Extensive, and collocated measurements of the mass concentrations (MB) of aerosol black carbon (BC) and (MT) of composite aerosols were made over the Arabian Sea, tropical Indian Ocean and the Southern Ocean during a trans-continental cruise experiment. Our investigations show that MB remains extremely low (<50 ng m-3) and remarkably steady (in space and time) in the Southern Ocean (20¿S to 56¿S). In contrast, large latitudinal gradients exist north of ~20¿S; MB increasing exponentially to reach as high as 2000 ng m-3 in the Arabian Sea (~8¿N). Interestingly, the share of BC showed a distinctly different latitudinal variation, with a peak close to the equator and decreasing on either side. Large fluctuations were seen in MT over Southern Ocean associated with enhanced production of sea-salt aerosols in response to sea-surface wind speed. These spatio-temporal changes in MB and its mixing ratio have important implications to regional and global climate.