Aerosol size characteristics over the Arabian Sea and Indian Ocean: Extensive sub-micron aerosol loading in the northern hemisphere

Aerosol size characteristics and columnar loading are deduced from spectral optical depths estimated over the Arabian Sea and south-western Indian Oce an, using a 10 channel multi-wavelength solar radiometer (MWR) on board the cruise #133 of ORV Sagar Kanya during the First Field Phase (FFP-98) of th e Indian Ocean Experiment (INDOEX), The columnar size distributions showed a clear and consistent bimodal nature over the oceanic areas of the norther n hemisphere (or due north of the inter-tropical convergence zone (ITCZ)) w ith a sub-micron accumulation mode at similar to 0.05 mu m and a coarse par ticle mode at similar to 1 mu m; both are attributed to different source me chanisms. There is a rapid decrease in aerosol mass loading as one moves to remote oceanic regions from the coast. Over the pristine environment, on t he south of the ITCZ, the accumulation mode tends to vanish leading to broa d unimodal distributions; the mass loading becomes < similar to 70 mg m(-2) column. Aerosol size characteristics show an extensive increase in the rel ative abundance of sub-micron (r < 0.5 mu m) aerosols in the northern hemis phere. The concentration of these is highest in the coastal and interior re gions of the northwestern Arabian Sea and decrease rapidly as one moves to the Indian Ocean. The results are discussed based on different aerosol gene ration mechanisms and the possible transport through air trajectories.