Dominant Role of NO₂ Oxidation in SO₂ Conversion to Sulfate Revealed by Synchronous Measurements of Gas and Particle Sulfur Isotopes in Haze Episodes

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The oxidation pathways of SO 2 conversion to sulfate remain controversial. Sulfur isotope (δ 34 S) has been used to trace the SO 4 2– formation pathways based on sulfur fractionation. Accurately assessing δ 34 S fractionation is crucial to quantify SO 4 2– formation pathways. However, previous studies have used particle-phase δ 34 S to estimate δ 34 S fractionation (α 34 S g→p -estimated) in SO 4 2– formation, leading to significant uncertainties. δ 34 S values of gas-to-particle (δ 34 SO 2 and δ 34 SO 4 2– ) were synchronously measured to uncover isotope fractionation (α 34 S g→p ). Results found that α 34 S g→p (−3.7‰ to +9.9‰) obtained by gas-to-particle δ 34 S showed a significant difference with α 34 S g→p -estimated(−6.4‰ to +1.4‰) obtained by δ 34 SO 4 2–, implying different results for SO 4 2– formation using the two methods. Among them, α 34 S g→p results indicated the prominent pathway of NO 2 oxidation (48–56%), while α 34 S g→p -estimated suggested the dominant role of transition metal ion (TMI)-catalyzed O 2 (54–80%). Additionally, α 34 S g→p results show a more reasonable response to SO 4 2– formation and consistent trends with oxidant concentrations. α 34 S g→p -estimated overestimated the TMI-catalyzed O 2 pathway contribution (38–47%) to SO 4 2– formation. This is the first study to employ gas-to-particle δ 34 S to demonstrate the dominant role of NO 2 oxidation in SO 4 2– formation. This approach provides new insight into using δ 34 SO 4 2– for the analysis of SO 4 2– formation.