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Organic and inorganic markers and stable C-, N-isotopic compositions of tropical coastal aerosols from megacity Mumbai: Sources of organic aerosols and atmospheric processing

by S. G. Aggarwal, K. Kawamura, G. S. Umarji, E. Tachibana, R. S. Patil, P. K. Gupta
Atmospheric Chemistry and Physics ()

Abstract

To better understand the sources of PM10 samples in Mumbai, India,\naerosol chemical composition, i.e., total carbon (TC), organic carbon\n(OC), elemental carbon (EC), water-soluble organic carbon (WSOC), and\ninorganic ions were studied together with specific markers such as\nmethanesulfonate (MSA), oxalic acid (C-2), azelaic acid (C-9), and\nlevoglucosan. The results revealed that biofuel/biomass burning and\nfossil fuel combustion are the major sources of the Mumbai aerosols.\nNitrogen-isotopic (delta N-15) composition of aerosol total nitrogen,\nwhich ranged from 18.1 to 25.4 %, also suggests that biofuel/biomass\nburning is a predominate source in both the summer and winter seasons.\nAerosol mass concentrations of major species increased 3-4 times in\nwinter compared to summer, indicating enhanced emission from these\nsources in the winter season. Photochemical production tracers, C-2\ndiacid and nssSO(4)(2-), do not show diurnal changes. Concentrations of\nC-2 diacid and WSOC show a strong correlation (r(2) = 0.95). In\naddition, WSOC to OC (or TC) ratios remain almost constant for daytime\n(0.37 +/- 0.06 (0.28 +/- 0.04)) and nighttime (0.38 +/- 0.07 (0.28 +/-\n0.06)), suggesting that mixing of fresh secondary organic aerosols is\nnot significant and the Mumbai aerosols are photochemically well\nprocessed. Concentrations of MSA and C-9 diacid present a positive\ncorrelation (r(2) = 0.75), indicating a marine influence on Mumbai\naerosols in addition to local/regional influence. Backward air mass\ntrajectory analyses further suggested that the Mumbai aerosols are\nlargely influenced by long-range continental and regional transport.\nStable C-isotopic ratios (delta C-13) of TC ranged from -27.0 to -25.4\n%, with slightly lower average (-26.5 +/- 0.3 %) in summer than in\nwinter (-25.9 +/- 0.3 %). Positive correlation between WSOC/TC ratios\nand delta C-13 values suggested that the relative increment in C-13 of\nwintertime TC may be caused by prolonged photochemical processing of\norganic aerosols in this season. This study suggests that in winter, the\ntropical aerosols are more aged due to longer residence time in the\natmosphere than in summer aerosols. However, these conclusions are based\non the analysis of a limited number of samples (n = 25) and more\ninformation on this topic may be needed from other similar coastal sites\nin future.

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