Dicarboxylic acids and water-soluble organic carbon in aerosols in New Delhi, India, in winter: Characteristics and formation processes

Day-and nighttime aerosol samples were collected at an urban site in New Delhi, India, in winter 2006-2007. They were studied for low molecular weight dicarboxylic acids and related compounds, as well as total water-soluble organic carbon (TWSOC). High concentrations of diacids (up to 6.03 mu g m(-3)), TWSOC, and OC were obtained, which are substantially higher than those previously observed at other urban sites in Asia. Daytime TWSOC/OC ratio (37%) was on average higher than that in nighttime (25%). In particular, more water-soluble OC (M-WSOC) to TWSOC ratio in daytime (50%) was twice higher than in nighttime (27%), suggesting that aerosols in New Delhi are photochemically more processed in daytime to result in more water-soluble organic compounds. Oxalic acid (C-2) was found as the most abundant dicarboxylic acid, followed by succinic (C-4) and malonic (C-3) acids. Contributions of C-2 to M-WSOC were greater (av. 8%) in nighttime than daytime (av. 3%). Positive correlations of C-2 with malic acid (hC(4)), glyoxylic acid (omega C-2), and relative humidity suggest that secondary production of C-2 probably in aqueous phase is important in nighttime via the oxidation of both longer-chain diacids and omega C-2. C-2 also showed a positive correlation with potassium (K+) in nighttime, suggesting that the enhanced C-2 concentrations are associated with biomass/biofuel burning. More tight, positive correlation between lesswater-soluble OC (L-WSOC) and K+ was found in both day- and nighttime, suggesting that L-WSOC, characterized by longer chain and/or higher molecular weight compounds, is significantly influenced by primary emissions from biomass/biofuel burning.