Strong Acceleration of SO₂ Autoxidation at the Subsurface of Deliquesced Aerosols with Surface-Active Organic Acids

Although the uncatalyzed oxidation of SO₂ by O₂ (autoxidation) is a ubiquitous SO₂ conversion pathway in the troposphere, its atmospheric significance has long been overlooked due to its low reaction rate. Here we show that SO₂ autoxidation is largely accelerated by surface-active organic acids such as dicarboxylic acids (DCAs) in deliquesced organic aerosol particles. Specifically, the sulfate formation rate in DCA particles (i.e., glutaric, succinic, and malonic acids) is 2-3 orders of magnitude higher than in inorganic particles under identical buffered pH and ionic strength conditions, and is 4-5 orders of magnitude higher than in solutions. We find that the reaction rate is proportional to the surface-to-volume ratio (1/R) of individual deliquesced aerosols by TEM-EDS, suggesting that the aerosol surface is involved. Combining experimental results with theoretical simulations, we reveal that the enrichment of DCAs at the air-water interface of aerosols enhances SO₂ autoxidation by promoting the surface uptake of SO₂ and accelerating the oxidation of HSO₃^- by O₂ within a distinct subsurface region. These findings suggest that the subsurface of aerosols may serve as a previously unrecognized hotspot for multiphase oxidation of SO₂, and that organic-acid-accelerated SO₂ autoxidation can occur at rates comparable to the H₂O₂ oxidation pathway.