13 Nov 2023
 | 13 Nov 2023
Status: this preprint is open for discussion and under review for Atmospheric Chemistry and Physics (ACP).

Negligible Temperature Dependence of the Ozone-Iodide Reaction and Implications for Oceanic Emissions of Iodine

Lucy V. Brown, Ryan J. Pound, Lyndsay S. Ives, Matthew R. Jones, Stephen J. Andrews, and Lucy J. Carpenter

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 were 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 Arrhenius parameters determined of A = 1.0 ± 4.6 ×1011 M−1 s−1 and Ea = 8.5 ± 10.9 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 ozoneoxidised iodide solution. This work has significance for global modelling of dry deposition of ozone to the ocean and the subsequent emissions of iodine-containing species, thus improving understanding of the feedbacks between natural halogens, air quality, and climate change.

Lucy V. Brown et al.

Status: open (until 25 Dec 2023)

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Lucy V. Brown et al.

Lucy V. Brown et al.


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Short summary
Ozone is deposited from the lower atmosphere to the surface of the ocean, however the chemical reactions which drive this deposition are not currently well understood. Of particular importance is the reaction between ozone and iodide, and this work measured the kinetics of this reaction, as well as its temperature dependence, which we found to be negligible. We then investigated the subsequent emissions of iodine-containing species from the surface ocean, which can further impact ozone.