Influence of line mixing on absorption by CH₄ in atmospheric balloon-borne spectra near 3.3 μm

The influence of line mixing on spectral signatures of atmospheric methane is studied in the region of the ν₃ band of CH₄ near 3.3 μm using balloon-borne stratospheric transmission measurements. Twenty-four solar occultation spectra are used which were recorded in the 2850-3115 cm^-1 region using a Fourier transform spectrometer with an unapodized resolution (FWHM) of 0.013 cm^-1. They were obtained at float (≈30 km) for tangent heights from about 13 to 30 km and involve a large range of optical thickness. Two models are used, one assuming purely Voigt line shapes and one accounting for collisional line mixing between lines of the ν₃ band of methane, respectively. The calculations are made with no adjustable parameter, using input data (spectroscopic and collisional parameters, mixing ratios, pressure and temperature profiles, etc.) obtained independently. Comparisons between calculated and measured transmissions are made in the vicinity of several methane features in the ν₃ band. They confirm the significant effect of line mixing expected from previous laboratory studies and validate a recently developed line-coupling model.