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 μ 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 ( ω 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 ω 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 less water‐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.