OF-CEAS laser spectroscopy to measure water isotopes in dry environments: example of application in Antarctica

Lauwers, Thomas; Fourré, Elise; Jossoud, Olivier; Romanini, Daniele; Prié, Frédéric; Nitti, Giordano; Casado, Mathieu; Jaulin, Kévin; Miltner, Markus; Farradèche, Morgane; Masson-Delmotte, Valérie; Landais, Amaëlle

doi:https://doi.org/10.5194/egusphere-2024-2149

Preprints

https://doi.org/10.5194/egusphere-2024-2149

Preprints

15 Aug 2024

| 15 Aug 2024

OF-CEAS laser spectroscopy to measure water isotopes in dry environments: example of application in Antarctica

Thomas Lauwers, Elise Fourré, Olivier Jossoud, Daniele Romanini, Frédéric Prié, Giordano Nitti, Mathieu Casado, Kévin Jaulin, Markus Miltner, Morgane Farradèche, Valérie Masson-Delmotte, and Amaëlle Landais

Abstract. Water vapour isotopes are important tools to better understand processes governing the atmospheric hydrological cycle. Their measurement in polar regions is crucial to improve the interpretation of water isotopic records in ice cores. In situ water vapour isotopic monitoring is however an important challenge, especially in dry places of the East Antarctic plateau where water mixing ratio can be as low as 10 ppmv. We present in this article new commercial laser spectrometers based on the optical feedback – cavity enhanced absorption spectroscopy (OF-CEAS) technique, adapted for water vapour isotopic measurement in dry regions. We characterize a first instrument adapted for Antarctic coastal monitoring with an optical cavity finesse of 64,000 (ringdown time of 54 µs), installed at Dumont d’Urville station during the summer campaign 2022–2023, and a second instrument with a high finesse of 116,000 (98 µs ringdown), to be deployed inland East Antarctica. 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.

Received: 12 Jul 2024 – Discussion started: 15 Aug 2024

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Journal article(s) based on this preprint

06 Mar 2025

OF–CEAS laser spectroscopy to measure water isotopes in dry environments: example of application in Antarctica

Atmos. Meas. Tech., 18, 1135–1147, https://doi.org/10.5194/amt-18-1135-2025,https://doi.org/10.5194/amt-18-1135-2025, 2025

Short summary

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2024-2149', Harro A.J. Meijer, 28 Aug 2024

This is a well-written, interesting paper about the 'next level' OF-CEAS instrumentation. I liked reading it, and is carries important new information. I recommend publication, but/and have a set of comments and questions that I invite the authors to respond to, preferably by adapting the paper where appropriate, or else in writing to the editor (and me).
First a general question: how come it is so difficult to perform isotope measurements on water vapour at 10 ppm or even higher, while for example precise isotope measurements on methane (2 ppm) are routine nowadays? Is it because of the humidity range that has to be accomodated?
Then in more detail, going through the paper:
Figure 1 The methane interference can potentially be severe for low humidity (1/100 to 1/1000 of the humidity in this plot). Apart from this plot, this interference has not been discussed or even mentioned anywhere else. Is the methane spectrum always included in, and corrected for in the fits?
Figure 2 It is not clear to me how the residuals in cm-1 relate to the arbitrary normalized intensity (in arbitrary units). Even the x-axis is in free spectra range steps, not in cm-1. Please clarify.
Compared to figure 1, there is a flat baseline here in figure 2. How can that be, as all lines in figure 1 will be proportionally lower?
Line 169 and elsewhere: suddenly decimal commas instead of points. Happens more often, with the declaration of the values for reference waters.
Figure 5 the number of points for the Picarro instrument are much higher than for the AP2E one. Why? DIfferent strategy?
Table 1 should include the number of measurements (so much more for the Picarro I presume)
Lines 190 further. This humidity / mixing ratio dependence is widely observed, not only for water vapour measurements, but also for isotopes in atmospheric CO2. In that field, two ways of dealing with it are in use: the 'ratio method', so building ratios first, and then correct for mixing ratio dependence (this is what you do here), the other method is the isotopologue method, which would first analyze the different isotopologues as different species, calibrating first them, and only then build ratios. See for instance papers:
Flores, et al. Calibration Strategies for FT-IR and Other Isotope Ratio Infrared Spectrometer Instruments for Accurate delta C-13 and delta O-18 Measurements of CO2 in Air. Analytical Chemistry {89}, {3648-3655} (2017).
Steur et al. Simultaneous measurement of δ 13C, δ 18O and δ 17O of atmospheric CO2 – performance assessment of a dual-laser absorption spectrometer. Atmos Meas Tech 14, 4279–4304 (2021).

