Directly measuring Fe(iii)-catalyzed SO₂ oxidation rate in single optically levitated droplets

Sulfate aerosols are produced in China's winter haze at an unresolved rapid rate. Such fast kinetics may arise from a heterogeneous SO₂ conversion in urban aerosols, which differs significantly from the aqueous S(iv) oxidation in bulk solutions. Given the uniqueness of aerosols as a multiphase reactor, it is preferable to measure the heterogeneous SO₂ conversion rate in situ, ideally in levitated microdroplets. Here, we directly measure the Fe(iii)-catalyzed SO₂ conversion in single microdroplets trapped and levitated with a gradient-force aerosol optical tweezer. The sulfate formation rate was inferred from the droplet's growth rate driven by the heterogeneous reaction. Our results show that the Fe(iii)-catalyzed SO₂ conversion in aerosols is 2 to 3 decades faster than that determined in bulk solutions. The SO₂ reactive uptake coefficient at pH ∼5.0 and 298 K is on the order of 10⁻⁴ to 10⁻³. The reaction rate scales with droplet surface area, indicating that the major reaction location is the air-water interface. This interfacial reaction is further corroborated by a positive kinetic salt effect, a trait of the interaction between ions and the neutral molecules, such as Fe(iii) ions and SO₂ molecules. The reaction rate decreases by up to a decade, as the Fe(iii)/S(vi) coexisting time increases, possibly owing to a complexation between Fe(iii) and S(vi) ions.