Analysis of the origins and implications of the18O content of stratospheric water vapor

Factors influencing the¹⁸O content of stratospheric H₂O are reviewed in order to provide a theoretical framework for the interpretation of measurements of this quantity, which are now becoming available. Depletions in¹⁸O of 5–10% in stratospheric H₂O are expected based on the known correlation between that of D and¹⁸O in tropospheric H₂O and observed measurements of large (typically 50%) depletions of D in stratospheric H₂O. H₂O formed in the stratosphere as a result of oxidation of CH₄ can be expected to reflect primarily the¹⁸O content of stratospheric O₂, which is the same as that of tropospheric O₂ (slightly enhanced with respect to standard mean ocean water). Thus, a reduction in the¹⁸O depletion is expected with increasing altitude, but not a large enhancement in¹⁸O in upper stratospheric H₂O as found in recent far infrared measurements. The observed large enhancement of¹⁸O in stratospheric O₃ is not expected to be reflected in stratospheric H₂O. Necessary laboratory data for the improved quantification of these effects are reviewed.