Quantification the diffusion-induced fractionation of (H2O)-H-1-O-17 isotopologue in air accompanying the process of water evaporation

Advances in laser technology enabling simultaneous analyses of three heavy water isotopologues ((HHO)-H-1-H-2-O-16, (H2O)-H-1-O-18 and (H2O)-H-1-O-17) triggered renewed interest in studies of the hydrological cycle using full isotopic composition of water. Such studies require adequate knowledge of equilibrium and kinetic isotope effects accompanying phase transitions of water. Here we report the results of laboratory experiments aimed at quantification of diffusion-induced fractionation of (H2O)-H-1-O-17 isotopologue in air, accompanying the process of water evaporation. Three evaporation experiments have been conducted: (i) evaporation under constant temperature and relative humidity levels, (ii) evaporation under constant temperature and at three different relative humidity levels, and (iii) evaporation under constant relative humidity level and at two different temperatures. Experiment (i) yield the value of diffusive fractionation coefficient of (H2O)-H-1-O-17 isotopologue in air equal 14.64 & PLUSMN; 0.24 parts per thousand & nbsp;& nbsp;at 20.4 & DEG;C. This value is indistinguishable within the quoted uncertainties from the value of 14.71 & PLUSMN; 0.05 parts per thousand & nbsp;derived experimentally for the first time by Barkan and Luz (2007). We confronted these experimental values with the results of state-of-the art quantum-mechanical calculations published recently by Hellmann and Harvey (2020). Experiments (ii) and (iii) yield for the first time the experimental evidence that, contrary to the d-excess, the O-17-excess parameter is largely insensitive to changes in temperature but is sensitive to changes of the relative humidity and this sensitivity increases with the degree of isotope enrichment of the evaporating water body. The linear dependence of kinetic fractionation coefficients of heavy water isotopologues in air on the relative humidity deficit, postulated by the Craig-Gordon isotope model of evaporation process, was confirmed by the results of experiment (ii) for all three isotope systems studied, pointing to correctness of the conceptual framework underlying this widely used model.(c) 2022 Elsevier Ltd. All rights reserved.

Automated summary

Advances in laser technology have allowed for simultaneous analyses of three heavy water isotopologues, leading to renewed interest in studying the hydrological cycle using the full isotopic composition of water. Laboratory experiments were conducted to quantify diffusion-induced fractionation of the (H2O)-H-1-O-17 isotopologue during water evaporation. The results showed that the O-17-excess parameter is largely unaffected by temperature changes but is sensitive to relative humidity changes. Additionally, it was found that the kinetic fractionation coefficients of heavy water isotopologues in air followed a linear dependence on the relative humidity deficit, confirming the validity of the widely used Craig-Gordon isotope model.