Stratospheric measurements of continuous absorption near 2400 cm(-1)

Theoretical far-wing line shape and absorption for high-temperature CO2

Theoretical results for the far-wing line shapes and corresponding absorption coefficients in the high-frequency wing of the nu(3) fundamental band of self-broadened CO(2) are presented for a number of temperatures between 218 and 751 K. These first-principles calculations are made assuming binary collisions within the framework of a quasi-static theory with a more accurate interaction potential than in previous calculations. The theoretical results are compared with existing laboratory data and are in good agreement for all the temperatures considered.

Infrared collision-induced absorption by N(2) near 4.3 μm for atmospheric applications: measurements and empirical modeling

Accurate measurements of collision-induced absorption by pure nitrogen in the fundamental band near 4.3 μm have been made in the 0-10 atm and 230-300 K pressure and temperature ranges, respectively. A Fourier-transform spectrometer was used with a resolution of 0.5 cm(-1). The current measurements, which agree well with previous ones but are more precise, reveal that weak features are superimposed on the broad N(2) continuum. These features have negligible temperature dependence, and their origin is not clear at the present time. Available experimental data in the 190-300 K temperature range have been used to build a simple empirical model that is suitable for use to compute atmospheric N(2) absorption. Tests indicate that this model is accurate unlike the estimates produced by widely used atmospheric transmission codes.

Transmission window near 2400 cm(-1): an experimental and modeling study

The absorption in the 2400-cm(-1) region is dominated by continuum absorptions from carbon dioxide and nitrogen, and it is important to be able to describe the temperature dependence of these two continua. A series of measurements of atmospheric transmission over a 5.7-km range were carried out during the summer and winter seasons of 1988. Following a brief description of the experiments a selected number of spectra, covering the temperature range from -21.4 to 30.3 degrees C, are presented. These measurements are compared with predictions by the atmospheric transmission model FASCOD2 and modified versions using models of the carbon dioxide and the nitrogen continua derived from experimental laboratory measurements. Finally, an improved temperature-dependent model for the nitrogen continuum is derived from atmospheric transmission measurements. The model covers the range of temperatures found in the troposphere and differs significantly from laboratory-based measurements.

Temperature dependence of the absorption in the region beyond the 4.3-microm band head of CO(2) . 2: N(2) and O(2) broadening

In this paper we present the first accurate determination, from laboratory measurements, of the temperature dependence of CO(2) continuous absorption beyond the v(3) band head, with N(2) and O(2) as perturbers. The chi form factors previously published have been tested and optimized, and new form factors are proposed. It has been clearly demonstrated for N(2), as well as for O(2) broadening, that a chi factor independent of T in the range 193 </= T </= 300 K gives calculated values of the absorption coefficient in fair agreement (+/-20% level) with observed values in the spectral region 2400 < sigma < 2500 cm(-1) .

Temperature dependence of the absorption in the region beyond the 4.3-microm band head of CO(2). 1: Pure CO(2) case

The temperature dependence of the high frequency far wings of the self-broadened CO(2) lines has been investigated in the 2400-2600-cm(-1) spectral region. The temperature dependence of the corrective shape factor X(sigma,T) is demonstrated for the first time.

Effect of interferogram smearing on atmospheric limb sounding by Fourier transform spectroscopy

A method is presented for analyzing an absorption spectrum obtained from the interferogram measured by an interferometer operated in the solar occultation mode. In this mode the complete interferogram is smeared with various components of individual interferograms generated by rays passing through different tangent altitudes. It is shown that the effective tangent altitude of the spectrum is the altitude at which the center fringe of the interferogram is recorded and that the other components of the interferogram only define the instrument line shape. The interferogram smearing effectively creates strong sidelobes on absorption lines so that a strong apodization on the interferogram is recommended for the solar occultation experiment. These concepts are applied to retrieve pressure and temperature simultaneously from stratospheric absorption spectra in the CO(2) 4.3-microm band obtained by a balloon-borne interferometer in 1976 over Palestine, Tex. Included in the analysis of the CO(2) 4.3-microm band are the continuum absorptions by the pressure-induced N(2) fundamental band and by far wings of the CO(2) v(3) band. The CO(2) absorption line is corrected by a sub-Lorentzian function. Excellent agreement is found between the observed and simulated spectra.