicinity in early 1980s, when acid rain problems became an important environmental issue. In late 1980s, another two problems were recognized to be serious; i.e., global warming due to the increased greenhouse gases and depletion of stratospheric ozone discovered over Antarctica. I have developed GC and GC/MS methods to determine various organic compounds in the atmosphere including water-soluble mono- (C₁-C₁₀) and di-carboxylic (C₂-C₁₀) acids. Using those methods, formic and acetic acids were found as major monoacids in rainwater and in gas phase, followed by propionic acid. Total monoacids were found to account for up to 12% of total organic carbon in the Los Angeles rainwater. Their concentrations were equal to 1/4 to 1/2 of those of inorganic acids (sulfate and nitrate). Similarly, oxalic acid (C₂) was found as the most abundant diacid species followed by malonic (C₃) and succinic (C₄) acids in aerosols and rainwater. Studies of diacids in the atmosphere of urban Tokyo showed a diurnal variation with daytime maximum, indicating that secondary photochemical production of small diacids is more important than the primary emission from fossil fuel combustion (motor exhausts). Although diacids (C₂-C₁₀) account for <2% of total aerosol carbon in the urban atmosphere, the values increased up to 9% in the Arctic and 26% in the marine aerosols from the Pacific. Polar sunrise experiments conducted at Alert in the high Arctic further confirmed a significant secondary production of small diacids in early spring when sun returns on the horizon. Based on the spatial distributions of diacids over the Pacific, photochemical transformation (aging) of organic aerosols was characterized by an enrichment of oxalic acid in the marine atmosphere. Because dicarboxylic acids are water-soluble and can act as cloud condensation nuclei (CCN) in the atmosphere, they likely contribute to the formation of cloud droplets and thus affect indirectly on radiation balance of the Earth and the climate changes. Recent advances in the organic aerosol studies are also reviewed together with the studies on their hygroscopic properties (CCN activity) with a scope for the future study.

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