The authors have addressed the comments of the two referees in a satisfactory way in the revised version. Yet I have some minor comments and language issues that need to be improved before the manuscript is ready for publication. The comments are listed below.

Best regards
Christof Ammann
AMT Associate Editor


COMMENTS

p1 line 3 / 19: At both locations, a better explanation is necessary. It needs to be specified that (nocturnal) stable stratification does not generally violate the assumptions for EC measurement, but only under 'calm and stable conditions' (or 'very stable conditions') without fully developed turbulence. Windy stable conditions, which can also frequently occur during nights at many sites, are not problematic for EC measurements.

p1 line 10: change to "an area in the order of"

p1 line 16: Why is "methane" introduced here but not mentioned in the rest of the introduction? I suggest to leave it away and fully concentrate on CO2.

p2 line 12: Why do you mix/combine "Biometric approaches, including enclosure-based methods". These are two totally different approaches in my view with different advantages/disadvantages.

p2 line 20/21: I do not agree that going from the EC scale to a larger scale is generally the better choice than going to the smaller scale (chambers). This is not supported by existing literature and also not by the present study (that uses small chambers as a reference). Instead the authors should argue that going to the larger scale is an 'alternative option' to chambers with other advantages and disadvantages. I would even suggest not to stress the potential of the NBL budget as direct alternative to nighttime EC (or chambers), but rather as (the only) method being able to measure fluxes on a larger (landscape) scales.

p2 line 25: Clarify to "Any tracer emitted from the surface into the atmosphere..."

p3 line 5: I suggest to write "...at low instrumental costs." The instrumentation may be low cost with this method, but the manpower for operation over certain time periods is definitely not low cost (in contrast to an EC or chamber system that is run automatically). Maybe you also want to comment on that latter issue.

p3 line 14: Replace "creation" by "determination"; line 19: Replace accordingly "created" by "determined"

p3 line 16/17: The sentence "In the following ..." can be omitted.

p3 line 29: Change "sowed" to "sown"

p3 line 33: It would be good to indicate the position and extension of this grid in Fig. 1 or 2.

p3 line 33: "5 cm depth" occurs twice in the sentence. Omit one.

p7 line 30: Replace "lower than 150m" by "less than 150 m"

p8 Eq. 2: It is not clear what the subscript "SC" of x_SC(t) means here.

p11 line 10: What do "l1, l2" mean here. I think they are not defined in the text.

p11 line 14/15: The sentence "The sign of the daytime CO2..." is not necessary and a bit confusing at this position. I suggest to omit it.

p11 line 21: The sentence "In a stable NBL, however, little turbulent exchange takes place" is not really correct. Du to (intermittent) turbulent exchange, the CO2 emitted at the surface is mixed up to heights of e.g. 100 m as shown in Fig. 7. Probably the authors wanted to say that "little (or no) turbulent exchange takes place across the top of the NBL" ?

p13 line 21/22: I think it should read here "...with a Gaussian filter ..."

p14 line 10/11: I do not really see the sense of smoothing the height z, as long as the filter for the concentration profile is symmetric and a function of height (and not of number of available measurement points). Was that not the case here?

p18 line 27: The term "stationary measurements" could be misleading in the micrometeorological context (it is usually used for temporal stationarity of measurements). Better use e.g. "tower-based measurements"

Figure 9: It should be clarified in the figure caption that the HPB data was a measurement at a single height above the NBL, which is assumed representative for the entire (daytime) profile. In order to distinguish this data from the fully measured COPAP profiles, I suggest to plot the HPB concentration as a dashed line.

Figure 9: The figure caption statement "for each 1-minute sampling period of the mast" is not fully clear. Did you mean "...during the individual UAS flight" ?

p19 line 10/11: The statement "that the stable NBL retains most of the surface fluxes." is sloppy and needs rephrasing. E.g. "...retains most of the surface emitted CO2."

Table 2: List also the duration of the flights in this table.

p30 line 13: Better write: "Firstly, this would support ..."

In the author's response and the revised version, all my comments have been adressed in a satisfactory way. Therefore the manuscript is now accepted for publication.

Best regards
Chrsitof Ammann