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

摘要

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, SO_4~(2-) (range: 0.6-35 u.g m~(-3)) is the most dominant. The non-sea-salt SO_4~(2-) (nss-SO_4~(2-)) constitutes a major fraction (55-65%) of the aerosol water-soluble ionic composition (WSIC), whereas contribution of NO_3~- is relatively minor. The magnitude of Cl-deficit (with respect to its sea-salt proportion) exhibits linear increase with the excess-nss-SO_4~(2-) (excess over NH_4~+). We propose that displacement of HC1 from sea-salt aerosols by H_2SO_4 is a dominant reaction mechanism for the chloride-depletion. These results also suggest that sea-salts could serve as a potential sink for anthropogenic SO_2 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).