Global reduction of in situ CO2 transfer velocity by natural surfactants in the sea-surface microlayer

For decades, the effect of surfactants in the sea-surface microlayer (SML) on gas transfer velocity (k) has been recognized; however, it has not been quantified under natural conditions due to missing coherent data on in situ k of carbon dioxide (CO2) and characterization of the SML. Moreover, a sea-surface phenomenon of wave-dampening, known as slicks, has been observed frequently in the ocean and potentially reduces the transfer of climate-relevant gases between the ocean and atmosphere. Therefore, this study aims to quantify the effect of natural surfactant and slicks on the in situ k of CO2. A catamaran, Sea Surface Scanner (S3), was deployed to sample the SML and corresponding underlying water, and a drifting buoy with a floating chamber was deployed to measure the in situ k of CO2. We found a significant 23% reduction of k above surfactant concentrations of 200 µg Teq l−1, which were common in the SML except for the Western Pacific. We conclude that an error of approximately 20% in CO2 fluxes for the Western Pacific is induced by applying wind-based parametrization not developed in low surfactant regimes. Furthermore, we observed an additional 62% reduction in natural slicks, reducing global CO2 fluxes by 19% considering known frequency of slick coverage. From our observation, we identified surfactant concentrations with two different end-members which lead to an error in global CO2 flux estimation if ignored.

I am writing to inform you that your manuscript RSPA-2019-0307 entitled "Global reduction of in-situ CO2 transfer velocity by natural surfactants in the sea surface microlayer" has been rejected in its present form for publication in Proceedings A.
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Detail Comment I think the authors need to improve their abstract. What are the main objectives of this study? Its is good if they can briefly mention the approach of their study.
The introduction is not well structured as well. The authors need to clearly mention the motivation for the study. What they already knew and what the want to cover for this study.
I suggest the authors improve their paragraphing in the Introduction.

Is the paper of sufficient general interest? Good
Is the overall quality of the paper suitable? Good Can the paper be shortened without overall detriment to the main message? Yes Do you think some of the material would be more appropriate as an electronic appendix? No

Recommendation? Accept as is
Comments to the Author(s) I am completely unpersuaded by the response of the authors and I think they are kidding themselves if they think the data in this paper is accurate. For instance, the authors use measurements of bulk-phase turbulence inside and outside their chamber as evidence that the presence of the chamber is not affecting the surface microlayer. However, it is well known that measurements of bulk phase turbulence do not accurately represent conditions within a few hundred microns of the water surface. So, sure, turbulence a few centimeters or tens of centimeters below the surface is the same inside and outside the chamber, but the authors have no way of knowing, and have provided no information, on whether the surfactant concentration in the surface microlayer is the same. However, I can't prove the chamber is affecting the microlayer and more than the authors can prove there is no effect.
Similarly, the authors' accusation that I am cherry-picking studies to support my case that their transfer velocities are outside what might be expected is insulting. Instead, they pick a study done from the very first time direct-variance flux measurements gave anything near a sensible answer as justification for their anomalous data. The point of picking the studies I did was they spanned a range of conditions and were done by different groups, but mostly it was simply that I happened to have those datasets in digital form at hand on my hard drive for plotting. Most importantly however, I could have picked nearly *any* credible open-ocean gas transfer dataset and it would have plotted almost on top of the data shown in the figure in my review. In contrast, the authors' data is clearly more that one sigma outside the variance. Again, however, I admit I cannot prove there is an issue with the gas transfer data presented in this manuscript. Just that it looks very different from nearly every other measurement of gas transfer made in the last 15 years.
Gas exchange research will survive yet another paper published with questionable data in it. This can be the new Smith and Jones.

Review form: Referee 2
Is the manuscript an original and important contribution to its field? Excellent

Is the paper of sufficient general interest? Excellent
Is the overall quality of the paper suitable? Excellent Can the paper be shortened without overall detriment to the main message? Yes Do you think some of the material would be more appropriate as an electronic appendix? Yes On behalf of the Editor, I am pleased to inform you that your manuscript entitled "Global reduction of in-situ CO2 transfer velocity by natural surfactants in the sea surface microlayer" has been accepted in its final form for publication in Proceedings A.
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Best wishes
Similarly, the authors' accusation that I am cherry-picking studies to support my case that their transfer velocities are outside what might be expected is insulting. Instead, they pick a study done from the very first time direct-variance flux measurements gave anything near a sensible answer as justification for their anomalous data. The point of picking the studies I did was they spanned a range of conditions and were done by different groups, but mostly it was simply that I happened to have those datasets in digital form at hand on my hard drive for plotting. Most importantly however, I could have picked nearly *any* credible open-ocean gas transfer dataset and it would have plotted almost on top of the data shown in the figure in my review. In contrast, the authors' data is clearly more that one sigma outside the variance. Again, however, I admit I cannot prove there is an issue with the gas transfer data presented in this manuscript. Just that it looks very different from nearly every other measurement of gas transfer made in the last 15 years.
Gas exchange research will survive yet another paper published with questionable data in it. This can be the new Smith and Jones.

