Balloon‐borne in situ measurements of stratospheric H 2 O, CH 4 and H 2 at midlatitudes

Using a new Lyman α hygrometer, balloon‐borne measurements of H 2 O were performed on September 20, 1993 at midlatitudes (43° N). A cryogenic whole air sampler to measure CH 4 and H 2 amongst other long‐lived trace gases was operated on the same payload. The profile of the water vapor mixing ratio showed a well‐pronounced hygropause of 4 ppmv at θ = 420 K increasing to 6.0 ppmv at θ = 920 K. For θ > 400 K, total hydrogen ΣH 2 = H 2 O + 2·CH 4 + H 2 appeared to be constant at a mean value of 7.72 ± 0.13 ppmv; the overall accuracy of this value is 0.65 ppmv. From the correlation functions of CH 4 with H 2 and H 2 O the water vapor yield from methane oxidation is determined with high precision to be 1.975 ± 0.030. The globally averaged water vapor mixing ratio entering the stratosphere from the troposphere was found to be 3.9 ± 1.0 ppmv. Intercomparison with H 2 O and CH 4 profiles obtained nearby from the Halogen Occultation Experiment (HALOE) on board the UARS satellite showed good agreement within the experimental errors.