Non-local thermodynamic equilibrium limb radiance from O 3 and CO 2 in the 9–11 μm spectral region

Abstract

Satellite remote sensing of mesospheric and thermospheric O 3 abundance in the terrestrial atmosphere often uses 9–11 μm thermal emission. In this paper, we apply a line-by-line non-local thermodynamic equilibrium (non-LTE) radiance model to this spectral region and investigate the conditions of LTE breakdown and the effect that this has on the limb radiance measured by an i.r. sounder. Monochromatic and band-integrated radiance calculations have been performed for limb view tangent heights between 55 and 105 km under daytime and nightime conditions. Non-LTE emission from both O 3 and CO 2 are shown to be important with the divergence of radiance from LTE values and the diurnal variation being band dependent. We have shown that the contribution of the CO 2 bands to the Limb Infrared Monitor of the Stratosphere O 3 channel is significant for daytime conditions at tangent heights above about 60 km. A study has been made to choose O 3 sounding channel spectral passbands for the High Resolution Dynamics Limb Sounder. High resolution calculations are required to determine those spectral intervals that will filter radiance from selected bands and characterize their non-LTE behavior. This will allow for improved O 3 retrievals above 70 km and non-LTE studies.