Stable Sulfur Isotopes Revealed a Major Role of Transition-Metal Ion-Catalyzed SO₂Oxidation in Haze Episodes

Abstract

Secondary sulfate aerosols played an important role in aerosol formation and aging processes, especially during haze episodes in China. Secondary sulfate was formed via atmospheric oxidation of SO2 by OH, O3, H2O2, and transition-metal-catalyzed (TMI) O2. However, the relative importance of these oxidants in haze episodes was strongly debated. Here, we use stable sulfur isotopes (δ34S) of sulfate aerosols and a Rayleigh distillation model to quantify the contributions of each oxidant during a haze episode in Nanjing, a megacity in China. The observed δ34S values of sulfate aerosols showed a negative correlation with sulfur oxidation ratios, which was attributed to the sulfur isotopic fractionations during the sulfate formation processes. Using the average fractionation factor calculated from our observations and zero-dimensional (0-D) atmospheric chemistry modeling estimations, we suggest that OH oxidation was trivial during the haze episode, while the TMI pathway contributed 49 ± 10% of the total sulfate production and O3/H2O2 oxidations accounted for the rest. Our results displayed good agreement with several atmospheric chemistry models that carry aqueous and heterogeneous TMI oxidation pathways, suggesting the role of the TMI pathway was significant during haze episodes.

Fast sulfate formation from oxidation of SO2 by NO2 and HONO observed in Beijing haze

“…Some studies have suggested that oxidation by NO 2 would be enhanced by fog 9,13,17 . Aqueous-phase autoxidation of SO 2 by molecular oxygen catalyzed by transition metal ions (TMI) has been proposed 22,23 but is poorly constrained due to the lack of information on TMI concentration, complexation, and solubility 24 . A recent study suggests that aqueous-phase oxidation by hydrogen peroxide (H 2 O 2 ) in haze could be significant 10 .…”

Effect of NO<sub>X</sub>, O<sub>3</sub> and NH<sub>3</sub> on sulfur isotope composition during heterogeneous oxidation of SO<sub>2</sub>: a laboratory investigation

“…It can be found that the δ 34 Saerosol values showed a~3.9 ‰ variation, and displayed a negative correlation with SOR. The variation of δ 34 Saerosol values was attributed to the isotope fractionation during the oxidation processes of SO2 (Li et al, 2020). To shed light on the mechanism of SO2 oxidation in sulfur isotope fractionation and sulfate formation, the SO2 oxidation processes on the surface of α-Fe2O3 in the presence of NOX, O3, and NH3 were carried out in laboratory.…”

“…The δ 34 S value of SO2 sources (δ 34 Semission) strongly depends on the origin of SO2, thus it is difficult to constrain . On the basis of the literature, +2.7±2.0 ‰ is used as the δ 34 Semission value during our sampling period (Li et al, 2020). Therefore, we used a Rayleigh distillation model and the isotopic enrichment factor (ԑ = (α-1)*1000‰) to explain the discrepancy between δ 34 S and δ 34 Semission values and quantify the contributions of each oxidant on the basis of the above results in the experiment.…”

“…Some investigations about sulfur isotope composition and fractionation have been performed for understanding the formation pathways of sulfate aerosols (Harris et al, 2013b;. However, most of these observational and modeling studies investigate the sulfur isotope composition in real atmosphere without physical boundaries (Han et al, 2016b;Li et al, 2020). Yet to date, a few experiments were performed in limited physical boundaries to explore the mechanism of sulfur isotope fractionation on the microscale (Harris et al, 2012a;Harris et al, 2012b).…”

Global modeling of heterogeneous hydroxymethanesulfonate chemistry

“…HiFA increases HCHO emissions from the transportation and residential sectors by a factor of 5. Chemical transport models commonly underestimate SO 4 2− during winter haze episodes in China, and thus some studies have adopted an apparent heterogeneous parameterization for SO2 reactive uptake in order to compensate for the missing SO 4 2− (Zheng et al, 2015;Cheng et al, 2016;Wang et al, 2016;Liu et al, 2019;Li et al, 2020).…”