Dominant role of NO2 oxidation in SO2 conversion to sulfate revealed by synchronous measurements of gas and particle sulfur isotopes in haze episodes

Abstract The oxidation pathways of SO2 conversion to sulfate remain controversial. Sulfur isotope (δ34S) has been used to trace SO42− formation pathways based on sulfur isotope fractionation. Accurately assessing the isotope fractionation is crucial for quantifying the oxidation pathways of SO42− formation. However, previous studies have used particle δ34S (δ34SO42−) to estimate the isotope fractionation (α34Sg→p-estimated), leading to significant uncertainties in SO42− formation. This study synchronously measured δ34S values of gas-phase SO2 and particle-phase SO42− to uncover isotope fractionation (α34Sg→p) in SO42− formation during haze episodes. Results found that the α34Sg→p values (-3.7 ~ + 9.9‰) obtained by gas-to-particle δ34S showed a significant difference with α34Sg→p-estimated values(-6.4 ~ + 1.4‰) obtained by δ34SO42−, implying different results for SO42− formation using the two methods. Among them, α34Sg→p results indicated the prominent contribution of NO2 oxidation(48–56%), followed by TMI-catalyzed O2 (transition-metal ions, 26–40%). While α34Sg→p-estimated (-6.4 ~ + 1.4‰) suggested the dominant role of TMI-catalyzed O2 (54–80%). Compared to α34Sg→p-estimated, α34Sg→p results show more reasonable response to SO42− formation and consistent trends with oxidant concentrations. α34Sg→p-estimated analysis 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, implying the importance of synchronous measurements gas-to-particle δ34S. Furthermore, a functional relationship between D-values (difference values of α34Sg→p and α34Sg→p-estimated) and impact factors was established, simulating actual α34Sg→p in the absence of gas-phase δ34S. This approach offers a possible correction for α34Sg→p-estimated values, providing new insight into using single δ34SO42− for the analysis of SO42− formation.