Non‐local thermodynamic equilibrium studies of the 15‐μm bands of CO2 for atmospheric remote sensing

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Abstract
A new line‐by‐line non‐local thermodynamic equilibrium (non‐LTE) radiance model based on the GENLN2 radiative transfer code is presented. This is capable of high resolution spectral radiance calculations for the upper atmosphere. We describe the non‐LTE model implementation and discuss the molecular state vibrational temperature input requirements for studies of the 15‐μm ν2 bands of CO2. Monochromatic and band‐integrated radiance calculations have been performed for atmospheric limb view tangent heights between 50 and 120 km for non‐LTE nighttime and daytime conditions. Two model atmospheres are considered, the U.S. 1976 Standard and a subarctic summer, which show, respectively, mean and large radiance differences from a reference LTE radiance calculation. Non‐LTE radiance considerations are shown to be important for the 15‐μm CO2 bands for tangent heights greater than 70 km, the magnitude of the divergence from LTE values and diurnal variation being dependent on the band and kinetic temperature profile. We show that the use of the Voigt line shape, rather than the Doppler, is important for tangent heights below 85 km, and that the inclusion of the effect of overlapping lines is a consideration for tangent heights below 75 km. We present calculations of synthetic radiance spectra showing the non‐LTE effect for two CO2 temperature sounding channels of instruments aboard the Upper Atmosphere Research Satellite as a demonstration of the model capability.

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