Chemical Drivers of Oceanic Ozone Uptake – Iodide vs Surfactants

Due to its position at the air-sea interface, the sea-surface microlayer (SML) modulates the exchange of gases, including the deposition of ozone to the ocean. While ozone deposition to the ocean is a large sink of ozone from the troposphere, the processes involved are not well understood. Previous work has focussed on seawater iodide as a driver of ozone uptake to the ocean, however the SML contains a complex mixture of organic material, which could also impact ozone uptake. The contribution of this organic material to ozone uptake remains particularly unclear.

During this project, ozone uptake to seawater was measured by eddy covariance from coastal towers near Penlee Point (Plymouth, UK) and Tudor Hill (Bermuda), and at sea aboard the RV Atlantic Explorer, operating at and around the Bermuda Atlantic Time-series Study site in the Sargasso Sea. Additionally, the chemical component of ozone uptake to seawater was measured using a flow reactor during a trans-Atlantic cruise. This suite of observations has been combined to investigate the driving forces of oceanic ozone uptake. We present data that demonstrate that iodide was not a strong predictor of ozone uptake, despite its fast chemical reaction with ozone and the ubiquitous presence of iodide in the surface ocean.

Organic compounds in the SML are of interest to this work because some organic compounds have ozone-reactive functional groups. An example of this is carbon-carbon double bonds, present in some oceanic fatty acids. By increasing chemical reactivity, organic material can therefore augment ozone uptake to the ocean. The contribution of chemical reactions between ozone and organic material to ozone uptake was investigated using the kinetic multilayer model of surface and bulk chemistry (KM-SUB). A simplified system of a monolayer of an unsaturated fatty acid (oleic acid) over seawater was modelled and demonstrated that a monolayer of ozone-reactive surfactants on the ocean surface could contribute substantially more to ozone uptake, compared to environmental levels of aqueous iodide.

This work indicates that the commonly applied iodide-based parameterisation for ozone uptake to seawater may not accurately represent the chemical processes involved in ozone deposition to the sea surface. This has implications not only for predicted spatial and temporal variations in the magnitude of ozone deposition, but also for the chemical profile of oxidised gases emitted from the sea surface to the remote marine troposphere.