The hydrogen isotopic composition of water vapor entering the stratosphere inferred from high-precision measurements of δD-CH4 and δD-H2

The hydrogen isotopic composition of water vapor entering the stratosphere provides an important constraint on the mechanisms for dehydration of air ascending through the tropical tropopause layer. We have inferred the annual mean hydrogen isotopic composition of water vapor entering the stratosphere (or δD‐H2O0) for the mid to late 1990s based on high‐precision measurements of the hydrogen isotopic compositions of stratospheric H2 and CH4 from whole air samples collected on the NASA ER‐2 aircraft between 1996 and 2000 and remote observations of δD‐H2O from the FIRS‐2 far infrared spectrometer. We calculate an annual mean value for δD‐H2O0 of −653 (+24/−25)‰ relative to Vienna standard mean ocean water (VSMOW). Previous inferences from balloon‐borne and spacecraft remote‐sensing observations are ∼20‰ lighter than the value from this analysis. We attribute the difference to an underestimation of deuterium in the molecular H2 reservoir in earlier work. This precise and more accurate value for the annual mean δD‐H2O0 will be useful as a 1990's benchmark for detecting future changes in the details of the dehydration of air due to the impact of climate change on convection intensity, cloud microphysics, or tropical tropopause layer temperatures. In addition, we report a value for the total deuterium content in the three main stratospheric hydrogen reservoirs HDO, HD, and CH3D of 1.60 (+0.02/−0.03) ppbv.