Iodide oxidation by ozone at the surface of aqueous microdroplets

Abstract

The oxidation of iodide by ozone occurs at the sea-surface and within sea spray aerosol, influencing the overall ozone budget in the marine boundary layer and leading to the emission of reactive halogen gases. A detailed account of the surface mechanism has proven elusive, however, due to the difficulty in quantifying multiphase kinetics. To obtain a clearer understanding of this reaction mechanism at the air–water interface, we report pH-dependent oxidation kinetics of I⁻ in single levitated microdroplets as a function of [O₃] using a quadrupole electrodynamic trap and an open port sampling interface for mass spectrometry. A kinetic model, constrained by molecular simulations of O₃ dynamics at the air–water interface, is used to understand the coupled diffusive, reactive, and evaporative pathways at the microdroplet surface, which exhibit a strong dependence on bulk solution pH. Under acidic conditions, the surface reaction is limited by O₃ diffusion in the gas phase, whereas under basic conditions the reaction becomes rate limited on the surface. The pH dependence also suggests the existence of a reactive intermediate IOOO⁻ as has previously been observed in the Br⁻ + O₃ reaction. Expressions for steady-state surface concentrations of reactants are derived and utilized to directly compute uptake coefficients for this system, allowing for an exploration of uptake dependence on reactant concentration. In the present experiments, reactive uptake coefficients of O₃ scale weakly with bulk solution pH, increasing from 4 × 10⁻⁴ to 2 × 10⁻³ with decreasing solution pH from pH 13 to pH 3.