Solar occultation spectra obtained with a balloon-borne interferometer have been used to study continuous absorption by N2 and CO2 near 2400 cm-1 in the lower stratosphere. Synthetic continuum transmittances, calculated from published coefficients for far-wing absorption by CO2 lines and for pressure-induced absorption by the fundamental band of N2, are in fair agreement with the observed stratospheric values. The continuum close to the ν3 R-branch band head of CO2 is sensitive to the CO2 far-wing line shape. Therefore, given highly accurate knowledge of the N2 continuum from laboratory data, high-resolution stratospheric spectra provide a sensitive means for in situ testing of various air-broadened CO2 line shapes at low temperatures.
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Quantitative absorpance measurements have been made in pure CO2 and mixtures of CO2 with N2 and O2 in a 10 m White Perkin-Elmer cell. With absorbing paths up to 50 m-atm, results have been obtained from the band head at 2397 cm-1 to 2575 cm-1. The continuous absorption, which is due to the extreme tails of the strong lines in the v3 band centered at 2349 cm-1, is much less than calculated with the Lorentz line shape. A good fit with the data is obtained with an empirical line-shape which retains the Lorentz pressure dependence but requires a nearly exponential modification of the frequency dependence.
