Multi-decadal source apportionment of South Asia wintertime and summertime sulfate using δ³⁴S–SO₄²⁻ and emission-inventory-based model estimates

Sulfate aerosols exert a strong negative effective radiative forcing and remain a major source of uncertainty in regional climate projections. In South Asia, sustained elevated sulfate loadings are subject to intensified mitigation efforts. Such legislation could result in uncertain intensification of near-term warming through unmasking of net cooling aerosols. Robust source attribution is therefore needed to interpret past variability and to evaluate emission inventories used in climate and air-quality assessments.

In this study, we quantify anthropogenic versus natural contributions using the stable sulfur isotope composition (δ³⁴S) of aerosol sulfate (SO₄²⁻) measured at the Maldives Climate Observatory Hanimaadhoo (MCOH), a receptor site for the South Asian outflow. The analysis targets winter and summer monsoon air masses over 2006–2025 to sample contrasting transport regimes influencing MCOH.

Initial δ³⁴S-constrained apportionment indicates that wintertime sulfate is consistently dominated by anthropogenic sources (≈90–99%), whereas the summer monsoon shows a substantially larger spread in anthropogenic influence (≈47–88%). Ongoing work couples the isotopic constraints with FLEXPART transport footprints and state-of-the-art regionally-tuned emission inventories to resolve dominant upwind source regions and diagnose as well as improve agreement between inventory-based bottom-up estimates and in situ top-down observations. As the record is extended toward multi-decadal coverage, it will provide improved observational constraints on sulfate sources in the South Asian outflow and support evaluation and improvement of emission inventories, intrinsic to effective climate policy.