Ozone dry deposition to the sea surface microlayer

Project Information

O3-SML

Grant agreement ID: 833290

DOI

10.3030/833290

Project closed

EC signature date 9 July 2019

Start date 1 October 2019

End date 30 September 2025

Funded under

• EXCELLENT SCIENCE - European Research Council (ERC)

Total cost

€ 2 479 602,00

EU contribution

€ 2 479 602,00

Coordinated by

UNIVERSITY OF YORK
United Kingdom

Periodic Reporting for period 4 - O3-SML (Ozone dry deposition to the sea surface microlayer)

Reporting period: 2024-04-01 to 2025-09-30

Summary of the context and overall objectives of the project

The sea surface microlayer (SML) – the top few millimetres of the ocean surface - is a critical yet little understood interface which covers more than 70% of the Earth’s surface. A key interaction of the SML with the atmosphere is via ozone (O3), which is reactive with chemicals present at the surface of the ocean. Estimates of O3 deposition velocities (vD) vary widely, in large part from a lack of direct field measurements of oceanic O3 fluxes, and translate into large differences in the predicted ocean dry deposition flux. Crucially, the lack of co-measured oceanic biogeochemical information means that current observations cannot be used to disaggregate the variability in vD that arises from chemical, physical or biological processes.

The deposition velocity of O3 is significantly enhanced by chemical reactions in seawater, particularly reactions involve iodide (I-) and organics. However, in the absence of more definitive observed values, most atmospheric chemistry models use a constant or near-constant vD. The nature and reactivity of the organic material in seawater which interacts with O3 is unknown. Interactions of O3 with the SML not only determine how quickly O3 may be irreversibly taken up at the ocean surface, but also influence the re-emissions of other reactive trace gases out of the oceans that go on and affect gas phase tropospheric O3 processes and/or constitute a source of secondary organic aerosols (SOA) to the marine atmosphere.

Tropospheric ozone is a significant climate gas in addition to having a major influence on air quality, public health, on the photochemical processing of atmospheric chemicals, and on food security and ecosystem viability. Dry deposition of O3 to the Earth’s surface is estimated to account for about 30% of overall tropospheric O3 removal, however losses to the ocean SML, which is believed to be the largest single sink, are subject to much greater uncertainties than deposition to land. Model calculations show that ocean dry deposition has the potential to reduce surface O3 mixing ratios by several ppb, which is of a magnitude where it can influence human exposure and impact on ecosystems and agricultural crop yields.

The major aim of “O3-SML” is to obtain substantial new insight, via lab and field observations, into O3 fluxes over the Earth’s oceans coupled with new fundamental understanding of its major biogeochemical controls and to translate these into policy relevant conclusions through the use of numerical models. This work will provide the first definitive constraint on O3 dry deposition over the ocean and an improved numerical representation for chemistry-transport models. A secondary objective is to better quantify the trace gas fluxes out of the ocean that arise from oceanic O3-SML interactions and assess their atmospheric importance, in comparison to other mechanisms for oceanic VOC production.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

WP1:
Demonstrated the impact of dissolved organic material on marine iodine emissions: Tinel, L. et al., 2020 https://pubs.acs.org/doi/abs/10.1021/acs.est.0c02736.
Measured the second-order rate coefficient for the ozone-iodide reaction, reducing the uncertainty and dramatically revising the knowledge of the temperature-dependence: Brown, L.V. et al. Atmos Chem. Phys., 24, p. 3905–3923, 2024. A runner up in the 2024 ACP Paul Crutzen Publication Award
Proposed a framework for understanding how the chemical DOM system interacts with that of iodide and together translates into ozone deposition: Brown , L.V. et al. Biogenic organic matter is a major control of oceanic ozone deposition, Paper in preparation.
Produced improved sampling methods for sampling surfactants: PhD thesis, “Surface Tension of the Ocean and its Relationship with Environmental Variables”, University of York, submitted September 2025.

WP2:
Obtained a coastal ozone flux dataset of more than one year duration from eddy covariance (EC) measurements : Loades , D.et al., https://amt.copernicus.org/articles/13/6915/2020/
Assembled, tested and deployed a sensitive ship-borne EC system: Stapleton, C. et al., ‘Shipborne Eddy Covariance Measurements of Oceanic Ozone Fluxes’, AGU Fall Meeting 2023,
Obtained a > year-long ozone flux (by EC) time series at Tudor Hill, Bermuda: Drysdale, W. et al. Measurements of Oceanic Ozone Deposition at Two Coastal Sites, AGU Fall Meeting 2023,

WP3:
Provided a perspective on global iodine biogeochemical cycles and the potential for future climate-induced changes in oceanic iodide: Carpenter, L.J et al. Proc.Roy. Soc.A: Mathematical, Physical and Engineering Sciences, 2021
Contributed further understanding of dissolved iodine species in the coastal ocean: Jones, M.R. et al. Frontiers in Marine Science, 11, 18 p., 1277595, 2024.
Found that the chemical uptake of ozone to underlying water is controlled by both iodide and organics: Yang, M., et al. Geophysical Research Letters, 52, 12, 2025. e2024GL113187.
Measured concentrations of unsaturated fatty acids, known to react rapidly with ozone, in the SML and underlying seawater: Weddell, K., et al. Dissolved fatty acids and potential degradation products in coastal and oceanic sea-surface microlayers, Paper in preparation.
Developed an improved technique for dissolved iodine analyses: Jones. M.R. et al. Analytica Chimica Acta, 1239, 11 p., 340700, 2023.

WP4:
Demonstrated that the photochemical release of molecular iodine from iodide in surface snow could provide an iodine emission flux to the Arctic atmosphere comparable to oceanic fluxes: Brown, L.V. et al. Faraday Discussions, 2025, doi: 10.1039/d4fd00178h
Developed a novel technique for the calibration of hypoiodous acid (HOI) and successfully measured ambient HOI and I2 using bromide-CIMS: Marden, L.M. et al., Submitted to Atmospheric Measurement Techniques,, 2025

WP5:
Incorporated a new scheme for oceanic ozone deposition into GEOS-Chem: Pound, R. J., et al. Atmos. Chem. Phys., 20, 4227-4239, 2020.
Highlighted the role of iodine in atmospheric chemistry : Pound, R., et al. Geophys. Res. Lett. . 50, 8, 11 p., e2022GL100997, 2023.
Developed a coupled surface microlayer box model: Pound, R. J. et al. https://doi.org/10.5194/acp-24-9899-2024
Proposed a revised two-layer scheme to represent oceanic ozone deposition: Stapleton, C. G. et al. Oceanic Ozone Deposition: A Revised Scheme Constrained by Eddy Covariance Measurements AGU Fall Meeting 2023, Poster No. 1826, id. OS43D-1826

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

Demonstrated a negligible positive temperature dependence of the reaction rate between ozone and iodide in seawater within the range of conditions found in the ocean.

Showed that the photochemical release of molecular iodine from iodide in surface snow could provide an iodine emission flux to the Arctic atmosphere that is comparable to oceanic fluxes, and is driven by deposition of iodine to the snow.

Developed a new model which couples oceanic ozone dry deposition with inorganic chemistry in the SML to predict inorganic iodine emissions.

For the first time, developed an experimental calibration technique for HOI that does not rely on theoretical calculations.

Showed that control by biologically-produced marine surfactants contributes to ozone dry deposition These results offer a new paradigm for modelling oceanic ozone dry deposition.

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