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On the distribution of CO 2 and CO in the mesosphere and lower thermosphere
Author(s) -
Garcia Rolando R.,
LópezPuertas Manuel,
Funke Bernd,
Marsh Daniel R.,
Kinnison Douglas E.,
Smith Anne K.,
GonzálezGalindo Francisco
Publication year - 2014
Publication title -
journal of geophysical research: atmospheres
Language(s) - English
Resource type - Journals
eISSN - 2169-8996
pISSN - 2169-897X
DOI - 10.1002/2013jd021208
Subject(s) - thermosphere , mesosphere , mesopause , atmospheric sciences , mixing ratio , atmosphere (unit) , gravity wave , eddy diffusion , thermal diffusivity , airglow , stratosphere , environmental science , physics , meteorology , astrophysics , thermodynamics , gravitational wave , geophysics , turbulence , ionosphere
We have used the Whole Atmosphere Community Climate Model (WACCM) to calculate the distribution of CO 2 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 CO 2 in the MLT and the rapid falloff of CO 2 above about 80 km. Analysis of the principal terms in the calculated budget of CO 2 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 CO 2 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. Simulated CO is also consistent with observations, except that in the standard version of the model, its mixing ratio is uniformly lower than observed above about 100 km. We conclude that WACCM likely underestimates the rate of production of CO in the lower thermosphere from photolysis of CO 2 at wavelengths < 121 nm, and we show that this stems from the use of a very large absorption cross section for O 2 in the wavelength range 105–121 nm. When a smaller cross section is used, photolysis of CO 2 increases by a factor of 2–3 at ~ 95–115 km and, as a result, CO mixing ratios become larger and agree much more closely with observations. We emphasize that the increase in CO 2 photolysis implies only minor changes in the vertical profile of CO 2 because photolytic loss is a minor term in the budget of CO 2 in the MLT.