Size-resolved airborne particulate oxalic and related secondary organic aerosol species in the urban atmosphere of Chengdu, China

• Enhanced production of dicarboxylic acids from ketocarboxylic acids and α-dicarbonyls is due to the increase of aerosol liquid water content (LWC) at night.
• Secondary organic and inorganic species grow to larger sizes at night due to LWC increase.
• Significant contribution from biomass burning emission to SOA production

Size-segregated (9-stages) airborne particles during winter in Chengdu city of China were collected on a day/night basis and determined for dicarboxylic acids (diacids), ketocarboxylic acids (ketoacids), α-dicarbonyls, inorganic ions, and water-soluble organic carbon and nitrogen (WSOC and WSON). Diacid concentration was higher in nighttime (1831 ± 607 ng m− 3) than in daytime (1532 ± 196 ng m− 3), whereas ketoacids and dicarbonyls showed little diurnal difference. Most of the organic compounds were enriched in the fine mode (< 2.1 μm) with a peak at the size range of 0.7–2.1 μm. In contrast, phthalic acid (Ph) and glyoxal (Gly) presented two equivalent peaks in the fine and coarse modes, which is at least in part due to the gas-phase oxidation of precursors and a subsequent partitioning into pre-existing particles. Liquid water content (LWC) of the fine mode particles was three times higher in nighttime than in daytime. The calculated in-situ pH (pHis) indicated that all the fine mode aerosols were acidic during the sampling period and more acidic in daytime than in nighttime. Robust correlations of the ratios of glyoxal/oxalic acid (Gly/C2) and glyoxylic acid/oxalic acid (ωC2/C2) with LWC in the samples suggest that the enhancement of LWC is favorable for oxidation of Gly and ωC2 to produce C2. Abundant K+ and Cl− in the fine mode particles and the strong correlations of K+ with WSOC, WSON and C2 indicate that secondary organic aerosols in the city are significantly affected by biomass burning emission.