Abstract

In view of the world-wide interest in the remote sensing of chloro-fluoro-carbons (CFC’s), especially CFC-11 (CFCl₃) and CFC-12 (CF₂Cl₂), hydro-chloro-fluoro-carbons (HCFC’s), HCFC-22 (CHFCl₂) in particular, the fluorocarbon CF₄, the chlorocarbon CCl₄, and sulphur hexaflouride (SF₆), we have been performing laboratory measurements of the spectral absorption cross-sections needed for the retrieval of the atmospheric vertical mixing ratios and column abudances of these chemicals from data obtained by satellite-based infrared instruments. These chemicals, which possess either a strong ozone-depletion potential or a global-warming potential, or both, have infrared absorption features that do not readily lend themselves to the conventional line-by-line analysis.^1,2 Therefore, we adapted a novel technique of measuring the absorption cross-section, k_ν (cm² molecule^-1), which is defined as k_ν = (-ln τ_ν)/pξL in terms of the spectral tranmittance τ_ν at the wavenumber ν, temperature T, and total pressure p along an optical path of length L through the absorbing chemical with a mixing ratio of ξ in N₂ (used in place of air without any measurable difference in the measured k_ν) directly. Highly accurate k_ν measured in the thermal infrared bands of CFC-11 (CFCl₃), CFC-12 (CF₂Cl₂), HCFC-22 (CHFCl₂), and SF₆ at several (T,p) combinations representing atmospheric layers³ have already been presented at the last Topical Meeting of this series.² We present now the significant enhancement that we have produced in the database on cross-sections by extending our earlier measurements to include additional chemicals such as CF₄ (CFC-14) and CCl₄ and conditions encountered in the atmosphere at Arctic and Antarctic latitudes. Our data (Figs. 1 and 2), which were obtained at pressures and temperatures given in commonly tabulated atmospheric models³ and represent tangent heights in solar occultation type remote sensing observations of the atmosphere as well as conditions encountered in the polar regions, are directly applicable to the analysis of the satellite data. The data were obtained at temperatures between 180 and 295 K using a high-resolution Fourier-transform spectrometer and are free from instrumental distortion, since the spectra were recorded at spectral resolution that was sufficiently high at broadening pressures corresponding to tropospheric and stratospheric layers. Various methods of extending these data to conditions not covered in the experiments will be discussed.

© 1997 Optical Society of America