Enhanced oxidation of SO2 by H2O2 during haze events: constraints from sulfur isotopes

Sulfate aerosols play a critical role in atmospheric chemistry and climate change. However, the mechanism of sulfate formation during haze events remains highly uncertain. In this work, sulfur isotopic compositions of SO2 and sulfate in PM2.5 samples collected in Tianjin, China as well as a Rayleigh distillation model are used to constrain SO2 oxidation pathways. Here, we show the sulfur isotopic compositions of SO2 and sulfate  as well as concentrations of major ions in PM2.5. Our results show that a large sulfur isotope fractionation between SO2 and sulfate observed in the first stage of haze events may be attributed to an enhanced oxidation of SO2 by H2O2. This study highlights the important role of SO2 oxidation by H2O2 during the haze events. In addition, it is particularly important to note that the sulfur isotopic fractionation factors during haze events dispaly a distinct trend to other periods, indicating that this isotopic approach is useful in understanding the sulfate formation during haze events.