Spectra calculations in central and wing regions of CO₂ IR bands between 10 and 20μm. III: Atmospheric emission spectra

A theoretical model for the prediction of CO₂ absorption in both central and wing regions of infrared absorption bands was presented in the companion paper I. It correctly accounts for line-mixing effects and was validated by comparisons with laboratory spectra in the 600-1000 cm^-1 region. This quality was confirmed using atmospheric transmissions measured by solar occultation experiments in the second paper. The present work completes these studies by now considering atmospheric emission in the 10-20 μm range. Comparisons are made between computed atmospheric radiances and measurements obtained using four different Fourier transform experiments collecting spectra for nadir, up-looking, as well as limb (from balloon and satellite) geometries. Our results confirm that using a Voigt model can lead to very large errors that affect the spectrum more than 300 cm^-1 away from the center of the CO₂ ν₂ band. They also demonstrate the capability of our model to represent accurately the radiances in the entire region for a variety of atmospheric paths. This success opens interesting perspectives for the sounding of pressure and temperature profiles, particularly at low altitudes. Another benefit of the quality of the model should be an increased accuracy in the retrieval of atmospheric state parameters from broad features in the measured spectra (clouds, aerosols, heavy trace gases).