Pressure sounding of the middle atmosphere from ATMOS solar occultationmeasurements of atmospheric CO2 absorption lines

A method for retrieving the atmospheric pressure corresponding to the tangent pointof an infrared spectrum recorded in the solar occultation mode is described andapplied to measurements made by the Atmospheric Trace Molecule Spectroscopy (ATMOS)Fourier-transform spectrometer. Tangent pressure values are inferred frommeasurements of isolated CO2 lines with temperature-insensitive strengthsby measuring the slant-column CO2 amount and by adjusting the viewinggeometry until the calculated column matches the observed column. Tangent pressuresare determined with a spectroscopic precision of 1\%--3\%,corresponding to a tangent-point height precision of 70--210 m. The totaluncertainty is limited primarily by the quality of the spectra and ranges between4 and 6 (280--420 m) for spectra with signal-to-noise ratiosof 300:1 and between 4 and 10 for spectra with signal-to-noiseratios of 100:1. The retrieval of atmospheric pressure increases the accuracy of theretrieved-gas concentrations by minimizing the effect of systematic errors introducedby climatological pressure data, ephemeris parameters, and the uncertainties ininstrumental pointing.