Ozonolysis of Aqueous Iodide at the Air-Water Interface Studied by Single Microdroplet Mass Spectrometry

ALEXANDER PROPHET, Kevin R. Wilson, Lawrence Berkeley National Laboratory

     Abstract Number: 198
     Working Group: Aerosol Physical Chemistry and Microphysics

Abstract
Iodide ozonolysis occurring in seawater and aqueous aerosol is a major sink for atmospheric ozone and a source of molecular iodine. The exceptionally fast reaction rate between ozone and iodide, together with the strong surface-propensity of aqueous iodide, suggests the importance of reactivity at the air-water interface in this system. However, previous reports differ in describing the surface composition and the relative contributions of surface and bulk reactions. Here, the ozonolysis of aqueous iodide in microdroplets is studied using a quadrupole electrodynamic trap (QET) coupled with single-droplet paper-spray mass spectrometry. A kinetic model is constructed to account for dynamic concentrations and reactions at the surface and in the bulk, leading to the production of aqueous and gas-phase products. This description allows for a detailed understanding of competitive surface-processes and their relation to the bulk that cannot be explained by widely used resistor model limiting cases. Experimental and modeling results indicate that surface-depletion of both ozone and iodide at the air-water interface critically determines the overall kinetics observed. Analytical expressions describing coupled reagent depletion at the interface are derived and applied to compute surface-specific uptake coefficients. A kinetic analysis of the results suggest that the experiments conducted, together with simulation, provide a novel approach to examining physical surface-processes such as desorption of ozone and solvation of interfacial ions.