Black carbon dynamics over the Northern Indian Ocean: Variability, emission sources, transport and future projections

This study presents an extensive long-term assessment of black carbon (BC) over the North Indian Ocean (NIO) by integrating multiple datasets, like reanalysis, emission inventory, satellite fire counts and trajectory model, highlighting the complex interplay among emission sources, atmospheric transport, and regional climate dynamics. The findings reveal persistent BC hotspots along the northeastern and western coast of India, particularly near the northern Bay of Bengal (BoB). These hotspots are mainly linked to intense anthropogenic activities and outflows from the Indo-Gangetic Plain (IGP). The study highlights a distinct land-to-sea gradient and strong seasonal variability in BC mass concentration, with peak values during winter (up to 3.20 μg/m³) and post-monsoon seasons (up to 2.05 μg/m³) due to reduced atmospheric dispersion. BC levels increased notably during the rapid economic expansion in the 1990s and 2000s (up to 0.036 μg m^-3 yr^-1) but flattened after 2011, particularly in eastern coastal regions, suggesting the substantial impact of emission control and air quality regulations policies. Regional discrepancies remain, with areas like sub-regions R1 experiencing both sharp increases and significant recent declines, highlighting heterogeneous emission patterns and the effects mitigation policies. Atmospheric processes, particularly long-range transport, extensively influence BC distribution. Cluster analyses of airmass back trajectories further confirm that the BC over the BoB predominantly originates from the Indian landmass, while the Arabian Sea (AS) receives BC loading from both India and the Middle East, along with ship emissions. Agricultural fires and forest degradation contribute significantly to BC emissions, with highest fire occurrences observed between 2001 and 2010 followed by a decline, except for a fire surge in 2021. Future projections of BC mass concentration using the Seasonal Autoregressive Integrated Moving Average (SARIMA) model suggest an increasing trend during 2023-2032 across all the sub-region (up to 0.035 μg m^-3 yr^-1) except sub-region R3 where almost negligible decreasing trend is found (-0.002 μg m^-3 yr^-1). Thus, present study concludes that effective BC mitigation demands integrated emission controls, targeted regional policies, and continuous monitoring to address both local sources and transported pollution, which is crucial for improving climate and air quality in South Asia and the NIO.