Comments
Response This is an interesting paper but I have several main issues with it. Primarily I do not think the authors have sufficiently demonstrated the efficacy of the floating-dome method for this particular application. The main issue, in my opinion, is that when you seal a small area of ocean surface underneath the dome and isolate it from the wind stress, it could allow formation of a more stable and enriched surface microlayer. The situation would be somewhat similar to stirredtank or wind tunnel experiments, which were found to be sensitive to contamination from surfactants since the surfactants accumulate on the surface and are never mixed into the bulk. Going back through the literature on headspace measurements, I do not see a study where anyone has used a headspace chamber in the laboratory in a wind tunnel with and without surfactants. If such an experiment were conducted it would provide evidence that sealing the surface from the wind stress is not affecting the surface chemistry of the water. Then it could be concluded that the gas flux inside the chamber is the same as the gas flux through the water surface (such a demonstration is certainly not in the references 23 and 24 from the manuscript).
We thank for reviewer comment.
We would like to emphasize that we do not use a simple chamber/dome, but an advanced, fully autonomous and free-drifting buoy with technology to monitor and correct biases. Our chamber is small in size meaning that any turbulence passes the chamber without diminishing significantly at the oceanic regimes we deployed it, that has been proven by our continuous monitoring of turbulent kinematic energy under and outside the chamber's perimeter. We have shown in previous studies that the Turbulent Kinetic Energy (TKE) inside (TKEin) and outside (TKEout) were not statistically significant (Reference 10 and 29). It also proves that the chamber itself does not decrease turbulence as reviewer suggest. We would like to point out that we have described this in the original manuscript in lines 105-140.
Overall, we disagree that our data are questionable simply based on the chosen technique. We have designed our chamber with much care over a two-year period undergoing various field tests and quality assurance procedure (see Ribas-Ribas et al., 2018). Furthermore, the chamber technique is the only applicable technique for our study to investigate small-scale variations of CO2 air-sea transfer including a comparison between slick and non-slick areas. Neither Eddy covariance nor dual tracer (all with their own biases and challenges to apply on sea) can assess gas transfer velocity on sufficient small spatial and temporal scales to apply on slicks.
In an another review process for our recent published paper in Geoscience There is also an issue that the transfer velocity data presented in the manuscript is very different from previous measurements. Below is a plot I generated of the data from Mustaffa et al. (Figure 1 Thank you for the additional data compilation. It clearly shows the complexity of gas exchange and its measurement, despite the rather selective choice of data source by the reviewer. As pointed out in our manuscript, our data are similar with Donelan, M.A. & Drennan (1995) (Reference 40), which was ignored by the reviewer. We suggest that the high k660 in our study, exclusively observed in the western Pacific (see Supplementary Fig. S2), is due to the low resistance of air-sea CO2 transfer by lowest concentration of surfactants observed on a global scale (text line 198-201).
In addition, Salter's et al. data under clean conditions and higher wind speeds (> 8 m s -1 ) are occasionally as high as with the "gradient technique" at a wind speed of 3-5 m s -1 . We believe it is difficult to compare data obtained from different techniques without cross-validation. Also, Salter's gas transfer velocity varies from about 8 cm h -1 to 27 cm h -1 at similar wind speeds, clearly indicating It is clear that the Mustaffa et al. data is very much larger than the other datasets, which are in relatively good agreement. The authors do not even attempt to explain this huge increase in gas transfer velocity, but move on to study the effects of surfactants. I find this troubling since it appears there is an issue with their fundamental data, and to this reviewer anyway, it alone calls into question their conclusions.
that an apparent reversed mismatch to our observations has been found in the past, e.g., lower gas transfer velocities at higher winds speeds compared to our higher gas transfer velocities are lower wind speed. However, with our surfactant data in the SML, we can provide a reasonable mechanism for the occurrence of the high k660, i.e. low resistance of air-sea CO2 transfer due to very low surfactant concentrations in the western Pacific The reviewer claims above that the floating chamber technique underestimate gas transfer, but here the reviewer realized, correctly, that our data provide a new upper range for the parameterization of gas transfer. It seems like that both main concerns raised by the reviewer contradict each other.
Due to long history of the wind-based parameterization based on Wanninkhof, it is likely a bias exists as we know (due to personal communications with the SOLAS community) that larger gas transfer velocities have been also measured by others, but omitted for publication as the parameterization has been set as the "true" reference. New technology, like our direct and proven floating chamber technique and catamaran S 3 , provides new insights into the complexity of the process. We need to consider that existing parameterization has been based initially on lake data, forced through a zero intercept and a 14C global budget, and later indirect measurements (mainly Eddy covariance and dual tracer) has shown a wide range of deviation (also shown by the reviewer's plot).
Another problem arises from the reviewer's comparison. If we look at the data from Salter et al. comparing oleyl alcohol and "clean" SML, a significant reduction is questionable due to the highly variable data. That is odd, as laboratory studies clearly showed a very strong reduction of oleyl alcohol as monolayer on gas exchange, which seems to be missing in Salter's data. Anyway, oleyl alcohol will form a monolayer and cannot be comparable with natural SML or slick. We already mention this in the text line 297 -300:

