On the distribution of CO₂ and CO in the mesosphere and lower thermosphere

We have used the Whole Atmosphere Community Climate Model (WACCM) to calculate the distribution of CO₂ and CO in the mesosphere and lower thermosphere (MLT), and we have compared the results with observations, mainly from the Atmospheric Chemistry Experiment Fourier Transform Spectrometer and Michelson Interferometer for Passive Atmospheric Sounding satellite-borne instruments. We find that WACCM can reproduce the observed distribution of CO₂ in the MLT and the rapid falloff of CO₂ above about 80 km.

Analysis of the principal terms in the calculated budget of CO₂ shows that its global-mean vertical profile is determined mainly by the competition between molecular diffusive separation and eddy mixing by gravity waves. The model underestimates somewhat the mixing ratio of CO₂ in the thermosphere compared to that in the observations, but we show that the discrepancy may be eliminated by a reasonable adjustment of the Prandtl number used to calculate the diffusivity due to gravity waves.