First-Principles Diffusivity Ratios for Kinetic Isotope Fractionation of Water in Air.

Robert Hellmann,Allan H Harvey

doi:10.1029/2020gl089999

Abstract

Kinetic isotope fractionation between water vapor and liquid water or ice depends on the ratio of the diffusivities of the isotopic species in air, but there is disagreement as to the values of these ratios and limited information about their temperature dependence. We use state-of-the-art intermolecular potential-energy surfaces for the water-nitrogen and water-oxygen pairs, along with the kinetic theory of molecular gases, to calculate from first principles the diffusivities of water isotopologues in air. The method has sufficient precision to produce accurate diffusivity ratios. For the HDO/H2O ratio, we find that the often used hard-sphere kinetic theory is significantly in error, and confirm the 1978 experimental result of Merlivat. For the ratios involving 17O and 18O, the simple kinetic theory is relatively close to our more rigorous results. We provide diffusivity ratios from 190 K to 500 K, greatly expanding the range of temperatures for which these ratios are available.

Highlights

Stable water isotopes, in particular the molecules HDO, H217O, and H218O, are widely used to model processes involving the atmosphere, ocean and fresh water, and ice (Gat, 1996)
Isotopic fractionation between the atmosphere and a condensed phase is determined by equilibrium thermodynamics and by a kinetic effect that depends on the relative diffusivities of the isotopic species in air
Since there seems to be no standard notation for these diffusivity ratios, for this work we define the relative diffusivities Dr; HDO ≡ DHDO=DH2O, Dr; ≡ DH217O=DH2O, and Dr; ≡ DH218O=DH2O, where Di is the diffusivity of species i in air

Summary

Introduction

In particular the molecules HDO, H217O, and H218O, are widely used to model processes involving the atmosphere, ocean and fresh water, and ice (Gat, 1996). Isotopic fractionation between the atmosphere and a condensed phase is determined by equilibrium thermodynamics and by a kinetic effect that depends on the relative diffusivities of the isotopic species in air. While the equilibrium fractionation is fairly well understood, at least for vapor‐liquid equilibria (Horita et al, 2008; Japas et al, 1995), there is significant disagreement, especially for D/H fractionation, regarding the correct diffusivity ratio for the kinetic effect. It is the purpose of this paper to resolve these disagreements. In some of the literature, the reciprocals of these ratios are used instead

Results

Discussion

Conclusion