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

Abstract

Stratospheric mixing ratios of CF 3 D from 100 mb to 17 mb (≈ 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‐1, ‐2, and ‐3 missions, extend in latitude from 70°S to 65°N. We find CH 3 D entering the stratosphere at an average mixing ratio of (9.9±0.8) × 10 −10 with a D/H ratio in methane (7.1±7.4)% less than that in Standard Mean Ocean Water (SMOW) (1σ combined precision and systematic error). In the mid to lower stratosphere, the average lifetime of CH 3 D 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 (1.0±0.1) molecules of stratospheric HDO are produced for each CH 3 D destroyed (1σ combined precision and systematic error), indicating that the rate of HDO production is approximately equal to the rate of CH 3 D destruction. Assuming negligible amounts of deuterium in species other than HDO, CH 3 D 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 CH 3 D destroyed.