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Keywords:

  • rain chemistry;
  • pH;
  • trace metals;
  • MSA;
  • major ions

[1] Rainwater samples for chemical analysis were collected over the Indian Ocean during the Indian Ocean Experiment (INDOEX) campaign January–March 1999 on board the research vessels Ronald H. Brown and Sagar Kanya. Samples were analyzed for major ions and some trace metals. The rainwater data are interpreted in terms of transport from potential source regions in Asia using air mass trajectories covering 10 days. A comparison is also made between the rainwater data and the concentration of aerosol components measured simultaneously on the ships. The concentrations of nonsea-salt (nss)-SO42−, NO3, NH4+, nss-K+, and nss-Ca2+ in rainwater over the Indian Ocean, while a factor of 2 to 3 lower than over the Indian continent, were still clearly influenced by pollution and soil sources in Asia. The concentration of nss-Ca2+ decreased more rapidly as the air moved southward from the continent out over the ocean, whereas the concentration of nss-SO42− became relatively more abundant. This was consistent with the observed higher acidity of the rainwater over the ocean (pH in the range 4.8 to 5.4) than over the Indian subcontinent, with NH4+ as the main cation (rather than Ca2+, as over land). Variations in the concentration of Al and Fe correlated well with those of nss-Ca2+, indicating a crustal source for these elements. The relation between Na+, Cl, and Br in the rainwater was close to that of seawater, implying no excess or deficit of the two halogen ions. The ratio between the concentration in rainwater and the concentration in surface air was systematically larger for aerosol components that exist in the coarse mode of sea-salt origin (Na+, Mg2+, and Cl) than those in the fine mode (NH4+, nss-K+, and nss-SO42−), indicating that fine-mode particles are scavenged mainly by in-cloud processes whereas coarse-mode sea-salt particles are scavenged also by falling raindrops under the clouds. Nss-Ca2+ and NO3 fall in a category in between, indicating that these compounds are not as effectively removed by below-cloud scavenging as sea-salt aerosols.