Influence of line mixing on absorption by CO2 Q branches in atmospheric balloon‐borne spectra near 13 μm

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Abstract
Line mixing in CO2 Q branches is studied in the 13.2 μm region from balloon‐borne atmospheric transmission measurements. Eighty‐five spectra were recorded (at 67°N, 22°E) in the 660–960 cm−1 region using a Fourier transform spectrometer with an unapodized resolution (full width at half maximum) of 0.013 cm−1. They involve balloon altitudes and tangent heights from 13 to 30 km and a large range of optical thicknesses. The results of computations assuming Voigt line shapes as well as accounting for line mixing are compared with measured transmissions in three CO2 Q branches of different intensities and symmetries. They confirm the significant effect of line mixing and validate recent line coupling models. As a consequence of the accuracy of the forward approach the Q branches can be used for remote sensing; this should lead to improved precision of pressure/temperature retrievals since absorption in the Q branch wing region is particularly pressure sensitive.