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.