"However, the insoluble properties of oleyl alcohol form a monolayer film that does not completely simulate a natural slick with its biofilm-like [7] and rheological properties [48], the latter through increased thickness (compared to non-slick SML or monolayers), and the presence of complex mixtures of soluble and insoluble surfactants [49]."
On a more metaphysical note, one thing I think would be good for the authors to explain is why they feel existing parameterizations need to be modified to account for surfactants. To my knowledge all of the commonly used parameterizations have been generated using field data. One thing that I think has been a true advancement in our understanding of the SML is that natural surfactants are ubiquitous in the ocean, seen everywhere anyone has measured them, with enrichment in the microlayer. The work of Frew et al. (2002, ref 15 in manuscript) shows that the effect of surfactants is seen at concentration levels well below the background surfactant concentration in the ocean. This implies that all the field measurements of gas exchange used to derive these parameterizations have included the effects of surfactants. It is not clear why an extra correction is required, and it would be good for the authors to discuss this effect.
We thank for the comment.
Our manuscript leads to a new thinking in a way that k parameterizations cannot be applied in a generic approach simply due to the occurrence of two end members in surfactant concentrations, i.e. low-surfactant regions (like the western Pacific) and slicks with extremely high surfactant concentrations.
Firstly, we highlight that slicks, a sea-surface phenomena of wave-damping, reduces the transfer of climate-relevant gases between the ocean and atmosphere. The phenomenon of slicks has been known for decades, but occurrence and dynamic behavior in size and shape has been an obstacle to assess its effect on airsea interaction. We developed the technology and combine different field studies to describe the role of slicks in the field of air-sea interaction with the aim that the occurrence of slicks will be considered in future field studies and models. We have found that at a threshold of 200 µg L -1 significant reduction of k occurs, but a further reduction occurs at concentrations 1000 µg L -1 (see Figure 2a and Figure 3b). We show in this paper that slicks with surfactant concentrations above 1000 µg L -1 reduces gas transfer velocities k660 for CO2 by 62% compared to ambient sea surfaces with concentrations of 200-600 µg L -1 , i.e. a further significant reduction above from the threshold of 200 µg L -1 . Reduction of k across slicks is the key finding of our manuscript, as clearly shown in Figure 4 in our manuscript. Considering that cyanobacterial slicks can, for example, occupy 20% of the Arabian Sea (Capone et al. 1997) implementation into Earth System Models of the effects by slicks is a necessary next step.
Secondly, the western Pacific Ocean is generally another extreme end member, i.e., with very low surfactant concentrations. As stated in our manuscript, wind-based parameterization has been developed elsewhere with typical surfactant concentrations exceeding those in the western Pacific at least two-fold. That means, the application of the parameterizations to regions of the western Pacific, or other low-surfactant regions, leads to an error of at least 20%.
For this reason, we disagree with the statement that effect of surfactants is seen below the background surfactant concentration in the ocean. At the time of Frew et. al (2002) surfactant concentrations in the SML were widely unknown (only occasional measurements were taken), and our study here provide the first surfactant data for the Pacific and identifying this region as low surfactant regime.
On a more technical note, the way the authors have plotted the data seems to done to obfuscate noise and issues with the data. (Figure 1 is an example of this, where previous gas exchange data are not shown). Figure 3 might be better if the k660 data were plotted as a function of wind speed, and then color coded as a function of surfactant concentration. Figure 5 might be more illustrative plotted in this manner as well. The benefit would be showing existing wind speed parameterizations of gas transfer velocity on the same plot would show the potential effect of including surfactants.
We did not intend to obfuscate noise and the data are available in PANGAEA data publisher as open access as outline in our manuscript (see references 59 and 60). In Figure 1, we intend to show the overall trend and for clarity we have binned the data, similar to McGillis et al. (2004).
In Figure 3, we intend to show the breaking point at 200 µg L -1 , and so we plotted against surfactants. Meanwhile in Figure 5, we intend to show the concentration of surfactant are influenced by different geographical location. Figure 5 is important for our claim that high transfer velocity occurred in the western pacific is due to low surfactant. However, we have added a new plot according to the reviewer suggestions to the supplementary material (see Supplementary Fig S2) autonomous drifting buoy in July 2017 in the Norwegian fjords and adjacent North Atlantic waters during cruise HE491.
In summary, in general I am supportive of work looking to understand the impact of surfactants on gas transfer. However, it is not productive to make sweeping claims on the role of surfactants using data that might be questionable (see figure above). The authors need to provide a far more convincing case that the headspace method provides reliable estimates of the transfer velocity, and that the values are not biased by an interaction between the windstress free surface under the headspace and existing surfactants. I recommend the manuscript be returned to the authors to deal with these issues, and final decision made after review of the revised manuscript.
Thank you for your support to understand the impact of surfactant on gas transfer velocity.
However, we disagree that our data are questionable simply based on the chosen technique. New techniques, or improving existing techniques, should not generally lead to the assumption that the data are questionable. We have published our technique and other data obtained with this technique, and, therefore, we don't understand why the reviewer request further convincing cases. Reviewer's data compilation does not justify to question our data as it shows also that low gas transfer velocity exists at high wind speeds (reverse from our observation).

