Abstract
In-situ observations of atmospheric Black Carbon (BC) aerosols over the northern Indian Ocean are sparse, especially during the Indian Summer Monsoon season due to harsh and challenging oceanic conditions associated with strong south-westerly winds. In the present study, we measured atmospheric BC over the Arabian Sea during June on board a research expedition. While BC is expected to decrease as we move away from continental landmass, surprisingly, despite negligible land-based combustion activities at this time of the year, a zone of high BC was observed offshore within a narrow band impacted by the Biparjoy cyclone. The median BC concentration was two-fold higher within the cyclone-impacted zone than in the region east of this zone. Airmass back-trajectory and satellite fire map analysis revealed that savanna burning in Africa significantly affects elevated BC signals over the Arabian Sea. The anti-correlation of BC with biomass-burning tracers inside the high-BC zone clearly suggested some other active source(s). Correlation analysis of BC with several deep-water vertical mixing indicators as well as atmospheric processes provides vital pointers that a significant amount of this BC can be regurgitated from waters brought to the surface from depth, as deep Ocean is one of largest reservoirs of BC aerosols. These findings demand a nascent scope for controlled simulation experiments to estimate regurgitated BC from the Ocean to reduce its uncertainty.