Toward the Origins of Quadruple Sulfur Isotope Anomalies in Modern Sulfate: A Multitracer Approach and Implications for Paleo- and Planetary Atmospheres

Sulfur isotope mass-independent fractionation signatures (S-MIF) have been widely found in modern sulfate aerosols. Their mechanistic origins and potential implications for atmospheric sulfate formation chemistry are however poorly understood. Of particular importance is that the mystery of modern sulfate S-MIF hinders precise interpretations of S-MIF in geological sediment and meteorite samples. Here, we examine several proposed origins of modern sulfate S-MIF using a multiple-tracer approach. Quadruple sulfur isotopes of sulfate aerosols collected from Guangzhou, a megacity in South China, were measured along with various chemical and isotopic tracers for high-altitude air masses, biomass burning, and mineral dusts in the same samples. Similar to previous studies, we found that negative Δ36S values in modern sulfate aerosols are linked to combustions, and to a larger extent, to high-temperature processes that may involve symmetry-dependent recombination reactions of sulfur. The origins of Δ33S anomalies widely observed in modern sulfate aerosols remain elusive and require further investigation before we can unambiguously use this proxy to quantify sulfur emission and transformation pathways in the modern atmosphere. From a comparative planetology perspective, we highlight that a complete understanding of S-MIF in modern aerosols is crucial for tracking paleo- and planetary atmospheres by multiple sulfur isotope measurements in Phanerozoic sediments and Martian meteorites because their S-MIF magnitudes are similar.