Atmos. Chem. Phys. Discuss., 10, 17569-17607, 2010
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Chemical composition of rainwater at Maldives Climate Observatory at Hanimaadhoo (MCOH)
R. Das1, L. Granat1, C. Leck1, P. S. Praveen2, and H. Rodhe1
1Department of Meteorology, Stockholm University, 106 91 Stockholm, Sweden
2Maldives Climate Observatory Hanimaadhoo, The Maldives

Abstract. Water soluble inorganic components in rain deposited at the Maldives Climate Observatory Hanimaadhoo (MCOH) were examined to determine seasonality and possible source regions. The study, which is part of the international Atmospheric Brown Cloud (ABC) project, covers the period June 2005 to December 2007. Air mass trajectories were used to separate the data into situations with transport of air from the Asian continent during winter (December–April) and those with southerly flow from the Indian Ocean during the monsoon season (June–September). The concentrations of nss-SO42−, NH4+, NO3 and H+ were more than a factor of 4 higher in winter than during the monsoon season. This shows a pronounced influence of continental pollutants during winter. The average rainwater pH was significantly lower in winter (4.7) than during the monsoon (6.0). The lower pH in winter is probably due to a more rapid decrease in the alkaline components than in the acidifying components as air is transported southwards over the Indian Ocean. The moderately high loadings of nss-SO42− during the monsoon season, supported by our measurements of Methane sulphonate (MSA), indicate that Dimethyl sulphide (DMS) is likely to contribute substantially to the nss-SO42− concentration during this season. The origin of the high concentration of nss-Ca2+ during the monsoon season – a factor of 4 to 7 higher than during the winter situations with trajectories from the continent – is unclear. We discuss various possibilities including long-range transport from the African or Australian continents, calcareous plankton debris and exopolymer gels emitted from the ocean surface. The occurrence of NO3 and NH4+ during the monsoon season suggests emissions from the ocean surface. Part of the NO3 could also be associated with lightning over the ocean. Despite the fact that the concentrations of nss-SO42−, NO3, and NH4+ were highest during the winter season their wet deposition was at least as high during the monsoon season reflecting the larger amount of rainfall in this season. The annual wet deposition of these components was comparable to that observed in the eastern US and in India but substantially higher than what has been observed on Amsterdam Island in the Southern Indian Ocean.

Citation: Das, R., Granat, L., Leck, C., Praveen, P. S., and Rodhe, H.: Chemical composition of rainwater at Maldives Climate Observatory at Hanimaadhoo (MCOH), Atmos. Chem. Phys. Discuss., 10, 17569-17607, doi:10.5194/acpd-10-17569-2010, 2010.
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