Targeting Atmospheric Oxidants Can Better Reduce Sulfate Aerosol in China: H₂O₂ Aqueous Oxidation Pathway Dominates Sulfate Formation in Haze

Abstract

Particulate sulfate is one of the most important components in the atmosphere. The observation of rapid sulfate aerosol production during haze events provoked scientific interest in the multiphase oxidation of SO 2 in aqueous aerosol particles. Diverse oxidation pathways can be enhanced or suppressed under different aerosol acidity levels and high ionic strength conditions of atmospheric aerosol. The importance of ionic strength to sulfate multiphase chemistry has been verified under laboratory conditions, though studies in the actual atmosphere are still limited. By utilizing online observations and developing an improved solute strengthdependent chemical thermodynamics and kinetics model (EF-T&K model, EF is the enhancement factor that represents the effect of ionic strength on an aerosol aqueous-phase reaction), we provided quantitative evidence that the H 2 O 2 pathway was enhanced nearly 100 times and dominated sulfate formation for entire years (66%) in Tianjin (a northern city in China). TMI (oxygen catalyzed by transition-metal ions) (14%) and NO 2 (14%) pathways got the second-highest contributions. Machine learning supported the result that aerosol sulfate production was more affected by the H 2 O 2 pathway. The collaborative effects of atmospheric oxidants and SO 2 on sulfate aerosol production were further investigated using the improved EF-T&K model. Our findings highlight the effectiveness of adopting target oxidant control as a new direction for sustainable mitigation of sulfate, given the already low SO 2 concentrations in China.

Significant formation of sulfate aerosols contributed by the heterogeneous drivers of dust surface

“…Aerosol liquid water serves as an efficient medium for multi-phase reactions (Wu et al, 2018;Yue et al, 2019), whereas the radius is negatively associated with the sulfate formation at the droplet interface (Hung et al, 2018;Chen et al, 2022). Furthermore, droplet acidity decreases as the liquid volume increases, thereby increasing the ionization of dissolved sulfur species, followed by the increased sulfate formation rate (Yue et al, 2019;Jin et al, 2020;Gao et al, 2022). Overall, how meteorological factors influence the relative importance of gas-phase, aqueous-phase, and heterogeneous pathways warrants further research.…”

“…T. Wang et al: Significant formation of sulfate aerosols Tilgner et al, 2021;Gao et al, 2022;S. Wang et al, 2022;Ye et al, 2022).…”

Directly measuring Fe(iii)-catalyzed SO2oxidation rate in single optically levitated droplets

“…The sulfate PM 2.5 concentration simulated with air quality models 13 were 3-4 times lower than that measured in the atmosphere. This gap has motivated many research efforts to discover the hitherto unidentified sulfate sources in polluted environments 11,[14][15][16][17][18][19][20][21][22] : New aqueous SO 2 conversion mechanisms have been identified, and the kinetics of existing mechanisms have been updated.…”

Multiple Sulfur Isotopic Evidence for Sulfate Formation in Haze Pollution

“…Recently, sulfur isotopic evidence from SO 2 and sulfate showed that the H 2 O 2 pathway is important for sulfate formation during the first stage of haze event . In addition, the oxidation experiments and thermodynamic considerations and kinetics modeling also suggest that the H 2 O 2 pathway is enhanced during haze events due to the high solute strength of the aerosol particles and general acid catalysis effects . The H 2 O 2 in haze events can be produced from iron–HULIS complex photochemistry and aqueous microdroplets (1–20 μm in diameter) .…”

“…Recently, several new pathways for rapid sulfate production in haze events have been proposed to address this issue, including aqueous oxidation of SO 2 by NO 2 , , black carbon-catalyzed sulfate formation, transition metal ions-catalyzed oxidation of SO 2 , , the heterogeneous production of hydroxymethanesulfonate by SO 2 and HCHO, enhanced sulfate formation by nitrate photolysis, and fast SO 2 oxidation by H 2 O 2 in deliquesced aerosol particles. , However, the oxidation rate of SO 2 is strongly affected by aerosol acidity (pH), ionic strength, and aerosol liquid water content, , leading to large uncertainties in the mechanisms of sulfate formation mechanism during haze events.…”