the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 22 Apr 2026
Air-sea gas exchange in the central Baltic Sea
Abstract. Air-sea CO2 fluxes play an important role in the global carbon cycle and impact Earth's climate. Knowledge of the gas transfer velocity (k) is needed to determine air-sea CO2 fluxes, and wind speed-based parametrizations of k often perform well in the open ocean under moderate winds. In the Baltic Sea, several parameterizations have been proposed to estimate k, and they yield a higher k compared with parameterizations commonly used in the open ocean. In this study, we measured k in the Baltic Sea using the 3He/SF6 dual tracer technique to assess the applicability of published parameterizations in the inland sea ecosystem. Observed k was similar to those in offshore regions at the same wind speeds, even with enhanced surfactant activity. Comparison with observations in the nearshore Baltic Sea suggests that commonly used open ocean parameterizations are applicable in the Baltic Sea under moderate wind speeds and developed wave fields.
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Status: open (until 17 Jun 2026)
- RC1: 'Comment on egusphere-2026-1984', Anonymous Referee #1, 30 May 2026 reply
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RC2: 'Comment on egusphere-2026-1984', Anonymous Referee #2, 02 Jun 2026
reply
The manuscript addresses parameterisation of gas exchange, an area being in focus for long - without being clearly resolved. It is important to address various types of basins, in this case an inland sea with lower salinity, being valid in addition to other studies.
There are some limitations to the manuscript, the description of the observations are to some extent limited - making the evaluatin of the results difficult. For example it would have been interesting to see a more clear comparison between the dual tracer data and the eddy covariance data from the study. The integration over time and space of dual tracer data (and the impact on such a comparison) should be included.
What are the variations in forcing parameters (wind speed, wave conditions and temperature) during each observational period with the dual tracer.
The discussion with respect to other forcing factors is too limited. For bubbles it is suggested that limited fetch inhibited wave development, but information about fetch (or wind direction) is not provided.
Surfactant activity is mentioned, I find one value given, bot no variation during the experiment. This would also have been a very interesting information.
it is stated in section 4.3 that k-U is different in the near-shore and central Baltic Sea but I cannot see the results supporting this statement.
Citation: https://doi.org/10.5194/egusphere-2026-1984-RC2
Data sets
Dataset: Sulfur hexafluoride and helium data from a tracer release experiment conducted in July 2022 in the central Baltic Sea during research cruise EMB295 on the R/V Elisabeth Mann Borgese David T. Ho and Ryo Dobashi https://www.bco-dmo.org/dataset/988658
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- 1
The manuscript by Dobashi et al. reports on gas exchange in the Baltic Sea. The authors report that wind-based parameterizations for the open ocean are also applicable to the inland sea that is the Baltic Sea. Since this is not always clear in scientific literature, and the parameterizations themselves contain uncertainties due to the complexity of gas exchange, these types of studies are certainly relevant. To capture the complexity of gas exchange, the authors measured additional important parameters: the concentration of surface-active substances at the sea surface, trubulence with ADCPs and the properties of air bubbles in the upper water column; the former influences gas exchange in relatively calm seas, and air bubbles rising in the water column during rough seas with breaking waves.
Main comments:
Line 15: Eddy covariance was also used, correct?
Since two techniques were used here to measure k—dual tracer and eddy covariance—the results from both should be compared directly, rather than just the coefficient of variation of the root mean square error.
The locations of the ADCP measurements are shown graphically in the figure 2, but the ADCP data itself is not presented or discussed. What effect did currents and TKE have on k, as well as on other parameters such as air bubbles and surfactants? These relationships should be illustrated, as they could provide a very good representation of the complexity of gas exchange.
Why was the camera used to detect air bubbles on only one day for a limited time? Technical issues? It should be briefly described for clarity, as reader may And wouldn’t one expect breaking waves—and thus air bubbles—at a wind speed of 6-7 m/s?
Line 112: Sampling surface films is problematic using various techniques due to the varying but critical film thicknesses. Sampling from large ships, even from the bow, is also problematic because of the ship’s pitching in rough seas; and I suspect that sampling was carried out from the ship precisely under those conditions.
Line 160: When calculated according to TEOS-10, the salinity must be expressed in units (i.e., g/kg). Even when calculated according to the older 1978 standard, salinity values should be expressed in PSU.
Line 163: Were surfactants concentration measured during this study or taken from Karnatz et al (2025). The authors need to explain why concentration are in a rather narrow range despite wide wind speed range
In Section 3.2, the results are already being compared with those of other studies and should therefore be moved to the discussion section. The authors should rather present the results in detail, e.g., a comparison of dual tracer and eddy covariance methods. I find a direct comparison to be very relevant for this and future studies.
Line 236: What is number of observation?
Line 237/278: The authors describe the concentration of surfactants as elevated. What is the basis for this assessment? Gas exchange is reduced at significantly higher concentrations of surfactants, e.g., in the presence of surface films (slicks). There are references in the literature supporting this. For example, Mustaffa et al. present high concentrations, and line 250 is somewhat misleading ignoring extreme - but not rare - cases
Line 244: That didn't surprise me, since the study was based on a single water mass (due to the fact that it was following the gas plume). The authors should address this in their discussion, as gradients in biological activity—or upwelling zones, rainfall, and other factors—would play a role in transects between coastal waters and the open ocean. So it should read "in this study" and not "in this region".
Line 264: The authors should briefly discuss the fundamental differences between the parameterizations from both Wanninkhof (1992) and Kuss et al. (2004), compared to Wanninkhof et al. (2009) and Takahashi et al. (2009). That may revela a more mechanistic understanding of biases.
Overall, the manuscript is relevant but requires major revisions.