Air-Sea Gas Exchange in the Coastal Baltic Sea: Implications for Marine Carbon Dioxide Removal

[Geoengineering News]

https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2025JC023324

Authors: Ryo Dobashi, David T. Ho, Christa A. Marandino, Peter Schlosser

First published: 11 February 2026

https://doi.org/10.1029/2025JC023324

Abstract

Air-sea gas exchange affects the biogeochemical cycling of trace gases such as CO2 and dimethyl sulfide (DMS) on a global scale, thereby influencing Earth's climate. In nearshore regions, differences in wind fetch and surfactants are expected to have an impact on gas transfer velocity (k). Accurate determination of air-sea gas exchange in nearshore regions is crucial for assessing the efficacy of carbon dioxide removal (CDR) techniques, as many CDR methods are expected to be deployed in these regions. In this study, we used the 3He/SF6 dual tracer technique to determine k and investigate factors that control air-sea gas exchange in a nearshore inland sea ecosystem, the coastal Baltic Sea. We found that k was, on average, about 39% lower than in other coastal and offshore regions at the same wind speed, with a more pronounced reduction at higher wind speeds and during developing wave conditions. Most of the wind speed/gas exchange parameterizations proposed for the Baltic Sea were found to overestimate k. The lower k was likely due to wind fetch limitation, wind-wave interactions, and the presence of surfactants.

Plain Language Summary

Atmospheric CO2 is increasing and influencing the Earth's climate. Ocean uptake of atmospheric CO2 via air-sea gas exchange is one of the processes that moderates the increase in atmospheric CO2. In the context of marine carbon dioxide removal (mCDR), which aims to enhance oceanic CO2 uptake, robust monitoring, reporting, and verification (MRV) is essential and requires accurate quantification of air-sea gas exchange. In this study, we investigated air-sea gas exchange in nearshore regions, where many mCDR projects are likely to be implemented in the future. In the open ocean, air-sea gas exchange is frequently predicted by wind speed. However, the air-sea gas exchange measured in this study was lower than that observed under the same wind speeds in the open ocean. In nearshore regions, additional factors such as wind fetch limitations, wind-wave interactions, and surfactants influence air-sea gas exchange. These factors introduce complexity into the accurate quantification of ocean CO2 uptake in nearshore environments, necessitating measurements of air-sea gas exchange for robust MRV.

Key Points

Gas transfer velocities (k) in the coastal Baltic Sea are lower than in other ocean regions at the same wind speeds

The lower k is attributed to wind fetch limitation, wind-wave interactions, and the presence of surfactants

Robust monitoring, reporting, and verification of marine carbon dioxide removal will require precise measurements of k in nearshore areas

Source: AGU