Both have their pros and cons.

Would that be something to try out here? Or at least to mention and discuss. Or is it not applicable at all in this context?
FIgure 8 The Allan deviation is only part of the final uncertainty: the humidty correction and the calibration error also contribute. While that is less important for tracking the diurnal cycle, the calibration error will influence the accuracy of the seasonal cycle.
Line 300 see above: the humidity dependence of dD is relatively speaking larger than that of d18O, and also it calibration uncertainty (fig 7) is, relatively, larger. Would that influence your conclusion?
Line 342 "applying a drying on the humidity generator" ?? unclear to me what you mean.
Lines 355-365 (and figure 6) I agree with your conclusion that while the gradual, linear dependence is indeed caused by a spectral 'misfit' (probably indeed the interference of the very strong lines further in the spectrum, or methane?), the low humidity part strongly indicates sample-to-sample memory effects. This is further supported by the fact that the depleted ref goes up, and the 'enriched' one goes down.

This effect will not fully vanish in the station, because (1) the residual water vapour is probably quite fractionated, and will thus still be different from outside humidity, and (2) you must calibrate your instrument regularly using two waters with very different isotope values.

I would like to see more discussion of these low-humidity part effects (after all, that is the truly innovative part of the instrument and your paper): for example, are the values (in fig 6) influenced by the 'sample history' before one of the reference waters and how would that be different in the station? Would you expect that sample water vapour with isotope values intermediate between the 'high' and 'low' ref waters scale linearly between them (as you suggest in lines 250-253)? What is the added uncertainty in this region?
Line 364-365: 'We insist thus on the importance of calibrating the instrument in the field to correct for those artefacts (Casado et al., 2016).' Indeed! May be change the word 'insist' into 'emphasize' ?
line 371-372 "The internal architecture of these analysers therefore reduces the risk of breakdowns during the deployment, but requires an expertise to finely tune them. "

To me this feels like a blessing in disguise, or may be just the exact opposite. Deploying these instruments thus always requires expertise (and time and some equipment) in the field. Any more comments to that? For example negative field experience with fixed mounted equipment such as the Picarro's ?

"Competing interests. The authors declare that they have no conflict of interest."
Two of the co-authors work with the (commercial) producer of the instrument. How do they avoid a conflict of interest?

Citation: https://doi.org/10.5194/egusphere-2024-2149-RC1
- AC1: 'Reply on RC1', Thomas Lauwers, 18 Oct 2024
  
  We thank Harro Meijer for his very pertinent questions concerning the general form of the manuscript as well as the scientific content, which allow us to be exhaustive on the instrumental limits presented in the paper, and give us interesting perspectives for future studies. The answers are in the file attached: they are inserted in the text in red and the citations from the manuscript are indicated in blue.
  
  Citation: https://doi.org/10.5194/egusphere-2024-2149-AC1
RC2:
'Comment on egusphere-2024-2149', Anonymous Referee #3, 10 Sep 2024

The manuscript titled "OF-CEAS laser spectroscopy to measure water isotopes in dry

environments: example of application in Antarctic" by Lauwers et al. is a much needed study on a promising technique to measure water vapor isotope composition at very low humidity. The manuscript includes all of the information one would hope for to employ their method and requires only minor revisions before publication. Indeed, the authors would go a long way towards publication readiness by clarifying the text and refining the grammar.

General comments for clarity:
- Make the figures and tables, including their captions, more independent of the main text. Telling a reader the same information twice (e.g., in the figure and in the main text) helps with clarity.

Instrumentation description:
- I expect this study and instrumentation to be popular among water isotope laboratories and researchers. As such, please describe the specific model of AP2E used. Or, if all three are custom, please indicate so. Consider including a brief description of how these particular OF-CEAS instruments differ from those in Casado et al., 2016. Are they the same?
- Use of the description "V-shape" does not help the reader understand the instrument. Perhaps it would be more informative to say something about the lack of fiber optic cabling in favor of highly reflective mirrors?

