Carbonaceous Aerosol Deposition over the Northern Indian Ocean: Agricultural Burning, Shipping, and Sustainability Challenges

Carbonaceous aerosols play a crucial role in climate forcing and the dynamics of the South Asian monsoon; however, their sources and deposition processes remain insufficiently understood. In this study, we used dual carbon isotopes (Δ¹⁴C, δ¹³C) to accurately trace water-insoluble carbon in rainwater samples collected from the Maldives Climate Observatory–Hanimaadhoo over four years, from 2019 to 2023.

Carbonaceous species in the rainwater exhibited pronounced seasonal contrasts. On average, black carbon concentrations were about five times higher in the winter monsoon than in the summer monsoon. In comparison, water-insoluble organic carbon was roughly twice as high in winter as in the summer monsoon.  In our dataset, black carbon varied from 1.7 to 76.3 µg L⁻¹ during the winter monsoon, from 0.8 to 20.1 µg L⁻¹ during the summer monsoon, and from 1.0 to 29.0 µg L⁻¹ during the transitional periods. Water-insoluble organic carbon dominated the insoluble carbon pool, consistent with the notion that black carbon typically has a lower wet scavenging efficiency compared to more hydrophilic organic carbon fractions. Radiocarbon analysis indicated that biogenic sources, especially from biomass burning, are the primary contributors to water-insoluble carbon, accounting for approximately 59 ± 13% of the total. Notably, C3  plants alone contributed about 87% of this biomass signal. We observed distinct seasonal variations in these contributions; during the winter monsoon, we recorded higher biomass fractions, correlating with agricultural residue burning in the Indo-Gangetic Plain. In contrast, the summer monsoon saw an increase in fossil-fuel contributions, coinciding with heightened shipping activity and fossil-fuel combustion in the region.

The acidity of the rainwater (pH ranging from 4.2 to 6.9) varied with the origin of the air masses, underscoring the significant impact of anthropogenic activities during continental outflows. These findings provide valuable insights into the complex interactions between aerosols and monsoon systems, highlighting that deposition patterns are closely tied to local agricultural practices and energy consumption. Addressing the issues stemming from residue burning and shipping emissions could offer a sustainable pathway with potential co-benefits for climate resilience, ecosystems, and food security throughout the Indian Ocean region.