High abundances of dicarboxylic acids, oxocarboxylic acids, and α-dicarbonyls in fine aerosols (PM2.5) in Chengdu, China during wintertime haze pollution

Daytime and nighttime fine aerosol (PM2.5) samples were collected during a haze episode in January 2013 within the urban area of Chengdu, southwest China. Aerosol samples were analyzed for low-molecular-weight homologous dicarboxylic acids, oxocarboxylic acids and alpha-dicarbonyls, as well as organic carbon and elemental carbon. Concentration ranges of diacids, oxoacids, and alpha-dicarbonyls were 1,400-5,250, 272-1,380, and 88-220 ng m(-3), respectively. Molecular distributions of diacids (mean 3,388 +/- 943 ng m(-3)) were characterized by a predominance of oxalic acid (C-2; 1,373 +/- 427 ng m(-3)), followed by succinic (C-4), terephthalic (tPh), and phthalic (Ph) acids. Such high levels of tPh and Ph were different from those in other Asian cities where malonic acid (C-3) is the second or third highest species, mostly owing to significant emissions from coal combustion and uncontrolled waste incineration. High contents of diacids, oxoacids, and alpha-dicarbonyls were detected on hazy days, suggesting an enhanced emission and/or formation of these organics during such a weather condition. Concentrations of unsaturated aliphatic diacids (e.g., maleic acid) and phthalic acids were higher in nighttime than in daytime. Good positive correlations of C-2 with C-3, C-4, ketomalonic (kC(3)), pyruvic (Pyr), and glyoxylic (E center dot C-2) acids in daytime suggest secondary production of C-2 via the photooxidation of longer chain diacids and E center dot C-2. This study demonstrated that both primary emissions and secondary production are important sources of dicarboxylic acids and related compounds in atmospheric aerosols in the Sichuan Basin.