Dominant role of NO2 oxidation in SO2 conversion to sulfate revealed by synchronous measurements of gas and particle sulfur isotopes in haze episodes
Discuss this preprint
Start a discussion What are Sciety discussions?Listed in
This article is not in any list yet, why not save it to one of your lists.Abstract
The oxidation pathways of SO 2 conversion to sulfate remain controversial. Sulfur isotope (δ 34 S) has been used to trace SO 4 2− formation pathways based on sulfur isotope fractionation. Accurately assessing the isotope fractionation is crucial for quantifying the oxidation pathways of SO 4 2− formation. However, previous studies have used particle δ 34 S (δ 34 SO 4 2− ) to estimate the isotope fractionation (α 34 S g→p -estimated), leading to significant uncertainties in SO 4 2− formation. This study synchronously measured δ 34 S values of gas-phase SO 2 and particle-phase SO 4 2− to uncover isotope fractionation (α 34 S g→p ) in SO 4 2− formation during haze episodes. Results found that the α 34 S g→p values (-3.7 ~ + 9.9‰) obtained by gas-to-particle δ 34 S showed a significant difference with α 34 S g→p -estimated values(-6.4 ~ + 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 contribution of NO 2 oxidation(48–56%), followed by TMI-catalyzed O 2 (transition-metal ions, 26–40%). While α 34 S g→p -estimated (-6.4 ~ + 1.4‰) suggested the dominant role of TMI-catalyzed O 2 (54–80%). Compared to α 34 S g→p -estimated, α 34 S g→p results show more reasonable response to SO 4 2− formation and consistent trends with oxidant concentrations. α 34 S g→p -estimated analysis 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, implying the importance of synchronous measurements gas-to-particle δ 34 S. Furthermore, a functional relationship between D-values (difference values of α 34 S g→p and α 34 S g→p -estimated) and impact factors was established, simulating actual α 34 S g→p in the absence of gas-phase δ 34 S. This approach offers a possible correction for α 34 S g→p -estimated values, providing new insight into using single δ 34 SO 4 2− for the analysis of SO 4 2− formation.
