Please find my review attached.

Review of A modular field system enabling cavity ring-down spectroscopy of in-situ vapor observations in harsh environments: The ISE-CUBE system

This manuscript describes a new modular box enclosure called ISE-CUBE that can be used to deploy water vapor isotopic analyzers and water vapor isotopic cold-trap systems in the field under extreme cold-weather conditions. The manuscript provides a short description of the enclosure and subsequently evaluates the isotopic analyzer’s housekeeping variables from a two-week winter deployment in Svalbard. The housekeeping data suggest the analyzer is able to maintain satisfactory ranges for its Data Acquisition System temperature, its cavity temperature and pressure, and its warm box temperature. The analyzer’s water isotopic measurement precision in the field is also comparable to its measurement precision while sampling calibration gas in a laboratory setting.

The manuscript also describes an optional “profiling module” for ISE-CUBE, consisting of a tripod with an articulating measurement arm, that can be used to position a heated inlet line for the isotopic analyzer anywhere from 4 to 205 cm above the ground surface. A 90-minute window of data is presented that shows water vapor concentrations and isotope ratios from six height levels within the articulating arm’s 2-m range. The paper argues that the articulating arm provides a means to resolve and study the water and isotopic gradients closest to the surface, although doing so requires repositioning the inlet height via a manual pulley every few minutes.

Comments

Where this paper really advances our measurement abilities is in the design and presentation of the box enclosure for the isotopic analyzer and cold trap; yet most of these details are in the Supplemental material instead of in the main paper. I would recommend revisiting Figures 1-2 and using these to convey specific details about the box connections and tubing materials, something more akin to the Connectors Template in the SI. As currently presented, Figure 1a is simply too dark to make out details, and Figure 1b requires more detail and explanation. For example, what are the “power out” and “data” ports used for? Where are the fan inlets and exhaust ports? Where do the boxes connect to one another? Is the CRDS inlet unheated after the check valve? Which lines are PTFE and which SS? In addition, the main text mentions components such as an “adapter,” an “exterior inlet bulkhead,” “incoming ventilation tubing,” and “manifold tubing.” Can these be labeled on Figs 1-2? The list of components in Appendix A is fantastic. Consider also a corresponding diagram (again, like the Connectors Template) that shows where all these components go and telling readers how many of each part are required to replicate the system. Another way to think about this: what would a purchase list look like?

It would be helpful if the manuscript discussed the relevance of the ISE-CUBE enclosure to the wider measurement community. Much of the manuscript is specific to the deployment of a Picarro CRDS water vapor isotopic analyzer. Would ISE-CUBE work for other types of isotopic analyzers? If the enclosure is specific to the size and shape of the Picarro systems, could ISE-CUBE work for other gas-phase Picarro analyzers? Moreover, based on the short two-week deployment in Svalbard, is there any sense whether ISE-CUBE could last for longer periods for unattended measurements? 

On a related note, the Data Processing section (Sect. 3.2, including Table 1) presents ISE-CUBE as producing three data streams generally, but these three streams are specific to the way the modular system was set up for testing during ISLAS2020. It would be helpful if the paper distinguished more carefully which aspects of the design are generic and applicable broadly vs. specific to the test case configuration.

The enclosure is presented as novel, in part, for minimizing disturbance to the environmental flow, but I think this claim might be overreaching, since most ground-based installations are designed to minimize flow disturbance (e.g. flux towers). The real draw of the enclosure in my mind is the ability to deploy a water vapor isotopic analyzer in an environment with minimal infrastructure support (e.g. nothing more than a power drop) and/or to reduce the length of inlet lines and thus measurement hysteresis.

To evaluate ISE-CUBE, the water vapor isotopic analyzer’s performance in the field is compared to its performance in the laboratory. The intention is to compare two distinct environmental settings. However, there is another relevant difference that needs to be communicated more transparently: in the laboratory, the analyzer samples reference gas continuously, whereas in the field, the analyzer is measuring real variability related to the environment. I would not be surprised if this difference in sampled air causes the differences in humidity-binned standard deviations presented in Fig. 10 or results in the differences in spectral-fit residuals (RS) presented in Fig. 8. The paper concludes that the field data are “marginally less precise,” but, again, I wonder if this is not just a reflection of the environmental air. Would one reach the same conclusion if the analyzer were measuring reference gas while deployed as part of ISE-CUBE in the Arctic?

While I’m not sure it is necessary for the point the paper is trying to make, it would be awfully interesting to see how the isotopic analyzer and cold trap compare during the ISLAS2020 deployment. Such a comparison could provide some indication of the accuracy of the isotopic analyzer when deployed with ISE-CUBE. 

