Rapid Sulfate Formation via Uncatalyzed Autoxidation of Sulfur Dioxide in Aerosol Microdroplets

Abstract

Severe winter haze events in Beijing and North China Plain are characterized by rapid production of sulfate aerosols with unresolved mechanisms. Oxidation of SO2 by O2 in the absence of metal catalysts (uncatalyzed autoxidation) represents the most ubiquitous SO2 conversion pathway in the atmosphere. However, this reaction has long been regarded as too slow to be atmospherically meaningful. This traditional view was based on the kinetic studies conducted in bulk dilute solutions that mimic cloudwater but deviate from urban aerosols. Here, we directly measure the sulfate formation rate via uncatalyzed SO2 autoxidation in single (NH4)2SO4 microdroplets, by using an aerosol optical tweezer coupled with a cavity-enhanced Raman spectroscopy technique. We find that the aqueous reaction of uncatalyzed SO2 autoxidation is accelerated by two orders of magnitude at the high ionic strength (∼36 molal) conditions in the supersaturated aerosol water. Furthermore, at acidic conditions (pH 3.5–4.5), uncatalyzed autoxidation predominately occurs on droplet surface, with a reaction rate unconstrained by SO2 solubility. With these rate enhancements, we estimate that the uncatalyzed SO2 autoxidation in aerosols can produce sulfate at a rate up to 0.20 μg m–3 hr–1, under the winter air pollution condition in Beijing.

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

“…For aerosol droplets, increased humidity elevates its liquid volume and radius (Wu et al, 2018;Ding et al, 2019). 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).…”

“…Apart from the dust surface drivers and inhibitors, the sulfate radical (SO q − 4 ) may also participate in the heterogeneous event. In the absence of solar irradiation, laboratory studies discovered the non-catalysed SO 2 autoxidation by SO q − 4 at the acidic droplet interface (Hung and Hoffmann, 2015;Hung et al, 2018;Chen et al, 2022). With an appropriate humidity and sufficient saline component, there could be an air-liquid interface occurring over the solid dust surface via water uptake (Gaston et al, 2017;Tang et al, 2019;, producing SO q − 4 .…”

“…The oxidation of SO 2 was found to proceed at the interfacial layer of a droplet with higher kinetics than the bulk process (Jayne and Davidovits, 1990). Recently, the interfacial roles of O 2 (Hung and Hoffmann, 2015;Hung et al, 2018;Chen et al, 2022), NO 2 Yu, 2021), and Mn 2+ (H. Zhang et al, 2021; have been quantitatively described. Figure 12 compares the dust-driven heterogeneous pathway with the documented gas-liquid interfacial oxidations.…”

“…As reported by Hung et al (2015Hung et al ( , 2018, the acidic droplet interface favours the non-catalysed oxidation by the presence of sufficient SO q − 3 and SO q − 4 when pH < 4.0, which can be further explained by the structure differences of water at the interface and in the bulk phase. Recently, Chen et al (2022) reevaluated the sulfate formation by interfacial O 2 within the pH range of 3.5-4.5 and discovered the positive dependence on ionic strength. The oxidation of SO 2 by interfacial NO 2 displays a similar pH dependence to that which occurs in bulk solution.…”

Spatiotemporally Resolved pH Measurement in Aerosol Microdroplets Undergoing Chloride Depletion: An Application of In Situ Raman Microspectrometry

“…Next, the sample was loaded in a cell pending characterization. The relative humidity (RH, 70 to 85%) in the sample cell was controlled with a mixture of dry and humidified nitrogen gases. , After a sample was loaded, it took approximately 20 min for the RH in the cell to stabilize.…”