Factors influencing the
18O content of stratospheric H
2O are reviewed in order to provide a theoretical framework for the interpretation of measurements of this quantity, which are now becoming available. Depletions in
18O of 5–10% in stratospheric H
2O are expected based on the known correlation between that of D and
18O in tropospheric H
2O and observed measurements of large (typically 50%) depletions of D in stratospheric H
2O. H
2O formed in the stratosphere as a result of oxidation of CH
4 can be expected to reflect primarily the
18O content of stratospheric O
2, which is the same as that of tropospheric O
2 (slightly enhanced with respect to standard mean ocean water). Thus, a reduction in the
18O depletion is expected with increasing altitude, but not a large enhancement in
18O in upper stratospheric H
2O as found in recent far infrared measurements. The observed large enhancement of
18O in stratospheric O
3 is not expected to be reflected in stratospheric H
2O. Necessary laboratory data for the improved quantification of these effects are reviewed.
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