Kinetische Isotopentrennung bei der Verdampfung von Wasser

Water evaporating into air not saturated with water vapour shows a stronger enrichment of D and O-18 than could be ascribed to the simple equilibrium isotopic fractionation. Kinetic effects are responsible for the increase in isotopic fractionation between liquid and gaseous phase. The so-called kinetic fractionation factor is calculated by means of a simple model. Comparison with the experimentally obtained fractionation factors proves the assumption that the kinetic fractionation is produced by the different diffusion velocities of H₂O-16, H₂O-18 and HDO-16 in the surrounding turbulent air. The consequences of the kinetic separation for the evaporation of the oceans are briefly discussed. Those for the evaporation of a limited water reservoir in isotopic exchange with the water vapour of the air are more extensively investigated. In the latter case calculation and experiments show that during evaporation the D- and O-18-contents approach stationary values as the water mass decreases.