Abstract

The study of CO in the terrestrial middle atmosphere is of considerable interest since it plays an important role in many photochemical processes involving species like OH and O₃. It can also be a very good tracer, especially in the mesosphere where its photochemical lifetime becomes longer^1,2. Its volume mixing ratio is very variable in altitude, latitude and time³ because of the variety of production and loss mechanisms. ISAMS is the only instrument on board UARS which measures CO atmospheric emission at 4.7 μm. This emission originates from the first vibrationally excited level of the CO molecule which is in non-LTE in the whole mesosphere⁴,⁵,⁶. Also the increase with height of the concentration of CO in the mesosphere gives rise to the important contribution of mesospheric levels to limb radiances at stratospheric tangent heights. Therefore, knowledge of the non-LTE source function is essential before CO abundances in the middle atmosphere can be retrieved from 4.7 limb radiances. We describe here the non-LTE model we have developed to compute the population of CO(1) and the way in which these calculations will be incorporated into the ISMAS retrieval scheme.

© 1991 Optical Society of America