Seasonal variations of carboxylic acids and their contributions to the rainwater acidity: A case study of Guiyang and Shangzhong,China

Low molecule weight carboxylic acids are ubiquitous and important chemical constituents in the troposphere. Seven carboxylic acids in the rainwater of Guiyang and Shangzhong were simultaneously determined by ion chromatography from April 2006 to April 2007. Formic, acetic and oxalic acids were found to be the predominant carboxylic acids. Their volume weighted average concentration (VWA) in the rainwater of Guiyang were 14.24 μmol/L, 9.35 μmol/L and 2.79 μmol/L, respectively; as compared to 4.95 μmol/L, 1.35 μmol/L and 2.31 μmol/L in the rainwater of Shangzhong. In Shangzhong it is witnessed that the concentrations of these acids were higher in the summer than in the winter and direct emissions from vegetations or soils may account for the main provenance of the acids. This is, however, not the case in Guiyang, where the concentrations of the carboxylic acids were higher during the non-growing season than during the growing season. The relatively weak scavenging affected by scarce and little rainwater as well as the particles accompanied with the rainfall may have some effect on the carboxylic acids during the wintertime in Guiyang. Carboxylic acids in the rainwater of Guiyang were estimated to account for 18.7% to the free acidity, where formic, acetic and oxalic acids accounted for 7.9%, 4.7%, 6.1%, respectively. In Shangzhong, the 3 primary carboxylic acids represented 58.1% to the free acidity where formic, acetic and oxalic acids represented 25.1%, 7.5%, 25.5%, respectively. Carboxylic anions accounted for 6.6% (1.7%–19.2%) to the total anions in the rainwater of Guiyang. Carboxylic anions represented 13.2% (0.5%–92.2%) to the total anions in the rainwater of Shangzhong. These results indicated that carboxylic acids were the important contributors to the rainwater acidity, especially in remote regions.