A new experimental approach for the determination of the diffusivity fractionations of water isotopes in air

Water isotopes are key tools to understand the dynamics of the global hydrological cycle. A well constrained hydrological model requires a precise definition of isotopic diffusion and equilibrium fractionation factors (DIFF and EIFF, respectively) among water phases. Although there is a consensus on the EIFFs in the community, there is still a debate regarding the precise DIFF. Few experimental studies were performed to define the DIFFs of water in air, however, their data are scattered significantly. Most of these experiments were designed to gradually evaporate a small body of water forced by a stream of either dry nitrogen or air. This design induced several complications, including turbulence, surface cooling and isotopic heterogeneity in the water body, which required additional steps of corrections that may affect the final calculation of the DIFF.

In this study, we performed a passive approach using a hygroscopic salt (Anhydrous CaCl2) to absorb water vapor from air (nitrogen) under controlled relative humidity at room temperature in a glove box. Once the CaCl2 granules exposed to humid air, it progressively absorbed water vapor until it dissolved into the water forming a brine. At the early stage of the absorption the thin water film upon the surface of CaCl2 reflects a pure isotopic diffusion of water vapor, while the final stage of absorption (i.e., brine) represents a pure isotopic equilibrium between water vapor and the brine. Hydrogen and oxygen isotope measurements of these two stages of absorption will be presented in the meeting.