2.1.2 Long-term stability at Dumont d’Urville station:

- Do the authors expect the variable water vapor concentration requiring "An additional filtering has been applied to remove the points with a non- stable humidity, i.e. with a humidity value standing outside the 2-𝜎 interval." is due to the LHLG unit or to the OF-CEAS instrument itself?

Figure 5:
- De-emphasize the green data by making it a lighter shade and placing them behind the OF-CEAS data.
- I suggest expressing the y-axes as residuals in the same way the authors have already done with Figs 3 and 6. The wide range that must necessarily be used when expressing the values in their absolute terms may hide drift or biases.
- The caption sentence "Each point represents the average of the final minimum 5 minutes of 1000 ppm humidity plateaus and the error bars correspond to the associated standard deviation" is unclear. Does "minimum" imply that sometimes it is longer? Be specific.
- Why would "plateaus" exist with this particular dataset? The caption currently reads as if all of these data were collected at 1000 ppm. Please clarify.

Table 1:

- If these variance estimates are from the data presented in Fig 5, I suggest deleting this table and stating these values in the Fig 5 caption text. If the authors choose to keep the table, state in the table caption that the variance estimates are from the data presented in Fig 5. If they are not from the data in Fig 5, please describe their origin.

Figure 6:
- Please describe in the caption what is meant by "ref" in the y-axis titles.

Table 2:
- Does "lightly depleted" and "highly depleted" refer to standards shown in Fig 6? Please describe in the table caption what is meant, or better yet, remove the ambiguous terms in favor of the actual reference water names.

Figure 8:
- The caption is confusing. The authors are not plotting Allan deviation after 24 hours, they are plotting the predicted standard deviation after 24 hours of integration as predicted from an Allan variance analysis.
- The wording describing the dotted lines is confusing. It currently sounds as if they are the predicted range one would expect during the course of a 24 hour period. The main text, however, suggests these dotted lines are the authors desired "noise threshold".
- Is this threshold one standard deviation or perhaps a 95 % confidence interval? Please reword the caption.
- Make the right side axis title and tick labels red, in the same way the authors made the left side blue.
- Lastly, the font size of the stations on the map need to be much larger.

Section 3.1

- This section would be better positioned after the discussion since it is a proposed application of what the authors intend to do after having completed the current study.

Figure 9:
- Similar to my comment for Fig 8, reword "The noise is obtained from short-term Allan deviations at 𝝉 = 2 min..." to something like "Standard deviation is predicted from an allan variance analysis for the 2 minute integration time...".

Discussion Line 325:
- The sentence "It shows an optimum stability range of ~ 15 min, followed by a drift period in the hour range and finally a stabilisation of the signal in the day range" is confusing. Are the authors referring to the #1169 instrument at 500 ppm H2O and d18O? The blue series is the only series that does seem to be achieving stability in the day range. All others continue to drift or have insufficient data to make any conclusions about their trajectory (e.g., dD #1169).

Interesting features for field operation:
- This is an important section and seems to compare OF-CEAS to Picarro's CRDS. The glue, the inability to clean, lack of needed software. I suggest the authors state plainly that AP2E's design and software are more conducive to remote field deployment than Picarro's CRDS.

Citation: https://doi.org/10.5194/egusphere-2024-2149-RC2
- AC2: 'Reply on RC2', Thomas Lauwers, 18 Oct 2024
  
  We thank the reviewer for his careful reading and recommendations. The answers are in the file attached: they are inserted in the text in red and the citations from the manuscript are indicated in blue.
  
  Citation: https://doi.org/10.5194/egusphere-2024-2149-AC2

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2024-2149', Harro A.J. Meijer, 28 Aug 2024

