Unraveling the sources of atmospheric organic aerosols over the Arabian Sea: Insights from the stable carbon and nitrogen isotopic composition

The isotopic composition of stable carbon (delta C-13) and nitrogen (delta N-15) in marine aerosols influenced by the continental outflows are useful proxies for understanding the aging and secondary formation processes. Every winter, the haze pol-lutants transported from South Asia significantly affect the chemical composition of marine atmospheric boundary layer of the Arabian Sea. Here, we assessed the delta C-13 of total carbon (TC) and delta N-15 of total nitrogen (TN) in marine aero-sols collected over the Arabian Sea during a winter cruise (6-24 December 2018). TC (2.1-13.4 mu g m(-3)) is strongly correlated with TN (0.9-5.0 mu g m(-3)), likely because of their common source-emissions, biomass burning and fossil-fuel combustion in the Indo-Gangetic Plain and South Asia (corroborated by backward-air mass trajectories and satel-lite fire counts). Besides, the linear relationship between the mass ratios of water-soluble organic carbon (WSOC) to TC (0.04-0.65) and delta C-13(TC) (-25.1 parts per thousand to -22.9 parts per thousand) underscores the importance of aging process. This means oxidation of organic aerosols during transport not only influences the WSOC levels but also affects their delta C-13(TC). Likewise, the prevalent inverse linear relationship between the equivalent mass ratio of (NH4+/non-sea-salt-or nss-SO42-) and delta N-15 (TN) (+15.3 parts per thousand to +25.1 parts per thousand) emphasizes the overall significance of neutralization reactions between major acidic ([nss-SO42-] >> [NO3-]) and alkaline species (NH4+) in aerosols. Higher delta N-15(TN) values in winter than the spring inter-monsoon clearly emphasizes the significance of the anthropogenic combustion sources (i.e., biomass burning) in the South Asian outflow. A comparison of delta(13)C(TC)and delta N-15(TN) with the source emissions revealed that crop-residue burning emissions followed by the coal fired power plants mostly dictate the atmospheric abundance of organic aerosols in the wider South Asian outflow.