Dear Authors,

I would like to thank you very much again for submission to AMT. Your manuscript underwent a thorough review process and both referees attest to the high quality of your revised submission. However, as pointed out in one of the reports, the grammar needs to be improved substantially. I therefore decide to publish subject to minor revisions (review by editor) and I ask you to consider all of the suggestions provided by the corresponding referee report as well as the comments that are detailed further below. I also have three minor topical questions that should be addressed in the revision.

1. Figure 2, which has been the subject of previous referee comments should be improved. At first sight it seems that scales of the axes on the right- and left-hand side are quite different, but as a matter of fact they are very similar. I therefore suggest that you improve readability of the graph by using units of 1E-10 1/cm on the right and on the left hand side and plot (absorption - 3164) E-10 1/cm on the left instead of just the absorption signal. To ease the mind of the spectroscopically aware reader and especially to allow quick location of eventual traces of the H218O peak, please also provide a second x-scale (x-axis on top) in wavenumber units. Finally, it is not immediately clear how the residual trace depicted in Fig. 2 has been obtained. Has the fit been applied directly to the absorption signal shown in the figure or to a background corrected version of the spectrum, and which fitting function has been used (Voigt profile, what about the empty cell signal, methane, baseline function, ... ) ?

2. The absolute numbers for the 2-day stability are certainly not critical for the outcome and conclusions of the paper, but the last data points of all four blue and red curves in Fig. 4 are likely artifacts and need to be verified and/or removed. At least, these points don't seem to be produced by a conventional Allan deviation analysis, which provides data points that are equally spaced on a logarithmic scale (at τ = Δt 2^n, where Δt = 8 000 s and n = 0, 1, 2, ...). While points at 8 000 s, 16 000 s, 32 000 s, 64 000 s and 128 000 s are shown correctly in Figure 4, the next point should be at 256 000 s, but it is at 172 800 s! Moreover, and this is even more puzzling on Figure 4, the y-values at t = 2 days are identical to the y-values at 128 000 s. This points to a critical problem in the analysis software or the production of the graph. Therefore, I need to ask you to carefully check your data using independently verified and freely available software, such as DATAPLOT from NIST or the avar function provided in the statistical software R. These software packages also provide error bars which for long integration times are currently missing in the graph.
On a related issue, when mentioning the stability over two days (lines 161-162), it would be more appropriate to use the maximum of the deviation curves between 10000 and 100000 s. Please also add "P. Werle, R. Mücke, and F. Slemr. The limits of signal averaging in atmospheric trace-gas monitoring by tunable diode-laser absorption spectroscopy (TDLAS). Appl. Phys. B, 57:131–139, 1993" to your list of references. These authors have brought the Allan variance stability analysis to the domain of environmental spectroscopy.

3. Five different standards are used. It would be convenient to summarise their isotope composition in one Table.

Wording & grammar suggestions
------------------------------------------------
(in the following, the abbreviation 'l.' is used to indicate a line number)

General comment: The use of commas and points in decimal numbers is confusing. For example, sometimes a comma is used to separate three digits (line 142 '8,000 s', l. 247 '50-1,500 ppm' ), sometimes it is not (eg. line 162 '1000 ppm', l. 241 'above 1000 ppm', l 264 '50 to 6500 ppm', etc. ). This even occurs on the same page of the manuscript and triggers the question whether the comma is a decimal comma or not. The general recommendation for scientific publications (see BIPM, IUPAC, etc.) is that thin spaces can be used to group digits and that 'neither dots nor commas are ever inserted in the spaces between groups'. I would like to ask you to comply with this convention.

