Model, software, and database for computation of line-mixing effects in infrared Q branches of atmospheric CO₂ - I. Symmetric isotopomers

A theoretical model based on the energy-corrected sudden approximation is used in order to account for line-mixing effects in infrared Q branches of symmetric isotopomers of CO₂. Its performance is demonstrated by comparisons with a large number (about 130) of CO₂-N₂ and CO₂-O₂ laboratory spectra recorded by several instrument setup: nine Q branches of different vibrational symmetries lying between 4 and 17 μm are investigated in wide ranges of pressure (0.05-10 atm) and temperature (200-300 K). The model is used to generate a set of suitable parameters and FORTRAN software (available by ftp) for the calculation of the absorption within ¹²C¹⁶O₂. ¹³C¹⁶O₂, and ¹²C¹⁸O₂ infrared Q branches under atmospheric conditions, which can be easily included in existing radiance/transmission computer codes. Comparisons are made between many (about 280) computed atmospheric spectra and values measured using two different balloon-borne high-resolution Fourier transform instruments: transmission (solar occultation) as well as radiance (limb emission) measurements of seven Q branches between 12 and 17 μm for a large range of atmospheric air masses and pressure/temperature conditions have been used, including the v₂ band of both ¹²C¹⁶O₂ and ¹³C¹⁶O₂. The results confirm the need to account for the effects of line-mixing and demonstrate the capability of the model to represent accurately the absorption in regions which are often used for temperature/pressure sounding of the atmosphere by space instruments. Finally, quantitative criteria assessing the validity of the widely used Rosenkranz first-order approximation are given.