Estimation of Black Carbon Wet Deposition Fluxes from the Marine Atmospheric Column over the Northern Indian Ocean

Black carbon (BC) is a strong short-lived climate forcer and an important pathway for atmospheric carbon input to the ocean. Although the Northern Indian Ocean (NIO) receives a strong outflow from the Indian subcontinent (high BC emission region), quantitative estimates of its wet deposition to the oceans which is a dominant atmospheric removal mechanism remains largely unavailable for the Bay of Bengal (BoB) and the Arabian Sea (AS). This study provides the first long-term, basin-scale assessment of BC wet deposition fluxes over the NIO for the period 2002–2022, focusing on their seasonal variability and inter annual trends.
BC wet deposition fluxes were estimated using a parameterized approach in which the flux is defined as the product of BC concentration, an empirical particle washout ratio, and the precipitation rate. Near-surface aerosol mass concentration was derived by normalizing MODIS/Aqua Level-2 MYD04_3K (Collection 6.1) derived columnar mass concentration with boundary layer height from ERA5 reanalysis. These near surface mass concentration is corrected by density followed by hygroscopic growth factor. Surface BC mass concentration is estimated by applying a black carbon mass fraction (f_BC) to the hygroscopicity-corrected near-surface aerosol mass concentrations, which is further used to compute the BC wet deposition fluxes.
Results show strong seasonal variability in BC wet deposition over the NIO, with flux maxima during the southwest monsoon driven mainly by enhanced precipitation. Inter annual variability in BC wet deposition correlates to precipitation variability, confirming rainfall as the dominant controlling factor for BC removal over the region. Basin-scale contrasts show higher wet deposition over the BoB than AS, reflecting closer proximity to major continental emission sources. Spatially, BC wet deposition is enhanced over coastal and nearshore regions compared to the open ocean, reflecting a sharp gradient from the coast toward the open ocean and highlighting a strong influence of meteorology and source proximity in BC deposition across the NIO. These results provide the constrained, long-term estimate of BC wet deposition to the BoB and AS, offering inputs for regional climate modeling and improved understanding of aerosol–monsoon–ocean interactions.