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 PM_2.5 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.硫酸盐气溶胶（Sulfate aerosols）在大气化学与气候变化中发挥着关键作用。然而，霾天气过程中硫酸盐的形成机制仍存在极高不确定性。本研究采集了中国天津地区细颗粒物（PM₂.₅）样品中的二氧化硫（SO₂）与硫酸盐的硫同位素组成，并结合瑞利蒸馏模型（Rayleigh distillation model）对二氧化硫的氧化路径进行约束。本文展示了细颗粒物样品中二氧化硫、硫酸盐的硫同位素组成以及主要离子浓度。研究结果表明，霾天气初期观测到的二氧化硫与硫酸盐之间显著的硫同位素分馏，可能由过氧化氢（H₂O₂）对二氧化硫的强化氧化作用所致。本研究凸显了H₂O₂介导的二氧化硫氧化在霾天气过程中的重要作用。此外，需特别注意的是，霾天气期间的硫同位素分馏因子与其他时段呈现出显著差异，这表明该同位素方法有助于理解霾天气下硫酸盐的形成机制。