l. 19-20 Please revise the last phrase 'The high finesse instrument demonstrates a stability up to two days of acquisition with a limit of detection down to 10 ppmv humidity for 𝛿D and 100 ppmv for 𝛿18O.' Please rephrase. Information given in the abstract should be concise and clear, but here one can only guess what a 2 day stability means if one has read the article and the numbers for the detection limits (δD, δ18O) are missing entirely.
l. 28 delete 'such'.
l. 31 'The CRDS method is commonly implemented by Picarro company, and gives a high stability through the measurement of the photon lifetime inside the optical cavity instead of the direct absorbed light.' If the main intention is to inform the reader on the physical principles, I suggest to write 'The CRDS method, which is commonly implemented by Picarro, achieves a high measurement stability through the ....'
l. 53 'This technique was first implemented for water vapour isotope analysis with a laboratory prototype (Landsberg et al., 2014) but never successfully deployed in the field, with stable working conditions.' -> 'This technique was first implemented for water vapour isotope analysis using a laboratory prototype under stable working conditions (Landsberg et al., 2014), but never successfully deployed in the field.
l. 54 Start phrase with 'In this paper, we present'
l. 67 Cite in text 'For a complete description of the ProCeas® system, the reader may refer to the recent article of Piel et al. describing the OF-CEAS spectrometer used for atmospheric O2 isotopes measurement (Piel et al., 2024).' -> 'For a complete description of the ProCeas® system, the reader may refer to the recent article of Piel et al. (2024) describing the OF-CEAS spectrometer used for atmospheric O2 isotopic measurement.'
l. 69 'Water isotopes OF-CEAS spectrometers use ...' -> 'The OF-CEAS spectrometers for the measurements of water isotopologues use ...'
l. 74 'retrieved from HITRAN database' -> 'calculated from the HITRAN 2020 database'
l. 78 'is performed' -> 'is achieved'
l. 81 'the shown spectrum' -> 'the spectrum'
l. 82 'Nitrogen' -> 'nitrogen'
l. 86 'in a various range of gas matrices (pure nitrogen, atmospheric dry air and finally synthetic air with a low water content).' -> 'for a wide range of different gas matrices (pure nitrogen, atmospheric dry air and finally synthetic air with a low water content).'
The titles of Figs 1 and 2 should be deleted. The second seems to be inappropriate as it is essentially the spectrum of a very dry cavity.
l. 94 'in 600 ms' -> 'within 600 ms'
l. 95 'correct' -> 'useful'
l. 96 'In order to keep a fast, real time data acquisition, the fitting algorithm is tuned so that most parameters are fixed.' -> 'In order to keep the data acquisition fast and in real time, the fitting algorithm is tuned by fixing most parameters.'
l. 113 'maximal' -> 'maximum'
l. 117 Start phrase with 'In this section we present'
l. 125 'from few hours' -> 'from a few hours'
l. 139 'two first' -> 'first two'
l. 143 8,000s -> '8000 s
l. 155 Use 'tau' instead of 't' in the white noise law.
l. 159 'arising along the laser to cavity optical path' -> 'arising along the optical path between laser and cavity'
l. 161 'After two days, we calculate an AD ...' -> 'At a delay of two days, we observe an AD ...'
l. 164 'hourly region' -> 'on the time scale of a few hours'
l. 177 Write 'In Figure 5, we present ...'
l. 200 'Indeed, large temperature variation' -> 'Indeed, large temperature variations'
l. 205 Replace 'lightly' by 'slightly' in Table 1, I likewise recommend to replace 'highly depleted' by 'strongly depleted'
l. 235 Start phrase with 'In Figure 6, we show'
L. 223 - 225 Use italics for mathematical symbols f, h.
l. 258 Avoid using the comma as a separator for
l. 271 'further away than the large water absorption peak' -> 'further away from the large water absorption peak'
l. 279 'linear, global' -> 'global linear'
l. 288 'inside' -> 'within'
l. 327 'we plotted the associated standard deviation after 24 hours σ(δi) as predicted by the Allan deviation study' -> 'we plotted the Allan deviation for 24 hours σ(24h) as predicted by the study in section xxx' If you want to indicate the delta values, use the subscript notation as in Fig. 4.
l. 328 use superscript for 18
l. 329 An Allan deviation is not a standard deviation: 'standard deviation' -> 'Allan deviation'. Please make according corrections also in the manuscript.
l. 335 'This threshold value is indicated on the figure by the horizontal dotted line' -> 'These threshold values are indicated on the figure by the horizontal dashed lines''
l. 340 'led' -> 'leads'
l. 330 'correct' -> 'proper'
l. 338 'that we should preferably consider 𝛿D to 𝛿18O in very dry environments.' -> 'that we should prefer acquisitions of 𝛿D over measurements of 𝛿18O in very dry environments.'
l. 342 'showed' -> 'shows'
l. 355 'We compare in Figure' -> 'In Figure , we compare'
l. 359 'This shows the ability for the OF-CEAS technique to capture with a high precision transient events' -> 'This shows the ability of the OF-CEAS technique to capture transient events at high precision'
l. 369 'on the Allan' -> 'by the Allan'
l. 376 'This leads for the OF-CEAS' -> 'This leads'
l. 394 replace by a more appropriate term the word 'global', or delete it.
l. 395 'The low humidity divergence' -> 'The divergence at low humidity'
l. 398 'to changing gas mixture can' -> 'to a changing gas mixture can'
l. 412 'and in particular possible methane contribution (see Figure 1) ...' -> 'and in particular a possible contribution from methane (see Figure 1) ...'
l. 416 'those'-> 'these'
l. 416 'Such artefacts are complicated to evaluate ...' -> 'Such artifacts are difficult to evaluate ...'
l. 422 'vapor'->'vapour', please check all instances
l. 423 'great interest for' -> 'great interest in'
l. 438 'through long-term' -> 'through a long-term'
l. 446 'like in-situ measurement of the isotopic composition of individual snowflakes.' -> 'like the in-situ measurement of the isotopic composition of individual snowflakes.'
l. 449 'The OF-CEAS analyser limitations highlighted in this article concern the instabilities encountered in the hourly range, which we attribute to parasitic interferences.'-> 'The OF-CEAS analyser limitations highlighted in this article are instabilities that develop at the time scale of a quarter-hour or so, which we tentatively attribute to the evolution of parasitic interferences.'

