Solar occultation sounding of pressure and temperature using narrowband radiometers

Atmospheric CO(2) monitoring from space

A spectroscopic method of monitoring the atmospheric CO(2) mixing ratio vertical profile from space is described. An experimental design is presented for a solar occultation mode with the O(2)A band in the visible region to retrieve pressure and temperature profiles first, and then several CO(2) bands in the infrared region at 4.3, 2.7, and 2.0 mum to obtain CO(2) mixing ratio profiles. Instrument techniques considered are low resolution Fourier transform spectrometry and radiometry of various bandwidths. Simulations indicate that the precision of the pressure, temperature, and CO(2) mixing ratio measurements for an altitude region 30-10 km are less than 1%, 1 K, and 1%, respectively, for the case of the Fourier transform spectrometer and approximately 1%, 1 K, and 2% for the case of the radiometer. With careful experimental design, measurements can be made with better precision and also can extend below 10 km. This inferred precision of CO(2) may be considered to be good enough for investigating atmospheric dynamics when CO(2) is used as a tracer and also for measuring spatial and temporal variations of CO(2) mixing ratios in the range of 0.5-6.5% of 350 parts per million by volume in the troposphere and the lower stratosphere.

Pressure sounding of the middle atmosphere from ATMOS solar occultation measurements of atmospheric CO(2) absorption lines

A method for retrieving the atmospheric pressure corresponding to the tangent point of an infrared spectrum recorded in the solar occultation mode is described and applied to measurements made by the Atmospheric Trace Molecule Spectroscopy (ATMOS) Fourier-transform spectrometer. Tangent pressure values are inferred from measurements of isolated CO(2) lines with temperature-insensitive strengths by measuring the slant-column CO(2) amount and by adjusting the viewing geometry until the calculated column matches the observed column. Tangent pressures are determined with a spectroscopic precision of l%-3%, corresponding to a tangent-point height precision of 70-210 m. The total uncertainty is limited primarily by the quality of the spectra and ranges between 4% and 6% (280-420 m) for spectra with signal-to-noise ratios of 300:1 and between 4% and 10% for spectra with signal-to-noise ratios of 100:1. The retrieval of atmospheric pressure increases the accuracy of the retrieved-gas concentrations by minimizing the effect of systematic errors introduced by climatological pressure data, ephemeris parameters, and the uncertainties in instrumental pointing.

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.

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

Solar occultation spectra obtained with a balloon-borne interferometer have been used to study continuous absorption by N(2) and CO(2) near 2400 cm(-1) in the lower stratosphere. Synthetic continuum transmittances, calculated from published coefficients for far-wing absorption by CO(2) lines and for pressure-induced absorption by the fundamental band of N(2), are in fair agreement with the observed stratospheric values. The continuum close to the nu(3) R-branch band head of CO(2) is sensitive to the CO(2) far-wing line shape. Therefore, given highly accurate knowledge of the N(2) continuum from laboratory data, high-resolution stratospheric spectra provide a sensitive means for in situ testing of various air-broadened CO(2) line shapes at low temperatures.