New insights into diffusive kinetic fractionation during liquid condensation under supersaturated environment: an alternative approach for isotope tagging of ground-level water vapour

ABSTRACT

Stable water isotopes in ground-level vapour are key to estimating water exchange between geospheres. Their sampling, however, is limited to laser-absorption spectrometers and satellite observations, having inherent shortcomings. This study investigates diffusive kinetic fractionation during liquid condensation under supersaturated environment, providing a cost-effective, reliable way of sampling ground-level vapour isotopes (¹⁸O, ²H). Experiments were undertaken at three locations in India with ‘liquid’ samples collected from condensation of ambient air at 0°C. Simultaneously, pristine ‘vapour’ was sampled via cryogenic-trapping using liquid nitrogen–alcohol slush at –78°C. The ‘liquid’ condensed under supersaturation was progressively more depleted in ¹⁸O, and less enriched in ²H than expected under equilibrium fractionation, with an increasing degree of supersaturation expressed as saturation index (S_i). This study revealed: (1) S_i, molecular density, Rh, T together control the extent of isotopic kinetic fractionation. (2) The presence of diffusive concentration gradient inhibits the flow of heavier isotopes during liquid condensation. (3) The stochastic nature of the process cannot be explained using a physics-based model alone. The artificial neural network model is hence deployed to sample δ¹⁸O (δ ²H) within –0.24 ± 1.79‰ (0.53 ± 11.23 ‰) of true value. (4) The approach can be extended to ground-validate isotope-enabled general circulation models and satellite observations.

KEYWORDS:

• Diffusive gradient
• hydrogen-2
• isotope hydrology
• kinetic isotope fractionation
• machine learning
• oxygen-18
• supersaturation
• vapour sampling

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Correction

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data

The research database is attached as a Supplement and can be used to reproduce the results. The code used to perform this study can also be shared upon request.

Authors’ contributions

RDD conceived the initial problem statement and the significance of the same. AG was responsible for the data analyses, modelling and scientific evaluation. VP was responsible for processing the samples. GS, RDD and AG drafted the manuscript. All authors contributed to finalizing the manuscript.

Disclaimer

An earlier version of this manuscript is available in the form of a pre-print at the SSRN pre-print server [72].

Correction Statement

This article was originally published with errors, which have now been corrected in the online version. Please see Correction (https://doi.org/10.1080/10256016.2023.2224469)

Additional information

Funding

The work reported here is carried out under the aegis of a National Programme on Isotope Fingerprinting of Waters of India (IWIN) which was initially funded jointly by the Department of Science and Technology, Government of India, vide grant number IR/S4/ESF-05/2004, and the Physical Research Laboratory (PRL), a Unit under the Department of Space, Government of India. The IWIN National Programme is currently sustained exclusively by the PRL.