Anthropogenic sulphate aerosols and large Cl-deficit in marine atmospheric boundary layer of tropical Bay of Bengal

Abstract

Our long-term study provides an unequivocal evidence for near-quantitative (80–100%) depletion of chloride from sea-salts in the marine atmospheric boundary layer (MABL) of tropical Bay of Bengal. During the late NE-monsoon (Jan-Mar), continental outflow from south and south-east Asia dominate the wide-spread dispersal of pollutants over the Bay of Bengal. Among anthropogenic constituents, SO42− (range: 0.6–35 μg m−3) is the most dominant. The non-sea-salt SO42− (nss-SO42−) constitutes a major fraction (55–65%) of the aerosol water-soluble ionic composition (WSIC), whereas contribution of NO3− is relatively minor. The magnitude of Cl-deficit (with respect to its sea-salt proportion) exhibits linear increase with the excess-nss-SO42− (excess over NH4+). We propose that displacement of HCl from sea-salt aerosols by H2SO4 is a dominant reaction mechanism for the chloride-depletion. These results also suggest that sea-salts could serve as a potential sink for anthropogenic SO2 in the downwind polluted marine environment. Furthermore, loss of hydrogen chloride, representing a large source of reactive chlorine, has implications to the oxidant chemistry in the MABL (oxidation of hydrocarbons and dimethyl sulphide).