This is a well-written, interesting paper about the 'next level' OF-CEAS instrumentation. I liked reading it, and is carries important new information. I recommend publication, but/and have a set of comments and questions that I invite the authors to respond to, preferably by adapting the paper where appropriate, or else in writing to the editor (and me).
First a general question: how come it is so difficult to perform isotope measurements on water vapour at 10 ppm or even higher, while for example precise isotope measurements on methane (2 ppm) are routine nowadays? Is it because of the humidity range that has to be accomodated?
Then in more detail, going through the paper:
Figure 1 The methane interference can potentially be severe for low humidity (1/100 to 1/1000 of the humidity in this plot). Apart from this plot, this interference has not been discussed or even mentioned anywhere else. Is the methane spectrum always included in, and corrected for in the fits?
Figure 2 It is not clear to me how the residuals in cm-1 relate to the arbitrary normalized intensity (in arbitrary units). Even the x-axis is in free spectra range steps, not in cm-1. Please clarify.
Compared to figure 1, there is a flat baseline here in figure 2. How can that be, as all lines in figure 1 will be proportionally lower?
Line 169 and elsewhere: suddenly decimal commas instead of points. Happens more often, with the declaration of the values for reference waters.
Figure 5 the number of points for the Picarro instrument are much higher than for the AP2E one. Why? DIfferent strategy?
Table 1 should include the number of measurements (so much more for the Picarro I presume)
Lines 190 further. This humidity / mixing ratio dependence is widely observed, not only for water vapour measurements, but also for isotopes in atmospheric CO2. In that field, two ways of dealing with it are in use: the 'ratio method', so building ratios first, and then correct for mixing ratio dependence (this is what you do here), the other method is the isotopologue method, which would first analyze the different isotopologues as different species, calibrating first them, and only then build ratios. See for instance papers:
Flores, et al. Calibration Strategies for FT-IR and Other Isotope Ratio Infrared Spectrometer Instruments for Accurate delta C-13 and delta O-18 Measurements of CO2 in Air. Analytical Chemistry {89}, {3648-3655} (2017).
Steur et al. Simultaneous measurement of δ 13C, δ 18O and δ 17O of atmospheric CO2 – performance assessment of a dual-laser absorption spectrometer. Atmos Meas Tech 14, 4279–4304 (2021).

Both have their pros and cons.

Would that be something to try out here? Or at least to mention and discuss. Or is it not applicable at all in this context?
FIgure 8 The Allan deviation is only part of the final uncertainty: the humidty correction and the calibration error also contribute. While that is less important for tracking the diurnal cycle, the calibration error will influence the accuracy of the seasonal cycle.
Line 300 see above: the humidity dependence of dD is relatively speaking larger than that of d18O, and also it calibration uncertainty (fig 7) is, relatively, larger. Would that influence your conclusion?
Line 342 "applying a drying on the humidity generator" ?? unclear to me what you mean.
Lines 355-365 (and figure 6) I agree with your conclusion that while the gradual, linear dependence is indeed caused by a spectral 'misfit' (probably indeed the interference of the very strong lines further in the spectrum, or methane?), the low humidity part strongly indicates sample-to-sample memory effects. This is further supported by the fact that the depleted ref goes up, and the 'enriched' one goes down.

This effect will not fully vanish in the station, because (1) the residual water vapour is probably quite fractionated, and will thus still be different from outside humidity, and (2) you must calibrate your instrument regularly using two waters with very different isotope values.

I would like to see more discussion of these low-humidity part effects (after all, that is the truly innovative part of the instrument and your paper): for example, are the values (in fig 6) influenced by the 'sample history' before one of the reference waters and how would that be different in the station? Would you expect that sample water vapour with isotope values intermediate between the 'high' and 'low' ref waters scale linearly between them (as you suggest in lines 250-253)? What is the added uncertainty in this region?
Line 364-365: 'We insist thus on the importance of calibrating the instrument in the field to correct for those artefacts (Casado et al., 2016).' Indeed! May be change the word 'insist' into 'emphasize' ?
line 371-372 "The internal architecture of these analysers therefore reduces the risk of breakdowns during the deployment, but requires an expertise to finely tune them. "

To me this feels like a blessing in disguise, or may be just the exact opposite. Deploying these instruments thus always requires expertise (and time and some equipment) in the field. Any more comments to that? For example negative field experience with fixed mounted equipment such as the Picarro's ?

"Competing interests. The authors declare that they have no conflict of interest."
Two of the co-authors work with the (commercial) producer of the instrument. How do they avoid a conflict of interest?

Citation: https://doi.org/10.5194/egusphere-2024-2149-RC1
- AC1: 'Reply on RC1', Thomas Lauwers, 18 Oct 2024
  
  We thank Harro Meijer for his very pertinent questions concerning the general form of the manuscript as well as the scientific content, which allow us to be exhaustive on the instrumental limits presented in the paper, and give us interesting perspectives for future studies. The answers are in the file attached: they are inserted in the text in red and the citations from the manuscript are indicated in blue.
  
