Kinetic fractionation of water isotopes during liquid condensation under super-saturated condition

Abstract

Results of experiments carried out to study the kinetic fractionation during liquid condensation of water vapor in super-saturated environment are reported. The ground level ambient atmospheric water vapor was condensed on an ice-cooled (∼0°C) metallic surface to obtain the liquid condensate. The ambient vapor was also sampled simultaneously by complete cryogenic (−78°C) trapping to obtain unfractionated liquefied vapor.Unlike equilibrium fractionation, the liquid condensate at 0°C was found to be depleted in 18O and moderately enriched in 2H compared to vapor. Consequently, the liquid had unusually high d-excess. This observation signifies kinetic fractionation, similar to that reported for ice condensation (Jouzel and Merlivat, 1984).The observed kinetic effect can be explained by diffusion of water isotopologues across super-saturated boundary layer formed when condensation temperature is lower than the dew point. The magnitude of kinetic fractionation depends on degree of super-saturation determined by temperature and relative humidity (Rh) in ambient air and the temperature of condensation. Limited observations indicate possibility of similar kinetic fractionation during natural condensation of dew. This suggests that kinetic fractionation may also be involved in various other natural condensation processes when super-saturated conditions are generated.