Spatial gradient of aerosol mass concentrations and size distributions over southeastern Arabian Sea and equatorial Indian Ocean during ICARB-2018

Abstract

Continuous measurements of the mass concentrations of composite (MT) and Black Carbon (MB) aerosols were carried out on-board an Oceanographic Research Vessel (ORV) Sagar Kanya (cruise number – SK345) during its cruise (from 16 January to 13 February, 2018) over the southeastern Arabian Sea and equatorial Indian Ocean as a part of the Integrated Campaign for Aerosols, gases and Radiation Budget (ICARB-2018). In general, mass concentrations of aerosols during the cruise period of northern hemispheric winter were found to be high over the southeastern Arabian Sea (MT > 50 μg m−3; MB > 3 μg m−3), which gradually reduced to low values over the far oceanic regions of equatorial Indian Ocean (MT < 20 μg m−3; MB < 1 μg m−3). Aerosol mass size distributions were bimodal in nature having a prominent accumulation mode over both near coastal and far oceanic regions. Interestingly, BC mass fraction (FBC) also showed significant higher values (3.7%) at the far oceanic equatorial Indian Ocean, indicating the continental anthropogenic influence even in the remote environment. Over the near coastal locations of Arabian Sea, FBC increased significantly during the return phase of the cruise after a rainfall event. Considering latitudinal gradient between 15°N and 2°S, within the longitude sector of 65°E to 75°E, both MT and MB showed a scaling distance of 17.08° and 14.28° respectively. Investigation of the long-term temporal change of MT and MB over the experimental domain indicates that the values observed in the present study are significantly higher than those reported elsewhere two decades back, which is very significant over the Arabian Sea.