  Citation: https://doi.org/10.5194/egusphere-2024-2149-AC1
RC2:
'Comment on egusphere-2024-2149', Anonymous Referee #3, 10 Sep 2024

The manuscript titled "OF-CEAS laser spectroscopy to measure water isotopes in dry

environments: example of application in Antarctic" by Lauwers et al. is a much needed study on a promising technique to measure water vapor isotope composition at very low humidity. The manuscript includes all of the information one would hope for to employ their method and requires only minor revisions before publication. Indeed, the authors would go a long way towards publication readiness by clarifying the text and refining the grammar.

General comments for clarity:
- Make the figures and tables, including their captions, more independent of the main text. Telling a reader the same information twice (e.g., in the figure and in the main text) helps with clarity.

Instrumentation description:
- I expect this study and instrumentation to be popular among water isotope laboratories and researchers. As such, please describe the specific model of AP2E used. Or, if all three are custom, please indicate so. Consider including a brief description of how these particular OF-CEAS instruments differ from those in Casado et al., 2016. Are they the same?
- Use of the description "V-shape" does not help the reader understand the instrument. Perhaps it would be more informative to say something about the lack of fiber optic cabling in favor of highly reflective mirrors?

2.1.2 Long-term stability at Dumont d’Urville station:

- Do the authors expect the variable water vapor concentration requiring "An additional filtering has been applied to remove the points with a non- stable humidity, i.e. with a humidity value standing outside the 2-𝜎 interval." is due to the LHLG unit or to the OF-CEAS instrument itself?

Figure 5:
- De-emphasize the green data by making it a lighter shade and placing them behind the OF-CEAS data.
- I suggest expressing the y-axes as residuals in the same way the authors have already done with Figs 3 and 6. The wide range that must necessarily be used when expressing the values in their absolute terms may hide drift or biases.
- The caption sentence "Each point represents the average of the final minimum 5 minutes of 1000 ppm humidity plateaus and the error bars correspond to the associated standard deviation" is unclear. Does "minimum" imply that sometimes it is longer? Be specific.
- Why would "plateaus" exist with this particular dataset? The caption currently reads as if all of these data were collected at 1000 ppm. Please clarify.

Table 1:

- If these variance estimates are from the data presented in Fig 5, I suggest deleting this table and stating these values in the Fig 5 caption text. If the authors choose to keep the table, state in the table caption that the variance estimates are from the data presented in Fig 5. If they are not from the data in Fig 5, please describe their origin.

Figure 6:
- Please describe in the caption what is meant by "ref" in the y-axis titles.

Table 2:
- Does "lightly depleted" and "highly depleted" refer to standards shown in Fig 6? Please describe in the table caption what is meant, or better yet, remove the ambiguous terms in favor of the actual reference water names.

Figure 8:
- The caption is confusing. The authors are not plotting Allan deviation after 24 hours, they are plotting the predicted standard deviation after 24 hours of integration as predicted from an Allan variance analysis.
- The wording describing the dotted lines is confusing. It currently sounds as if they are the predicted range one would expect during the course of a 24 hour period. The main text, however, suggests these dotted lines are the authors desired "noise threshold".
- Is this threshold one standard deviation or perhaps a 95 % confidence interval? Please reword the caption.
- Make the right side axis title and tick labels red, in the same way the authors made the left side blue.
- Lastly, the font size of the stations on the map need to be much larger.

Section 3.1

- This section would be better positioned after the discussion since it is a proposed application of what the authors intend to do after having completed the current study.

Figure 9:
- Similar to my comment for Fig 8, reword "The noise is obtained from short-term Allan deviations at 𝝉 = 2 min..." to something like "Standard deviation is predicted from an allan variance analysis for the 2 minute integration time...".

Discussion Line 325:
- The sentence "It shows an optimum stability range of ~ 15 min, followed by a drift period in the hour range and finally a stabilisation of the signal in the day range" is confusing. Are the authors referring to the #1169 instrument at 500 ppm H2O and d18O? The blue series is the only series that does seem to be achieving stability in the day range. All others continue to drift or have insufficient data to make any conclusions about their trajectory (e.g., dD #1169).

Interesting features for field operation:
- This is an important section and seems to compare OF-CEAS to Picarro's CRDS. The glue, the inability to clean, lack of needed software. I suggest the authors state plainly that AP2E's design and software are more conducive to remote field deployment than Picarro's CRDS.

