Stratospheric observations of CH3D and HDO from ATMOS infrared solar spectra Enrichments of deuterium in methane and implications for HD

Stratospheric mixing ratios of CH3D from 100 mb to 17mb (=15 to 28 km) and HDO from 100 mb to 10 mb (=15 to 32 km) have been inferred from high resolution solar occultation infrared spectra from the Atmospheric Trace MOlecule Spectroscopy (ATMOS) Fourier-transform interferometer. The spectra, taken on board the Space Shuttle during the Spacelab 3 and ATLAS-l,-2, and -3 missions, extend in latitude from 70øS to 65øN. We find CH3D entering the stratosphere at an average mixing ratio of (9.9_+0.8)x10 -lø with a D/H ratio in methane (7.1_+7.4)% less than that in Standard Mean Ocean Water (SMOW) (1 • combined precision and systematic error). In the mid to lower stratosphere, the average lifetime of CH3D is found to be (1.19_+0.02) times that of CH 4, resulting in an increasing D/H ratio in methane as air "ages" and the methane mixing ratio decreases. We find an average of (1.•.1) molecules of stratospheric HDO are produced for each CH3D destroyed (1c• combined precision and systematic error), indicating that the rate of HDO production isapproximately equal to the rate of CH3D destruction. Assuming negligible amounts of deuterium in species other than HDO, CH3D and HD, this limits the possible change in the stratospheric HD mixing ratio below about 10 mb to be _+0.1 molecules HD created per molecule CH3D destroyed.