Carbon dioxide concentrations in the lower thermosphere

Carbon dioxide number densities were measured for the first time in the sunlit lower thermosphere on June 29, 1974, above Wallops Island (38°N). The CO 2 concentrations were obtained with a mass spectrometer that employed a cryo‐cooled ion source. Below the turbopause the CO 2 densities correspond to the ground level mixing ratio; above the turbopause the observed n (CO 2 )/ n (Ar) ratio decreases more rapidly with altitude than is expected for diffusive equilibrium. Photolysis and reactions with the O + ion alone remove sufficient amounts of CO 2 to explain the observed deviation from a pure diffusive equilibrium separation.