Dear Authors,

Thank you for the revised version of your manuscript which takes into account all previously raised questions. Please note that there are still three minor or very minor issues:

1. Figure 2.
The y-axis label should read "Absorption (10^-10 cm^-1) - 3416" to be dimensionally consistent.

2. Allan Variance
L171 "At a delay of two days, we observe an AD for δ18O of 1 ‰ at 100 ppm and 0.09 ‰ at 500 ppm (2.5 ‰ and 0.7 ‰
for δD at 100 ppm, and 500 ppm, respectively). For comparison, the maximum values of AD for δ18O between 104 and 105 s
are 1 ‰ (100 ppm) and 0.23 ‰ (500 ppm); for δD, 3.9 ‰ (100 ppm) and 1.3 ‰ (500 ppm). "
Since there are no data points at 48 h, nothing can be observed at a delay of two days. The text must be adapted accordingly (l 171, eventually also the text in the abstract). Moreover, the last few points at the end of an Allan Variance analysis are usually not very reliable and the observed levelling-off of the red AD curves might not be valid. If required, a safer way to extrapolate to 2 days would be to assume a linear drift of the instrument (slope = +1/2).
Finally, please show all data in Fig. 4 with error bars (or none). It is mostly the long tau-values that are interesting.

3. Reference Time
In the abstract, the discussion of the Allan deviation and the discussion of the instrument use in the AWACA project, different reference times are used. While it is 48 h for the former, section 3.1 (and Fig. 8) refers to 24 h. This is confusing. The presentation should either be homogenized or the choice of different reference periods explained better.

Dear Authors,

Thank you for submitting the revised manuscript. Given that you have taken into account all previously requested changes, I am glad to decide that the article be published subject to two technical corrections:

1/ Taking the scaling of the top axis in Fig. 2 as a reference, the frequency decreases from the left to the right. This is conflicting with the FSR (= 188 MHz) being a positive number and the frequency scaling on the bottom given in units of FSR. Please check the scaling of both axes (either the top axis has to be reversed or negative numbers have to be used on the bottom axis).
2/ replace 'two days' by 'almost two days' in line 153 of the corrected manuscript.

Please take into account the requested changes so that the manuscript can go into the next production stage.
This is also the occasion to thank you again for considering AMT/EGUSPHERE for publication of your scientific work.

With kind regards
Christof Janssen