CO2 Q-Branch Lineshapes for Atmospheric Remote Sensing

The high spectral resolution of today’s infrared remote sensing instruments puts great stress on our understanding of atmospheric spectral lineshapes. This is especially true for nadir sounders that use channels between spectral lines and in the far line wing, where the weighting functions are narrowest. The atmospheric emissions in these wing regions are sensitive to the molecular lineshape, which is difficult to measure in the laboratory over the wide range of atmospheric conditions. Since satellite temperature sounders utilize CO2 emission features we will concentrate on CO2 spectral lineshapes and their effect on atmospheric spectra in this paper. Two regions of the CO2 spectrum are often chosen for sounding channels; (1) Q-branch wings in the 15 µm region, and (2) the v 3 R-branch bandhead at 4.3 µm. Both of these regions are extremely sensitive to aspects of CO2 lineshapes that have only been studied in some detail in the last several years. For example, absorption coefficients in the CO2 Q-branch wing deviate from a Lorentz lineshape by up to 50% because of line mixing. Even larger deviations from a Lorentz lineshape are found in the 4.3 µm R-branch wing. This paper will concentrate on Q-branch spectral lineshapes which we are now able to model with high accuracy. Physically-based lineshape models that provide a basis for accurate spectral calculations under wide ranging conditions are still under development for the 4.3 µm region.