Isotopic composition of stratospheric water vapor: Measurements and photochemistry

We present a photochemical model describing the changes in the isotopic com position of stratospheric water vapor that result from methane oxidation an d reactions with O(D-1). We then compare model calculations with measuremen ts made with the Smithsonian Astrophysical Observatory far-infrared spectro meter during seven balloon flights that took place between 1989 and 1997. T he agreement between the model calculations and measurements establishes th e validity of the model and the internal consistency of the data set. Final ly, we use the model and measurements together to estimate the mixing ratio and isotopic composition of water vapor entering the stratosphere and find that the average over all flights of the water vapor mixing ratio, deltaD, delta O-18, and delta O-17 are 3.48 +/- 0.15 parts per million by volume, -679 +/- 20 parts per thousand, -128 +/- 31 parts per thousand, and -84 +/- 31 parts per thousand, respectively, where the errors indicate the estimat ed accuracy.