Chemical composition of rainwater at Maldives Climate Observatory at Hanimaadhoo (MCOH)

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 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 India and adjacent parts of the Asian continent during the months December and January (Indian group) and those with southerly flow from the Indian Ocean during the summer monsoon season June to September (Marine group). A third trajectory group was identified with transport from the northern parts of the Arabian Sea and adjacent land areas during the months March, April and October (Arabian Sea group). The concentrations of nss-SO₄²⁻, NH₄⁺ and NO₃⁻ were more than a factor of 4 higher in the Indian group than in the Marine group. The average rainwater pH was significantly lower in the Indian group (4.7) than in the Marine group (6.0). This shows a pronounced influence of continental pollutants during December and January. The origin of the very high concentration of nss-Ca²⁺ found in the Marine group – a factor of 7 higher than in the Indian group – is unclear. We discuss various possibilities including long-range transport from the African or Australian continents, local dust from nearby islands and calcareous plankton debris and exopolymer gels emitted from the ocean surface. The occurrence of NO₃⁻ and NH₄⁺ in the Marine group 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 in the Indian group the wet deposition was at least as big in the Marine group reflecting the larger amount of rainfall during the monsoon season. The annual wet deposition of NO₃⁻, NH₄⁺ and nss-SO₄²⁻ at MCOH is about a factor of three lower than observed at rural sites in India.