A review of CO₂ and CO abundances in the middle atmosphere

A review is presented of CO₂ and CO in the middle atmosphere. Knowledge of their abundances is important for understanding the thermal budget and transport processes of the middle atmosphere, and for its remote sounding. For both CO and CO₂, several techniques have been used to measure their abundance. Significant improvements have occurred over the last two decades with high quality satellite data now available from the Atmospheric Trace Molecule Spectroscopy (ATMOS) and Improved Stratospheric and Mesospheric Sounder (ISAMS) experiments. The CO₂ observations are well explained by models that account for diffusive separation, but a small overestimate above 95 km still remains. TIME-GCM calculations suggest a large depletion in the polar winter, down to near the stratopause which is as yet unconfirmed but which could be significant for remote sensing. The CO observations confirm the basic features of the seasonal and latitudinal variability predicted by dynamical models, particularly the enhancements in polar winter. However, the models underpredict the mesospheric CO abundance at mid-latitudes by a factor of 1.5-3 and fail to reproduce the strong latitudinal and vertical gradients in the polar regions. A reduction in the OH abundance from standard chemical models may partially solve this discrepancy. Finally, a comparison of simultaneously measured CO₂ and CO in the upper mesosphere/lower thermosphere with models suggests an eddy diffusion coefficient about 2-3 times smaller than those currently used.