Comparison of Atmospheric Monocarboxylic and Dicarboxylic Acids in Xi’ an, China, for Source Apportionment of Organic Aerosols

To investigate the characteristic distributions and formation mechanism of the polar organic components in atmospheric aerosols, atmospheric particulates were simultaneously sampled at three typical sites in urban(U), suburban(S) and rural(R) of Xi’an in summer and winter. Organic compositions, including dicarboxylic acids (DCAs), monocarboxylic acids (MCAs) were emphatically analyzed to explore the seasonal, spatial, and other variations. The results showed that total concentrations of DCAs and MCAs in winter were higher than those in summer. Athough the total content of DCAs in PM10 in winter is lower than that of MCAs, the total concentration of DCAs in PM2.5 and PM10 in summer are higher than that of MCAs. The total concentration of DCAs in PM2.5 in winter and summer were 1878 ± 1425 ng/m 3 and 1334 ± 493 ng/m 3 , 1294 ± 943 ng/m 3 and 728 ± 477 ng/m 3 in PM10. While the total concentration of MCAs in PM2.5 in winter and summer were 1193 ± 1142 ng/m 3 and 541 ± 367 ng/m 3 , 1659 ± 1162 ng/m 3 and 492 ± 424 ng/m 3 in PM10. Urban subtotal concentrations of DCAs from PM2.5 were the highest, and mass of organic acids in sunny days were significantly lower than the hazy days. Strong correlations were observed among the DCAs with ambient oxidants and precursors. Moreover, DCAs concentrations were vulnerable to the environmental factors, such as light intensity and relative humidity. Comparisons demonstrated that MCAs mainly comes from coal combustion, vehicle exhaust and other primary emissions, while the secondary photochemistry and liquid phase oxidation are the main sources of DCAs.