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.

There is a large gap between the simulated and observed sulfate concentrations during winter haze events in North China. Although multiphase sulfate formation mechanisms have been proposed, they have not been evaluated using chemical transport models. In this study, the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) was used to apportion sulfate formation. It was found that Mncatalyzed oxidation on aerosol surfaces was the dominant sulfate formation pathway, accounting for 92.3 ± 3.5% of the sulfate formation during haze events. Gas-phase oxidation contributed 3.1 ± 0.5% to the sulfate formation due to the low OH levels. The H 2 O 2 oxidation in aerosol water accounted for 4.2 ± 3.6% of the sulfate formation, caused by the rapid consumption of H 2 O 2 . The contributions of O 3 , NO 2 oxidation, and transition metal ion-catalyzed reactions in aerosol water could be negligible owing to the low aerosol water content, low pH, and high ionic strength. The contributions from in-cloud reactions were negligible due to the barrier provided by stable stratification during winter haze events.

<p><strong>Abstract.</strong> Gaseous sulfuric acid is known as one of the key precursors for atmospheric new particle formation processes, but its measurement remains a major challenge. A proxy method that is able to derive gaseous sulfuric acid concentrations from parameters that can be measured relatively easily and accurately is therefore highly desirable among the atmospheric chemistry community. Although such methods are available for clean atmospheric environments, a proxy that works well in a polluted atmosphere, such as those in Chinese megacities, is yet to be developed. In this study, the gaseous sulfuric acid concentration was measured in February–March, 2018, in urban Beijing by a nitrate based – Long Time-of-Flight Chemical Ionization Mass Spectrometer (LToF-CIMS). A number of atmospheric parameters were recorded concurrently including the ultraviolet radiation B (UVB) intensity, concentrations of O<sub>3</sub>, NO<sub><i>x</i></sub>, SO<sub>2</sub> and HONO, and aerosol particle number size distributions. A proxy for atmospheric daytime gaseous sulfuric acid concentration was derived using a statistical analysis method by using the UVB intensity, [SO<sub>2</sub>], condensation sink (CS), [O<sub>3</sub>], and [HONO] (or [NO<sub><i>x</i></sub>]) as the predictor variables. In this proxy method, we considered the formation of gaseous sulfuric acid from reactions of SO<sub>2</sub> and OH radicals during the daytime, and loss of gaseous sulfuric acid due to its condensation onto the pre-existing particles. In addition, we explored formation of OH radicals from the conventional gas-phase photochemistry using ozone as a proxy and from the photolysis of heterogeneously-formed HONO using HONO (and subsequently NO<sub><i>x</i></sub>) as a proxy. Our results showed that the UVB intensity and [SO<sub>2</sub>] are dominant factors for the production of gaseous sulfuric acid, and that the simplest proxy could be constructed with the UVB intensity and [SO<sub>2</sub>] alone, resulting in up to 29 % relative deviations when sulfuric acid concentrations were larger than 2.0 × 10<sup>6</sup> molecules cm<sup>−3</sup>. When the OH radical production from both homogenously- and heterogeneously-formed precursors were considered, the relative deviations were lower than 24 %.</p>