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First‐Principles Diffusivity Ratios for Kinetic Isotope Fractionation of Water in Air
Author(s) -
Hellmann Robert,
Harvey Allan H.
Publication year - 2020
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1029/2020gl089999
Subject(s) - isotopologue , thermal diffusivity , kinetic energy , isotope fractionation , kinetic isotope effect , thermodynamics , fractionation , isotope , range (aeronautics) , water vapor , mass independent fractionation , diffusion , chemistry , equilibrium fractionation , materials science , deuterium , molecule , atomic physics , physics , chromatography , nuclear physics , quantum mechanics , organic chemistry , composite material
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/H 2 O 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 17 O and 18 O, 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.