Role of aerosol liquid water content on the production of dicarboxylic acids in the dust-laden air masses over the Arabian Sea: Implications for heterogeneous chemistry

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Oxalic acid and the related organics are ubiquitous water-soluble constituents of ambient organic aerosols (OA). Their molecular distributions and relative abundances provide crucial insights on the secondary OA formation processes in the marine atmosphere and are important for understanding the cloud-aerosol interactions. Here, we measured the concentrations of dicarboxylic acids, pyruvic acid, ω-oxocarboxylic acids and α-dicarbonyls alongside aerosol chemical composition in the South Asian outflow to the Arabian Sea. Oxalic acid (C2) was the most abundant dicarboxylic acid followed by succinic (C4) and malonic (C3) acids; this distribution is typical of “less aged OA”. Oxalic acid levels correlated well with aerosol liquid water content (ALWC) and non-sea-salt (nss)-SO42− (a tracer of coal combustion influence), suggesting an important aqueous phase oxidation of successive higher homologs. This was inferred based on the prevailing inverse relationships between the relative abundances of oxalic acid in total aliphatic dicarboxylic acids and those of malonic, succinic, glutaric, adipic, and azelaic acids. Likewise, the concentrations of glyoxal (Gly: a major compound emitted from vehicular exhaust) and glyoxylic acid (ωC2) correlated strongly with C4, nss-Ca2+ (dust tracer) and elemental carbon (EC: a combustion tracer from biomass/fossil-fuels). These observations suggest gas/particle partitioning and subsequent aqueous phase oxidation of Gly to ωC2 and then elongation (oligomerization) to succinic acid associated with the mineral dust particles. This means relative abundances of anthropogenic SO42− in the dust-laden air masses govern the secondary OA formation over the Arabian Sea.