Literature data suggest that the main species controlling the deposition velocity of ozone to seawater is iodide [Chang et al., 2004], with organic species contributing to a much lesser extent, because of the lower kinetic rate constants (compared to iodide), combined with low (sub-nanomolar) concentrations. For example, the second-order kinetic rate constants for the reaction of ozone with alkenes and with iodide are 2–8 × 105 M−1 s−1 and 2 × 109 M−1 s−1, respectively [Dowideit and von Sonntag, 1998; Garland et al., 1980; see Chang et al., 2004, Table 1]. Because alkenes are present at sub-nanomolar concentrations, and iodide concentrations are of the order of 50–200 nM in seawater, the contribution of iodide to the ozone chemical loss rate is ∼106times higher than that of alkenes. Although alkenes are certainly representative of non-methane hydrocarbons produced in seawater, these gases represent only a minimal fraction of the overall content of organic material in seawater. The marine DOM pool in seawater, present at concentrations of 40–80 μM DOC [Hansell et al., 2009], is composed of a wide variety of compounds. A high but variable fraction of these comprises unsaturated and aromatic moieties, all potential sinks for atmospheric ozone. Although the contribution of these individual chemical species to ozone uptake is low (because of their low concentrations), their combined effect is shown here to be significant, and comparable to that of iodide. The more “empirical” approach followed in this study considers the contribution of DOM as a whole, rather than the contribution of individual chemical species present at sub-nM concentrations. This likely explains why our results indicate a more significant role for organic material in ozone deposition than estimated previously. However, it should be noted that because of the higher degree of unsaturation in Suwannee River DOM with respect to marine DOM, our results might overestimate the effect of organic matter compared to that of iodide. The approach used in this study does not allow determination of the reactivities of individual compounds. The value of 0.012 cm s−1 determined for chemical ozone deposition velocity for average ambient concentrations of iodide (150 nM) and DOM (80 μM DOC) in this study lies in the lower part of the range of deposition velocities measured at sea (0.01–0.1 cm s−1 [Fairall et al., 2007]). However it should be noted that whereas we measure the sole effect of the addition of specific chemical species to water, the overall deposition velocities measured at sea include a combination of chemical, as well as physical (e.g., wind speed) effects. On the other hand, values of deposition velocity determined in our experiments at values of 3–6 mg L−1 DOM (which correspond to 120–250 μM DOC, typical of coastal waters [Cauwet, 2002, and references therein]) compare well with those determined using a similar setup by McKay et al.  for UK coastal sites. Our values also compare well with the range (10−5 to 10−3 m s−1) estimated by Clifford et al.  for the contribution of chlorophyll a to ozone deposition velocity to oceanic surfaces, but are significantly lower than those estimated for humic acid films (0.4–0.9 cm s−1 [D'Anna et al., 2009]).