Elucidating the Mechanism on the Transition-Metal Ion-Synergetic-Catalyzed Oxidation of SO2 with Implications for Sulfate Formation in Beijing Haze

GEHUI WANG, East China Normal University

     Abstract Number: 37
     Working Group: Aerosol Chemistry

Abstract
Currently, atmospheric sulfate aerosols cannot be predicted reliably by numerical models, because the pathways and kinetics of sulfate formation are unclear. Here, we investigated systematically the synergetic catalyzing role of transition metal ions (TMIs, Fe³⁺/Mn²⁺) in the oxidation of SO₂ by O₂ on aerosols using chamber experiments. Our results showed that the synergetic effect of TMIs is critically dependent on aerosol pH due to the solubility of Fe(III) species sensitive to the aqueous phase acidity, which is effective only under pH<3 conditions. The sulfate formation rate on aerosols is two orders of magnitude larger than that in bulk solution and increases significantly on smaller aerosols, suggesting that such a synergetic catalyzed oxidation occurs on the aerosol surface. The kinetic reaction rate can be described as R=k* [H⁺ ]^-2.95 [Mn(II)][Fe(III)][S(IV)] (pH≤3.0). We found that TMIs-synergetic-catalyzed oxidation is the dominant pathway of sulfate formation in Beijing when haze particles are very acidic, while heterogeneous oxidation of SO₂ by NO₂ is the most important one when haze particles are weakly acidic. Our work for the first time clarified the role and kinetics of TMIs-synergetic-catalyzed oxidation of SO₂ by O₂ in haze periods, which can be parameterized into models for future study on sulfate formation.