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

05 July 2022, Version 1
This content is a preprint and has not undergone peer review at the time of posting.


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.


Heterogeneous Chemistry
Multiphase Chemistry
Surface Reactions
Atmospheric Chemistry

Supplementary materials

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


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