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

The oxidation pathways of SO2 conversion to sulfate remain controversial. Sulfur isotope (δ34S) has been used to trace the SO42– formation pathways based on sulfur fractionation. Accurately assessing δ34S fractionation is crucial to quantify SO42– formation pathways. However, previous studies have used particle-phase δ34S to estimate δ34S fractionation (α34Sg→p-estimated) in SO42– formation, leading to significant uncertainties. δ34S values of gas-to-particle (δ34SO2 and δ34SO42–) were synchronously measured to uncover isotope fractionation (α34Sg→p). Results found that α34Sg→p (−3.7‰ to +9.9‰) obtained by gas-to-particle δ34S showed a significant difference with α34Sg→p-estimated(−6.4‰ to +1.4‰) obtained by δ34SO42–, implying different results for SO42– formation using the two methods. Among them, α34Sg→p results indicated the prominent pathway of NO2 oxidation (48–56%), while α34Sg→p-estimated suggested the dominant role of transition metal ion (TMI)-catalyzed O2 (54–80%). Additionally, α34Sg→p results show a more reasonable response to SO42– formation and consistent trends with oxidant concentrations. α34Sg→p-estimated overestimated the TMI-catalyzed O2 pathway contribution (38–47%) to SO42– formation. This is the first study to employ gas-to-particle δ34S to demonstrate the dominant role of NO2 oxidation in SO42– formation. This approach provides new insight into using δ34SO42– for the analysis of SO42– formation.