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

摘要

To better understand the sources of PM_10 samples in Mumbai, India, aerosol chemical composition, i.e., total carbon (TC), organic carbon (OC), elemental carbon (EC), water-soluble organic carbon (WSOC), and inorganic ions were studied together with specific markers such as methanesulfonate (MSA), oxalic acid (C_2), azelaic acid (C_9), and levoglucosan. The results revealed that biofuel/biomass burning and fossil fuel combustion are the major sources of the Mumbai aerosols. Nitrogen-isotopic (δ~(15)N) composition of aerosol total nitrogen, which ranged from 18.1 to 25.4‰, also suggests that biofuel/biomass burning is a predominate source in both the summer and winter seasons. Aerosol mass concentrations of major species increased 3–4 times in winter compared to summer, indicating enhanced emission from these sources in the winter season. Photochemical production tracers, C_2 diacid and nssSO_4~(2?), do not show diurnal changes. Concentrations of C_2 diacid and WSOC show a strong correlation (r~2 = 0.95). In addition, WSOC to OC (or TC) ratios remain almost constant for daytime (0.37 ± 0.06 (0.28 ± 0.04)) and nighttime (0.38 ± 0.07 (0.28 ± 0.06)), suggesting that mixing of fresh secondary organic aerosols is not significant and the Mumbai aerosols are photochemically well processed. Concentrations of MSA and C_9 diacid present a positive correlation (r~2 = 0.75), indicating a marine influence on Mumbai aerosols in addition to local/regional influence. Backward air mass trajectory analyses further suggested that the Mumbai aerosols are largely influenced by long-range continental and regional transport. Stable C-isotopic ratios (δ~(13)C) of TC ranged from ?27.0 to ?25.4‰, with slightly lower average (?26.5 ± 0.3‰) in summer than in winter (?25.9 ± 0.3‰). Positive correlation between WSOC/TC ratios and δ~(13)C values suggested that the relative increment in ~(13)C of wintertime TC may be caused by prolonged photochemical processing of organic aerosols in this season. This study suggests that in winter, the tropical aerosols are more aged due to longer residence time in the atmosphere than in summer aerosols. However, these conclusions are based on the analysis of a limited number of samples (n=25) and more information on this topic may be needed from other similar coastal sites in future.