I am pleased to present this review of "Mechanisms of air-sea CO2 exchange in the central Baltic Sea" by Yuanxu Dong and co-authors. The paper presents a well-designed and well-executed field study of gas exchange in an interesting coastal region. Very few similar studies have been conducted, and most of those have occurred (for good reasons) in the open ocean. By focusing this study in the Baltic Sea, important new insights and clarifications about how we should be determining gas exchange in coastal waters are revealed. I thought the manuscript was technically solid. The field methods were well described, and utilized the current state-of-the-art. The choices of data analysis techniques were good, and reasonings used to interpret key factors like the impact of fetch-restricted wave fields and surfactant effects were well justified and acknowledged relevant uncertainties. I really don't have much negative to say about this paper... I think it will be an important contribution to the field, and I support its publication.

I have a few minor comments that I think the authors should consider addressing before publication:

Line 95: Wave parameters extracted from the ERA5 reanalysis product are an important component of this paper. I think the authors should explain a bit more the uncertainties in these products, and how that may affect their analyses.

Line 111: The authors choose to use a 10-minute flux averaging interval, but do not discuss the potential ramifications of this. I would assume it would result in some loss of low-frequency flux. Can this choice be better justified?

Line 190: is this u* value actually observed? Just previously, and after this, u* from COARE models are discussed. Can you clarify in the text whether this u* is coming from eddy covariance, or the COARE model?

Overall, I found it a bit strange that the dual-tracer results were included in this paper. They are not discussed at great length, and apparently there is another manuscript in review (or perhaps being drafted) that will deal with this in greater detail. I think it is worth keeping the measurements in, but maybe the authors could find a place to more explicitly explain why it is important to have this data in this study.

Line 218: Appendix A2 is about surfactants, not dual-tracers.

Line 224 and Line 236: qualitative statements about agreement between DT and EC fluxes are made... I find this a bit weak; there are clearly some DT points that fall below or above the EC fluxes. Can these statements be made a bit more quantitatively?

Line 273: I was confused by the statement "applying equation 3 to the equivalent wind speeds"... Equation 3 only includes u* and Hs as variables.

Section 3.6: The origin of some of the data is a bit unclear... in particular the source of the fCO2 data is not well explained.

Figure 7: Why not show us the computed flux? A lot of the text (eg Line 430-434) describes patterns in the computed flux, but this is not presented.

Line 490: There is a reference to a "bubble buoy" which I don't think is described anywhere else in the manuscript? What is this? Is it important to keep in?