| 23 Jun 2026
Technical note: A new monitoring approach to measure water vapor isotopes in high altitude regions
Abstract. Water vapor isotopes provide a comprehensive perspective on the moisture source dynamics for tracing the physical processes in hydrological and climatic studies. Continuous real-time water vapor isotopic measurements using Cavity Ring-Down Spectroscopy (CRDS) techniques are mostly based at stations located in high latitudes and the low-lying tropical regions. Such investigations from high-altitude, tropics, subtropical – mid-latitude transition zone – the Himalayas is limited owing to challenging physical conditions and multiple forms of precipitation occurring in the region. In this study, we report the establishment of the first continuous high-altitude isotope-monitoring laboratory in Northwest Himalayas windward side Manali (2,050 above msl) and leeward side Sissu (3,120 above msl) using the Picarro L2140-i Cavity Ring-Down Spectroscopy (CRDS) analyzer. This instrument enables real-time measurements of δ¹⁷O, δ¹⁸O, and δ2H in local atmospheric water vapor. Our laboratory setup integrates installation of Picarro analyzer, a heating air inlet system, meteorological sensor, lightning arrester and calibration protocols suited for optimum performance of the instrument in such challenging high-altitude Himalayan environment. Our laboratory setup protocols integrate the best practice and published guidelines with some additional modifications to mitigate the challenges in water vapor isotopic measurements in high altitude environment. A limitation of the current dataset is that no calibration has been performed since July 2025, due to relocation of JRF recruited to Delhi resulting in the unavailability of the trained personnel to carry out routine calibration cycles. We acknowledge this as a significant shortcoming and highlighted here for transparency. In addition to these continuous water vapor isotope measurements, precipitation events are also recorded, which could be helpful in investigating serious calibration problems should they arise.
I have reviewed the manuscript entitled "Technical note: A new monitoring approach to measure water vapor isotopes in high altitude regions" by Kumar et al. The authors developed a stable oxygen and hydrogen isotope monitoring station for atmospheric water vapor at a relatively remote high-altitude site in the Himalayas. The objective of the study is clear and relevant, and it has the potential to provide valuable insights into the hydroclimatic processes controlling the isotopic composition of precipitation and atmospheric water vapor in this region, with important implications for both the modern and past hydrological cycle.
Despite the considerable effort invested by the authors, I find that the methodological approach used to address these scientific questions is not sufficiently robust. The main limitation of this study lies in the calibration of the CRDS isotopic measurements, not only because calibration could not be performed during part of the monitoring period, but also because the calibration protocol itself is not appropriate in light of previous studies that monitored water vapor isotopes using the same instrument. For this fundamental reason, I cannot recommend this manuscript for publication based on the current dataset. Below I provide detailed comments explaining the reasons for this assessment, which I hope will help the authors improve future studies.
Introduction
The authors should explain in greater detail the advantages of measuring stable isotopes in atmospheric water vapor rather than in liquid water, which is technically much less challenging. This rationale is not sufficiently clear in the Introduction.
It would also be useful for the authors to mention alternative methods, besides continuous in situ monitoring, that have been used to measure stable isotopes in atmospheric water vapor. These include cryogenic trapping techniques and, more recently, the use of hygroscopic salts (El-Shenawy et al., 2024).
Methods
In line 110 and throughout the manuscript, the authors refer to monitoring at a single location. However, the abstract states that measurements were conducted at two sites ("windward side Manali (2,050 m a.s.l.) and leeward side Sissu (3,120 m a.s.l.)"). This discrepancy between the abstract and the main text is confusing and should be clarified.
The authors devote considerable attention to describing the air sampling system, which is indeed a critical aspect in such an environment. According to the description, they constructed a heated sampling line that passes through the roof structure of the building, where the tubing appears to be unheated. It is not clear how this temperature gradient might affect condensation processes within the sampling line. Moreover, this section is excessively detailed while providing little information that would improve reproducibility.
