Diffusivity fractionations of H₂¹⁶O/H ₂¹⁷O and H₂¹⁶O/H₂ ¹⁸O in air and their implications for isotope hydrology

We have determined the isotope effects of ¹⁷O and ¹⁸O substitution of ¹⁶O in H₂O on molecular diffusivities of water vapor in air by the use of evaporation experiments. The derived diffusion fractionation coefficients ¹⁷α_diff and ¹⁸α_diff are 1.0146 ± 0.0002 and 1.0283 ± 0.0003, respectively. We also determined, for the first time, the ratio In(¹⁷α_diff)/In(¹⁸α_diff) as 0.5185 ± 0.0002. This ratio, which is in excellent agreement with the theoretical value of 0.5184, is significantly smaller than the ratio in vapor-liquid equilibrium (0.529). We show how this new experimental information gives rise to ¹⁷O excess in meteoric water, and how it can be applied in isotope hydrology.