Rapid Sulfate Formation via Mn²⁺-Catalyzed SO₂ Oxidation on the Surface of NaCl Microdroplets

Sulfate formation is arguably the most important chemical process during urban haze events in China; however, the complex mechanism of sulfate formation from SO₂ remains unresolved. Recent studies showed regionally transported sea spray aerosols (SSAs) and transition-metal ions (TMI)-catalyzed SO₂ oxidation are major contributors to sulfate in haze events near coastal regions. To better understand the role of SSA and TMI catalysis, we investigated the reaction kinetics of Mn²⁺-catalyzed SO₂ oxidation in NaCl microdroplets using confocal Raman spectroscopy coupled with a relative humidity (RH) and trace gas control system. We demonstrated that the reaction occurred on the droplet surface according to the linear correlation between the reaction rate and the droplet radius. The saturation concentration for both Mn²⁺ (0.001 mol/L at 1 ppm SO₂) and SO₂ (2 ppm at 10^-5 mol/L Mn²⁺) implies the existence of an upper limit of the SO₂ uptake coefficient, which is intercorrelated with Mn²⁺ and SO₂ concentrations. The uptake coefficient increased with high ionic strength, while it remained constant when the pH ranged from 5 to 8. The uptake coefficient was on the order of 10^-5 to 10^-4, which is 10 to 100 times larger than that of the uncatalyzed autoxidation of SO₂ and is overall higher than other mineral dust oxidation pathways.