Fractionation coefficient measurements on water isotopologues using an innovative absorption spectrometer

Isotopic fractionation governs the relative change in the isotopic composition of water during atmospheric phase transitions. Therefore, it is imperative to characterize it precisely to better understand the atmospheric phenomena leading to ice formation in Antarctica.

The fractionation coefficient values currently in use were established over 50 years ago by Merlivat and Nief (1967) and Majoube (1971). However, these values now conflict with more recent findings by Ellehoj et al. (2013). Consequently, new measurements of isotopic fractionation between vapor and ice have been conducted for δ¹⁸O and δD using an absorption spectrometer that combines a V-shaped Cavity with Optical Feedback (VCOF) and the Cavity Ring-Down Spectroscopy (CRDS) technology. This experimental setup is particularly effective for low-temperature measurements, as the VCOF provides high optical stability and the CRDS offers high measurement precision.

The results on δ¹⁸O and δD differ from previous publications, raising many questions about the physical phenomena influencing isotopic fractionation as studied in the laboratory. Among these questions, gaining a better understanding of the equilibrium during fractionation appears to be the most crucial aspect of these measurements.

*Fig. 1. Comparison of fractionation coefficient measurements for the* δ¹⁸O. The measurements done by Majoube (1971) are plotted in red whereas the measurements of Ellehoj (2013) are plotted in blue. Each green cross represents a point of measurement, and for some crucial temperatures, such as 233 K and 248 K, measurements were conducted multiple times to ensure accuracy. Each measuring point (in green) is below the measurements taken in the literature. However, the measurements are coherent for different temperatures and repeatable.

Auteurs: Louis Cros-Wieczorek, Mathieu Casado, Justin Chaillot