Quantifying ozone deposition over the open oceans
Covering over 70 % of the Earth’s surface and holding roughly 97 % of the planet’s water, the world’s oceans support marine ecosystems, produce essential oxygen and regulate the climate. They also act as a natural sink for ozone (O3), a greenhouse gas and air pollutant that is harmful to human health, plant ecosystems, food security and the economy. This happens via dry deposition, the direct, non-precipitation transfer of atmospheric particles and gases onto the ocean surface. “Dry deposition to the ocean surface microlayer has the potential to reduce surface ozone mixing ratios by several parts per billion – a magnitude where it can limit human exposure and the impact the gas has on ecosystems and crop yields,” explains Lucy Carpenter, a professor of Atmospheric Chemistry at the University of York(opens in new window). With the support of the EU-funded O3-SML(opens in new window) project, Carpenter is leading an effort to quantify O3 deposition over the open oceans. “We aim to deliver new insights into oceanic ozone flux, a better understanding of its major biogeochemical controls, and, based on this, improved numerical representation for chemistry-transport models,” she adds. The oceanic surface microlayer (SML) consists of the top few millimetres of the ocean surface containing large chemical, physical and biological gradients that separate it from the underlying seawater. Oceanic O3 flux is the rate at which O3 is deposited into the SML.
A combination of laboratory experiments and field observations
To achieve its goals, the project, which received support from the European Research Council(opens in new window), used a combination of laboratory experiments and field observations. This included taking field measurements of oceanic deposition fluxes of O3 by eddy covariance, along with conducting flow tube studies of O3 uptake in seawater over a range of locations. “Our study was the first to combine these techniques with comprehensive observations of ocean biogeochemistry,” notes Carpenter. According to Carpenter, measuring eddy covariance O3 flux over the ocean was particularly challenging. Not only did they have to be done via ship, they had to be done with very little foundational research to build from. “I am very proud that the team gained the enhanced skills and expertise needed to successfully take such measurements over several research cruises and collect long-term data from two coastal stations,” she says.
Oceanic processes an important driver of deposition
While the full results of the project have not yet been fully realised, they are expected to fundamentally change the way oceanic O3 fluxes are parameterised in global atmospheric chemistry models. “These measurements have substantially increased the available observational estimates of O3 deposition fluxes and revealed that ocean biological processes are an important driver of deposition,” concludes Carpenter. Researchers are currently constructing a model framework for calculating O3 dry deposition over the world’s oceans, which will enable a more accurate assessment of its role in modulating tropospheric O3 concentrations.