Modification of Ozone Deposition and I<sub>2</sub> Emissions at the Air–Aqueous Interface by Dissolved Organic Carbon of Marine Origin ShawMarvin D. CarpenterLucy J. 2016 The reaction between gaseous ozone (O<sub>3</sub>) and aqueous iodide (I<sup>–</sup>) at the surface microlayer (SML) is believed to be a major chemical contributor to the oceanic dry deposition of O<sub>3</sub> over open ocean waters and has also recently been shown to produce environmentally significant quantities of gaseous molecular iodine (I<sub>2</sub>). Here we investigate how this reaction is affected by the presence of dissolved organic carbon (DOC) of marine origin, using a heterogeneous flow reactor and detection of gaseous I<sub>2</sub> by solvent trapping and UV/vis spectroscopy. Ozone deposition measurements over coastal seawater implied an O<sub>3</sub> reactivity (λ) toward coastal marine DOC of ∼500 (420–580) s<sup>–1</sup>, 2–5 times higher than that toward iodide at typical ocean concentrations (∼0.5–1 × 10<sup>–7</sup> M). We added varying amounts of highly concentrated DOC extracted from coastal seawater to I<sup>–</sup> solutions (1 × 10<sup>–5</sup> M) such that the relative reactivities of DOC and I<sup>–</sup> toward O<sub>3</sub> (λ<sub>DOC</sub>/λ<sub>I</sub>) were in the expected range for natural seawater. The evolution of gaseous I<sub>2</sub> and the loss of aqueous I<sup>–</sup> both reduced as DOC concentrations increased, with an overall suppression of I<sub>2</sub> emissions of about a factor of 2 under conditions of λ<sub>DOC</sub>/λ<sub>I</sub> representative of open ocean waters (0.5–1). A kinetic model of the SML suggested that neither competition of DOC with I<sup>–</sup> for reaction with interfacial O<sub>3</sub>, nor direct loss of I<sub>2</sub> and hypoiodous acid (HOI) through reaction with increasing quantities of DOC, can fully explain these results. We conclude that the suppression of I<sub>2</sub> emissions by DOC is largely a physical effect arising from a decrease in the net transfer of I<sub>2</sub> from the aqueous to gas phase, as suggested by recent laboratory studies.