However, the most serious methodological issue, and the main reason why I consider this dataset unreliable, is the calibration protocol. The authors calibrate the isotopic composition of water vapor using the Picarro vaporizer operated under the conventional liquid-water analysis protocol with synthetic AIR ZERO. Over the past several years, multiple studies by Voigt et al. and Bradley et al., among others, have demonstrated that synthetic air produces an optical matrix effect in CRDS analyzers, making 17O-excess measurements, and also δ¹⁸O and δ²H measurements, unreliable. Previous studies have overcome this issue by calibrating the instrument using dried natural air (obtained through cryogenic trapping) or compressed dry atmospheric air. Alternatively, the Picarro Delivery Module can be used, as it relies on atmospheric air rather than synthetic dry air for calibration.
Compelling evidence that the calibration approach used in this manuscript is inappropriate is the fact that the authors frequently report 17O-excess values above 0.2‰ (200 per meg). Such values are physically unrealistic. To my knowledge, no published study has reported 17O-excess values this high in either liquid water or atmospheric water vapor. Typical values are generally below 60–70 per meg. This strongly indicates a calibration artifact.
As an additional recommendation, calibrating the instrument with liquid water standards only once per month is not appropriate. Most laboratories measuring 17O-excess in liquid water perform calibrations every two or three days. At the very least, a quality-control standard should be analyzed much more frequently than once a month to assess instrumental stability. Otherwise, it is impossible to know whether instrumental drift occurred during the intervening period.
Another major methodological concern is the dependence of the raw isotopic measurements on water vapor concentration. This effect can significantly reduce analytical precision, particularly at vapor concentrations below approximately 1,500 ppm. As shown in Figure 4, vapor concentrations during the dry season frequently dropped below 500 ppm, making the measurements highly uncertain and potentially unreliable during those periods. This issue should have been addressed by performing a linearity test to quantify the dependence of all isotopic parameters, including the secondary parameters d-excess and 17O-excess, on water vapor concentration. Such an assessment would have allowed appropriate corrections to be applied. However, this important issue does not appear to have been considered in the study design.
Other methodological aspects also remain unclear. For example, it is not specified whether a quality-control standard was measured regularly to evaluate instrumental drift, particularly for the secondary isotope parameters. Furthermore, the authors do not explain how 17O-excess values were assigned to the calibration standards, most of which appear to be in-house standards. Did they use the approach proposed by Schoenemann et al. 2013, to assign 17O-excess values?
The authors also state that instrument calibration could not be performed during part of the monitoring period. This effectively invalidates all measurements collected after calibration ceased. Although this issue is mentioned in the abstract, it is surprisingly scarcely discussed in the remainder of the manuscript.
Finally, it is noteworthy that although the instrument likely acquired measurements every 2–3 seconds, resulting in a dataset comprising lots of observations, the authors only present monthly averages. It would have been much more informative to present the complete time series, perhaps together with appropriate moving averages or smoothing techniques, so that the temporal variability and trends throughout the monitoring period could be properly evaluated.
References
Brady, M. P., & Hodell, D. A. (2021). Continuous and simultaneous measurement of triple-oxygen and hydrogen isotopes of liquid and vapor during evaporation experiments. Rapid Communications in Mass Spectrometry, 35(10), e9078. https://doi.org/10.1002/rcm.9078
El-Shenawy, M. I., Herwartz, D., & Staubwasser, M. (2024). A Passive Method for Sampling Water in the Soil-Plant-Atmosphere Continuum for Stable Hydrogen and Oxygen Isotope Analyses. Rapid Communications in Mass Spectrometry, 38(2).
Schoenemann, S.W., Schauer, A.J., Steig, E.J., 2013. Measurement of SLAP and GISP δ17O and proposed VSMOW-SLAP normalization for 17O-excess. Rapid Commun. Mass Spectrom.27, 582–590.
Voigt, C., Vallet-Coulomb, C., Piel, C., & Alexandre, A. (2022). 17O-excess and d-excess of atmospheric water vapor measured by cavity ring-down spectrometry: Evidence of a matrix effect and implications for the calibration procedure. Rapid Communications in Mass Spectrometry, 36, e9227. https://doi.org/10.1002/rcm.9227