Dominant Roleof NO₂ Oxidation in SO₂ Conversion to SulfateRevealed by Synchronous Measurementsof 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 SO₄^2– formation pathways based on sulfur fractionation. Accurately assessing δ³⁴S fractionation is crucial to quantify SO₄^2– formation pathways. However, previous studies have used particle-phase δ³⁴S to estimate δ³⁴S fractionation (α³⁴S_g→p-estimated) in SO₄^2– formation, leading to significant uncertainties. δ³⁴S values of gas-to-particle (δ³⁴SO₂ and δ³⁴SO₄^2–) were synchronously measured to uncover isotope fractionation (α³⁴S_g→p). Results found that α³⁴S_g→p (−3.7‰ to +9.9‰) obtained by gas-to-particle δ³⁴S showed a significant difference with α³⁴S_g→p-estimated(−6.4‰ to +1.4‰) obtained by δ³⁴SO₄^2–, implying different results for SO₄^2– formation using the two methods. Among them, α³⁴S_g→p results indicated the prominent pathway of NO₂ oxidation (48–56%), while α³⁴S_g→p-estimated suggested the dominant role of transition metal ion (TMI)-catalyzed O₂ (54–80%). Additionally, α³⁴S_g→p results show a more reasonable response to SO₄^2– formation and consistent trends with oxidant concentrations. α³⁴S_g→p-estimated overestimated the TMI-catalyzed O₂ pathway contribution (38–47%) to SO₄^2– formation. This is the first study to employ gas-to-particle δ³⁴S to demonstrate the dominant role of NO₂ oxidation in SO₄^2– formation. This approach provides new insight into using δ³⁴SO₄^2– for the analysis of SO₄^2– formation.