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

Published

September 23, 2020

Author(s)

Robert Hellmann, Allan H. Harvey

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 hard-sphere kinetic theory often used in the literature 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.

Citation

Geophysical Research Letters

Volume

47

Issue

18

Pub Type

Journals

Download Paper

https://doi.org/10.1029/2020GL089999

Local Download

Keywords

air, diffusion, isotopes, kinetic fractionation, water

Citation

Hellmann, R. and Harvey, A. (2020), First-Principles Diffusivity Ratios for Kinetic Isotope Fractionation of Water in Air, Geophysical Research Letters, [online], https://doi.org/10.1029/2020GL089999, https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=930541 (Accessed October 9, 2025)