Abstract
Photochemical reactions of oleic acid (C18:1ω9) that is ubiquitous atmospheric constituents have been conducted in the quartz vessel with ozone and OH radical under UV irradiation. The reaction products were derived to dibuthyl esters and determined using a capillary gas chromatography (GC) and GC/mass spectrometry (GC/MS). During a laboratory oxidation, we observed the production of homologous series of low molecular weight dicarboxylic acids (C2-C9) with azelaic acid (C9) as a dominant species. Azelaic acid is produced by the oxidation of the double bond at C-9 position. Other dicarboxylic acids (C2-C8) were thought to be produced by the oxidation of azelaic acid with OH radical. Dicarboxylic acids except for oxalic (C2) and malonic (C3) acids showed an increase in the amounts with irradiation time, but they decreased after 20 hours, suggesting that not only production but also degradation of dicarboxylic acids occur. This experiments have clearly showed that biogenic unsaturated fatty acids, widely present in the atmosphere, are oxidized by ozone and OH radical, and produce low molecular weight dicarboxylic acids. These results are generally consistent with the distributions of dicarboxylic acids that have been reported in marine and urban aerosols and rain samples. However, oxalic acid is much more abundant than azelaic acid in the natural atmospheric samples. This suggests that short chain dicarboxylic acids, such as oxalic acid, have sources other than unsaturated fatty acids.