Citation: https://doi.org/10.5194/egusphere-2024-2149-RC2
- AC2: 'Reply on RC2', Thomas Lauwers, 18 Oct 2024
  
  We thank the reviewer for his careful reading and recommendations. The answers are in the file attached: they are inserted in the text in red and the citations from the manuscript are indicated in blue.
  
  Citation: https://doi.org/10.5194/egusphere-2024-2149-AC2

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

AR by Thomas Lauwers on behalf of the Authors (24 Oct 2024) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (08 Nov 2024) by Christof Janssen

RR by Harro A.J. Meijer (18 Nov 2024)

RR by Anonymous Referee #3 (25 Nov 2024)

Suggestions for revision or reasons for rejection

I think the authors have adequately responded to my previous review, except for improving the grammar. I think the manuscript will be ready for publication after incorporating the below edits or completing a thorough proofread themselves. The below suggested clerical edits use line numbers from the track-changes document with file name "egusphere-2024-2149-ATC1.pdf".

line 22 - "Stable water vapour isotopes" should read "Water vapour stable isotope", and note 'isotope' is singular in this case

line 31 - either change "by Picarro company" to read "by the Picarro company" or "by Picarro". Below, on line 55, you have "with the AP2E company". Be consistent.

line 33 - "stable water vapour isotopes" should read "water vapour stable isotopes"; check throughout manuscript for this change; you are measuring the stable isotopes in water vapour, not the isotopes in stable water vapour.

add comma after "e.g." throughout manuscript; you are saying "for example, cold fronts, cyclones"

line 36 - change "on board of boats" to read "on board boats"

line 37 - change "number of studies is also" to read "number of studies are also"

line 41 - change "isotopic records in ice core which" to read "isotopic records in ice cores, which"

line 43 - change "However, CRDS struggle" to read "However, CRDS struggles"

line 45 - change "or in altitude" to read "or at altitude"

line 48 - change ", allowing to narrow down" to read ", to narrow down"

line 49 - change "the cavity resonances (Morville" to read "the cavity resonance (Morville" OR if you wish to keep resonances plural, then change "laser emission frequency by locking it to the cavity resonances" to read "laser emission frequencies by locking them to the cavity resonances". The plural form is consistent with the following sentences.

line 58 - change "water vapour isotopes monitoring" to read "water vapour isotope monitoring"

line 65 - change "in term of robustness" to read "in terms of robustness"

line 68 - change "atmospheric O2 isotopes measurement " to read "atmospheric O2 isotopic measurement "

line 69 - change "Water isotopes OF-CEAS" to read "Water isotope OF-CEAS"

line 74 - change "from HITRAN database" to read "from the HITRAN database"

Figure 1 minor suggestion - place the dotted red sum line behind the H2O blue line.

Figure 2 caption - change the comma at the end of "than 52 ms," to a period.

line 97 - change "time is of 52 ms in" to "time is 52 ms in" or "time is approximately 52 ms in"

lines 109, 110 - change "gives an easy access" to "gives easy access"

line 114 - change "This permits to assess the spectrometer" to read "This permits the spectrometer to assess"

line 117 - change "by AP2E company." to read "by AP2E."

line 124, 125 - change "perform Allan deviations (AD) measurements" to read "perform Allan deviation (AD) measurements"

line 148 - reword "The use of the second dataset enables to reach a time range" to read "Using the second dataset allows for a time range"

line 157 - This sentence is rather confusing as structured; consider rewording it to make the comparison more apparent: "The ADs of the low-humidity analyser follow a white noise decay during several minutes, with a minimal value of 0.1 ‰ (resp. 0.06 ‰) for δ18O at 100 ppm (resp. 500 ppm) and 0.5 ‰ (resp. 0.2 ‰) for δD at 100 ppm (resp. 500 ppm)."

line 161 - Similarly, this sentence has the same confusing comparison structure: "After two days, we calculate an AD of 1 ‰ (resp. 0.09 ‰) for δ18O at 100 ppm (resp. 500 ppm) and of 2.5 ‰ (resp. 0.7 ‰) for δD at 100 ppm (resp. 500 ppm)."

line 174 - change "two standards injections" to read "two standard injected"

line 177 - change "performed all over" to read "performed over"

line 182 - change "After a filtering" to read "After filtering"

line 184 - change "calibrations for the CRDS analyser" to read "calibrations for the CRDS analysers"

