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

Sulfate formation is arguably the most important chemicalprocessduring urban haze events in China; however, the complex mechanismof sulfate formation from SO2 remains unresolved. Recentstudies showed regionally transported sea spray aerosols (SSAs) andtransition-metal ions (TMI)-catalyzed SO2 oxidation aremajor contributors to sulfate in haze events near coastal regions.To better understand the role of SSA and TMI catalysis, we investigatedthe reaction kinetics of Mn2+-catalyzed SO2 oxidationin NaCl microdroplets using confocal Raman spectroscopy coupled witha relative humidity (RH) and trace gas control system. We demonstratedthat the reaction occurred on the droplet surface according to thelinear correlation between the reaction rate and the droplet radius.The saturation concentration for both Mn2+ (0.001 mol/Lat 1 ppm SO2) and SO2 (2 ppm at 10(-5) mol/L Mn2+) implies the existence of an upper limit ofthe SO2 uptake coefficient, which is intercorrelated withMn(2+) and SO2 concentrations. The uptake coefficientincreased with high ionic strength, while it remained constant whenthe pH ranged from 5 to 8. The uptake coefficient was on the orderof 10(-5) to 10(-4), which is 10 to100 times larger than that of the uncatalyzed autoxidation of SO2 and is overall higher than other mineral dust oxidation pathways.