Isotopic apportionment of sulfate aerosols between natural and anthropogenic sources in the outflow of South Asia

Abstract. Sulfate aerosols cool the climate and thus temporarily mask climate warming, but at a cost to air quality. Their short atmospheric lifetime leads to heterogeneous global coverage, with sulfate concentrations over South Asia being especially elevated and continuing to increase. It remains challenging to constrain the relative importance of different emission sources due to poor observational coverage and uncertainties in bottom-up technology-based emission estimates. The stable sulfur isotope composition (δ³⁴S-SO₄^2-) quantitatively distinguishes natural and anthropogenic sources. This study aimed to constrain the sources of sulfate arriving at the Maldives Climate Observatory Hanimaadhoo (MCOH), which is ideally situated for intercepting the outflow from airsheds over the Indian subcontinent. The results show that anthropogenic sources of sulfate contributed 94 ± 11 %, 88 ± 9 %, and 67 ± 13 % in winter (post-monsoon), spring (pre-monsoon), and summer (monsoon), respectively. There was also a moderate to strong correlation (r² = 0.79, p << 0.05, n = 7) between continental anthropogenic (winter and spring) sulfate (δ³⁴S) and black carbon aerosols from fossil fuel combustion (pinpointed by Δ¹⁴C). This study provides improved constraints on sulfate sources for South Asia – a key region for aerosol pollution and aerosol masking of climate warming.