A revisit of the interaction of gaseous ozone with aqueous iodide. Estimating the contributions of the surface and bulk reactions

Abstract

The main source of atmospheric iodine is the heterogeneous reaction of aqueous iodide (I⁻) with ozone (O₃), which takes place in surface seawater and probably in sea-salt aerosols. However, there are seemingly contradictory conclusions about whether this heterogeneous reaction occurs in the bulk of the aqueous phase, via O₃ dissolution, or at the aqueous surface, via O₃ adsorption. In this work, the ozone uptake coefficient has been calculated as a function of the concentration of aqueous iodide ([I⁻]_aq) and gaseous ozone near the aqueous surface ([O₃]_gs) by estimating parameters of the resistor model using results of previous studies. The calculated uptake coefficients suggest that the aqueous-phase reaction dominates at low I⁻ concentrations (about <10⁻⁴ mol L⁻¹), regardless of [O₃]_gs, and also at sufficiently high [O₃]_gs (about >80 ppm), regardless of [I⁻]_aq. In contrast, the surface reaction dominates at high [I⁻]_aq (about >10⁻⁴ mol L⁻¹) as long as [O₃]_gs is low enough (about <80 ppm). This trend is able to reconcile previous studies of this reaction, and is a consequence of several factors, including the high surface excess of both reactants ozone and iodide. Given the typical O₃ concentrations in the troposphere and the possible I⁻ concentrations and O₃ solubilities in sea-salt aerosols, the surface reaction may compete with the aqueous-phase reaction in accumulation-mode aerosols, unlike in surface seawater, where the aqueous-phase reaction probably prevails. The rate constant of the surface reaction has been estimated as (3–40) × 10⁻¹³ cm² molecule⁻¹ s⁻¹.