Extensive measurements of aerosol number size distributions (in the size range of 10 to 875 nm) carried out over the oceanic regions of the Bay of Bengal and Arabian Sea during two large cruise experiments (one during pre-monsoon and the other during winter) are used to investigate the spatial distribution of aerosol size distributions, in general, and that of fine particles, in particular, within the marine atmospheric boundary layer. The size distributions over the northwestern Bay of Bengal, lying downwind of the continental outflow from the Indo-Gangetic Plain, and over the eastern Bay of Bengal, under the influence of the East Asian outflow, showed a bimodal structure with prominent mode (100–125 nm) in the accumulation regime and weak mode (30–40 nm) in the Aitken regime during both pre-monsoon and winter seasons. While the Aitken mode was found to be relatively quite weak in the Indo-Gangetic Plain outflow during both seasons, it was prominent in the East Asian outflow regions, especially during the winter season. The distributions over the northern Arabian Sea, a region quite prone to advection of dust during pre-monsoon and summer seasons, showed a prominent Aitken mode (~45 nm) followed by a weaker accumulation mode during pre-monsoon season. Analysis of SeaWiFS data revealed a systematic collocation of Aitken mode aerosols and the high chlorophyll concentration in the northern Arabian Sea, implying the role of ocean biogeochemistry in influencing the aerosol size distributions. Such a feature implying biogeochemical influence was not seen over the eastern and northern Bay of Bengal during pre-monsoon and winter seasons.