Competitive Adsorption and Reaction at the Air-Water Interface studied by Iodide Ozonolysis in Microdroplets

The ozonolysis of iodide in seawater and sea-salt aerosol is a primary sink for ozone in the marine boundary layer and a major source of atmospheric iodine. While the chemical composition of the air/water interface has been shown to influence the overall chemistry in this system, it remains unclear to what extent the reaction occurs at the interface and how non-reactive solutes (i.e., surfactants and other salts) might alter the multiphase reaction mechanism of O3 with I-. Using a quadrupole electrodynamic trap (QET) and single-droplet paper-spray mass spectrometry, we examine the competition between solute adsorption and reaction at the air-water interface by measuring the ozonolysis kinetics of I- in aqueous microdroplets in the presence of surface-active chlorate ions (ClO3-). Iodide consumption kinetics depend upon both pH and the gas phase ozone concentration [O3], with a transition from zero to first-order kinetics (in [I-]) with increasing [O3]. To explain these observations, a kinetic model is constructed that accounts for reaction and mass-transport of both I- and O3 and the competitive adsorption of iodide and chlorate at the microdroplet surface. Under our experimental conditions the reaction occurs at the air-water interface, where significant depletion of both ozone and iodide produces the observed shift from zero to first order kinetics with increasing [O3]. Analytical expressions for surface concentrations are derived to accurately predict the reactive uptake coefficients obtained from experiments (γ = 2×10-4). These predictions are extended over a range of [O3] and [I-] to assess the impact of competitive adsorption on the multiphase reaction mechanism under more dilute reaction conditions.