Reviewer 2
The manuscript discusses the effect of surfactants in the sea-surface microlayer (SML) on gas transfer velocity (k) under natural conditions. An error of flux estimation in the western pacific is induced by applying wind-based parameterization not developed in low surfactant regimes. Reduction in natural slicks reduces the global flux of CO2 by 19%. Overall the study is very interesting and significant. The manuscript needs to be improved before it can be accepted for publication.

Comments
Response I think the authors need to improve their abstract. What are the main objectives of this study? Its is good if they can briefly mention the approach of their study.
We thank for the comment.
Our manuscript aims to compare the reduction of gas transfer velocity (k) between non-slick and slick condition. The novelty of our study lies in the fact that we bridge decades-old laboratory studies to the real environment allowing the implementation of slicks to global carbon models; rather than using lab studies on artificial monolayers. Finally, we present a nearly 20% reduction of global CO2 fluxes considering known frequency of slick coverage; representing a significant error if slicks are further ignored. Therefore, we revised our abstract by adding the main objective in line 18 -23: "Moreover, a sea-surface phenomena of wavedamping, known as slicks, has been observed frequently in the ocean and potentially reduces the transfer of climate-relevant gases between the ocean and atmosphere. Therefore, this study aims to quantify the effect of natural surfactant and slicks on the in-situ k of CO2. A catamaran, Sea Surface Scanner (S 3 ), was deployed to sample the SML and corresponding underlying water (ULW), and a drifting buoy with a floating chamber was deployed to measure the in-situ k of CO2." The introduction is not well structured as well. The authors need to clearly mention the motivation for the study. What they We thank the reviewer for the comment and we have improved paragraphing (in original already knew and what the want to cover for this study. I suggest the authors improve their paragraphing in the Introduction. manuscript line 26 -55) of our introduction accordingly (now in line 33 -67).
The motivation of our study was to present the first in situ data on air-sea CO2 transfer velocity in dependence to different levels of surfactants in surface films. In-situ measurements have been the most reasonable next step in understanding air-sea gas exchange processes as, and that is most critical, observations from the field are most credible to the implementation into global models.
We also included literature on slick in our introduction (in text line 39-45).
"Meanwhile, slick is a sea-surface phenomena of wave-damping effect by the excessive accumulation of organic matter. Slicks are frequently observed in the ocean [6] and potentially reduces the air-sea CO2 exchange by 15% [7] based on data obtained from artificial monolayers. Natural SML and slicks have not been well explored in past research programs that estimate the fluxes of CO2 into and out of the ocean. However, known bias of 20-50% in theoretical approaches [8,9] controlled tank [10][11][12] and field experiments involving artificial SMLs [13] justify observation under natural conditions." Line 276: name the study that considers the effect of slick on K660.
To our knowledge, there is no study considering the effect natural slick on the k660. However, we already included the study considering the effect of artificial slick on k660 in the text line 294-297: "For example, the field measurements using artificial slicks of oleyl alcohol, reducing microscaled turbulence under the surface by damping capillary waves, indicated suppression of k660 up to 30% and 55% at low (1.5 -3.0 m s -1 ) [46] and high wind speeds (6.9 -7.6 m s -1 ) [11], respectively."