Figure 5 caption, line 194 - the sentence starting with "The blue line (resp. green line)" is confusing as worded similar to the above line 157, and 161 comments.

line 199 - change "that the analysers calibrations" to read "that the analysers' calibrations"

line 201 - change "variation have been registered" to read "variation has been registered"

line 202 - change "This points out the need for a" to read "This underscores the need for a"

line 212 - change "such as spectroscopic effect affecting the fitting procedure, or memory effect)" to read "such as spectroscopic fitting or memory)"

line 214 - remove "contents"

line 221 - change "using the additionnal calibrated" to read "using the additional calibrated"

line 224 - remove "then"

line 239 - change "various series of measurement" to read "various measurements"

line 241 - change "and finish with the" to read "and finishes with a"

line 246 - change "A first humidity sequence" to read "An initial humidity sequence"

Figure 6 caption, line 264 - change "The y-axis are" to read "The y-axes are"

line 272 - change "observe a larger noise" to read "observed increased noise"

line 284 - change "and for longer time period" to read "and for longer time periods"

line 292 - I believe a negative sign is missing in the phrase "0.7 ‰ for δ18O". Based on Figure 7, TD3 is below the expected value.

Figure 7 caption, line 297 and elsewhere - You have many notations to express "VSMOW-SLAP". I see " IAEA VSMOW/SLAP" and "VSMOW-calibrated" and "VSMOW-SLAP". Pick one notation and change throughout. I prefer one without "IAEA".

line 323 - By now I am accustomed to the "(resp. )" structure. However, I still think the authors should consider an alternative. Furthermore, the "(resp. dD)" needs a delta symbol rather than a lower case d.

line 328 - superscript the 18 in "δ18O"

line 332 - change "are of the order of 10 ‰ " to read "are of order 10 ‰ "

line 340 - change "water vapour isotopes monitoring" to read "water-vapour-isotope monitoring"

line 355 - I suggest changing "permits to reach extreme levels of precision at low water concentrations" to read "allows for high isotope-ratio precision at low water concentrations"

line 356 - change "at 2 minutes of the" to read "at 2 minutes integration of the"

line 359 - change "to capture with a high" to "to capture high"

line 372 - change "acquisition time – enabled to keeping enough precision to " to read "acquisition time, allowing for enough precision to "

line 375 - change "quantify the water isotopes concentration" to read "quantify the water isotope concentration"

line 388 - change "and tubings before" to read "and tubing before"

line 390 - change "over all the year" to read "over a year"

line 415 - change "We emphasize thus the " to read "We emphasize the "

line 427 - change "offers also the" to read "offers the"

line 428 - change "like for example various" to read "like various"

line 433 - change "spectrometers based on the OF-CEAS" to read "spectrometers using the OF-CEAS"

line 440 - change "drift on neither instrument" to read "drift on either instrument"

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ED: Publish subject to minor revisions (review by editor) (29 Nov 2024) by Christof Janssen

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.'

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AR by Thomas Lauwers on behalf of the Authors (17 Dec 2024) Author's response Author's tracked changes Manuscript

ED: Publish subject to minor revisions (review by editor) (20 Dec 2024) by Christof Janssen

AR by Thomas Lauwers on behalf of the Authors (28 Dec 2024) Author's response Author's tracked changes Manuscript

ED: Publish subject to technical corrections (03 Jan 2025) by Christof Janssen

AR by Thomas Lauwers on behalf of the Authors (06 Jan 2025) Author's response Manuscript

Journal article(s) based on this preprint

06 Mar 2025

OF–CEAS laser spectroscopy to measure water isotopes in dry environments: example of application in Antarctica

Atmos. Meas. Tech., 18, 1135–1147, https://doi.org/10.5194/amt-18-1135-2025,https://doi.org/10.5194/amt-18-1135-2025, 2025

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Water vapour isotopes are important tools to better understand processes governing the atmospheric hydrological cycle. In polar regions, their measurement helps to improve the interpretation of water isotopic records in ice cores. However, in situ water vapour isotopic monitoring is an important challenge, especially in dry places of East Antarctica. We present here an alternative laser spectroscopy technique adapted for such measurements, with a limit of detection down to 10 ppm humidity.


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OF-CEAS laser spectroscopy to measure water isotopes in dry environments: example of application in Antarctica

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