Extensive, collocated measurements of the mass concentrations of composite and black carbon (BC) aerosols were made over coastal Arabian Sea, adjoining Indian Peninsula, for the first time during the inter-monsoon and summer monsoon periods, of 2003, as part of Arabian Sea Monsoon Experiment (ARMEX). Results showed that the diurnal variations are weak in March, and vanish completely by May/June, associated with the change in the synoptic circulations. The concentration of BC (and its share to total aerosol mass) decreases continuously, from ~700 ng m-3 (2.5%) in March to ~104 ng m-3 (0.5%) by June. Consequently, the net atmospheric forcing (heating) efficiency decreases from ~70 W m-2 (for reported winter conditions) to ~30 W m-2 for inter-monsoon and to ~15 W m-2 for summer monsoon seasons. These will have implications on regional climate forcing.