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

Severe winter haze events in Beijing and North China Plain are characterized by rapid production of sulfate aerosols with unresolved mechanisms. Oxidation of SO₂by O₂in the absence of metal catalysts (uncatalyzed autoxidation) represents the most ubiquitous SO₂conversion 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 SO₂autoxidation in single (NH₄)₂SO₄microdroplets, by using an aerosol optical tweezer coupled with a cavity-enhanced Raman spectroscopy technique. We find that the aqueous reaction of uncatalyzed SO₂autoxidation 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 SO₂solubility. With these rate enhancements, we estimate that the uncatalyzed SO₂autoxidation in aerosols can produce sulfate at a rate up to 0.20 μg m^-3hr^-1, under the winter air pollution condition in Beijing.