Lastly, for Fig. 12, it appears there are environmental data missing during the period highlighted in the text (9:07, onwards). In addition, it would be helpful to know, are these data from the AWS? And can the figure be made larger?

Overall, the paper is very clearly written; however, a few minor comments on presentation are provided below:

L 15 - perhaps “components” instead of “compartments”

L 29-32 is a bit awkward and could be presented more clearly

L 39 and elsewhere - “pneumatically” might be the wrong word as this implies compressed air

L 165 - is there a reference for ISLAS2020?

L 177 - Does the ISLAS2020 data span 21 Feb to 14 Mar?

L 187 - “reliable” seems like the wrong word for what is intended

L 219 requires clarification

L 239 and elsewhere - “minutely” means “meticulously.” I think the paper intends to say “1 minute”

L 375 - since “field” and “laboratory” have specific meanings, I would use “remote observatory” or some other phrase here

Fig. C1 - caption says observatory margins are for the same “time” but not the same dates, right? Perhaps clarify.

L 521 - says “could be” but should it say “is” or does it really mean “might be comparable” (as in it’s unknown)?

Fig. D1 seems to be missing the gray shading mentioned in the caption

General comments:

I found this to be a clearly written paper, well-structured and systematic. It is a little long for the content so an attempt to shorten some sections would be welcome.

It provides a detailed account of the design and testing of a well engineered set of enclosures for the isotope analyser and some peripheral equipment. It looks to be quite costly - please provide the cost of the parts to build this. Many users of these instruments would look for low-cost solutions to building enclosures and peripherals and in many cases sensible cost cutting can be made without substantially affecting performance.

The paper would benefit from a better justification of the need for this design, i.e the advantages of this design compared to other solutions to obtain such data. For example, given that a power source must be nearby (presumably within a few 10’s of meters) why couldn’t instruments be installed indoors with inlet tubing to the outdoors? Long inlet tubings are routinely used on tall masts with an appropriate pump rate? Are there particular problems regarding lag time? Or memory effects? Or disturbance of the air stratification? Why couldn’t an existing mast be used with multiple inlet heights and the use of a manifold? A nearby EC tower is mentioned in the paper.

The publication could be seen as premature given the preliminary and incomplete parts (cold trap, pivot arm and standard gas supply module). However, it may be that the authors had limited opportunities to test the device in this remote location.

The paper is mainly an account of environmental measurements (T and P, spectral characteristics etc) of the analytical instrument when placed inside the housing under cold and windy conditions, rather than a more complete account of a test of actual isotopic measurements. Ultimately the most critical isotope test, i.e. a comparison of one or more constant gas supplies both in the field and the laboratory, could not be carried out due to the lack of a working standards gas supply module. Also, there seems to have been a missed opportunity to compare the real time isotope data with isotope data obtained from the cold trap sampling, why wasn’t this done (or presented) here?

Given the anticipated degradation of isotope ratio precision at low H2O mixing ratios (typical of polar regions), and the possible instrument drift due to the changing environmental conditions, it would be important to check standard gasses a regular interval (likely several times daily) in an actual measuring campaign, hence the need for a standard gas supply device. As mentioned, this has not been demonstrated in the current manuscript.

A more thorough explanation of the stratified air column data obtained would benefit the paper as it would demonstrate the useful application of the enclosure and pivot arm. The need for an operator to use the pivot arm seems to risk disturbing the stratified air column, depending on wind direction (creating turbulence)

Specific comments:

P1 L12: I don’t think you can claim it would be satisfactory in all environments, e.g. in the tropics, overheating, condensation of humidity may be a different challenge, in environments with high day – night temperature contrast drift may be problematic

P2 L41: What are the conventional approaches?

P2 L49: But you have a nearby power source so why not house the instrument there?

P2 L52: But long inlet lines (fluorinated plastics) are routinely used in tall towers

P3 L64: Is it pneumatic? Maybe just gas or airtight connectors?

P4 L85: Isn’t a lower flow rate preferable to increase precision in dry air? Especially since there’s a separate high-flow pump to deliver the air sample close to the inlet?

P5 L106: So why wasn’t this done when there was no available standard gas module available?

P6 L144: Does the presence of an operator disturb the stratification?

P7 L161: This would have been the most complete test of the system

P10 L211: Please state if these are liquid or vapour values

P10 L220: This sentence is unclear – does it mean that measurements were within +/- 1 sigma?

P10 L229: Not sure this is correct, there are numerous field applications documented on the Picarro web site.

P11 L247: Don’t think ‘minutely’ is a word (?)

P11 L259: Please specify where DAS is measured

P16 L355: As mentioned above, wouldn’t the normal low flow setup have been preferable?

P22 L454: add ‘for d18) and dD, respectively