Negligible temperature dependence of the ozone–iodide reaction and implications for oceanic emissions of iodine

Abstract

The reaction between ozone and iodide is one of the main drivers of tropospheric ozone deposition to the ocean due to the ubiquitous presence of iodide in the ocean surface and its rapid reaction with ozone. Despite the importance of this sea surface reaction for tropospheric ozone deposition and also as the major source of atmospheric iodine, there is uncertainty in its rate and dependence on aqueous-phase temperature. In this work, the kinetics of the heterogeneous second-order reaction between ozone and iodide are investigated using conditions applicable to coupled ocean–atmosphere systems (1 × 10−7–1 × 10−5 M iodide; 40 ppb ozone; 288–303 K; 15.0 psi). The determined Arrhenius parameters of A = 5.4 ± 23.0 × 1010 M-1s-1 and Ea = 7.0 ± 10.5 kJ mol−1 show that the reaction has a negligible positive temperature dependence, which could be weakly negative within errors. This is in contrast to a previous study that found a strong positive activation energy and a pre-exponential factor many orders of magnitude greater than determined here. The re-measured kinetics of ozone and iodide were used to constrain a state-of-the-art sea surface microlayer (SML) model. The model replicated results from a previous laboratory study of the temperature dependence of hypoiodous acid (HOI) and molecular iodine (I2) emissions from an ozone-oxidised iodide solution. This work has significance for the global modelling of the dry deposition of ozone to the ocean and the subsequent emissions of iodine-containing species, thus improving the understanding of the feedback between natural halogens, air quality and climate change.