Fourier transform infrared spectroscopy measurements of H₂O-broadened half-widths of CO₂ at 4.3 μm

摘要

To support remote sensing of carbon dioxide in the troposphere, H₂O pressure-broadened half-widths were obtained for 182 lines of CO₂ in the 2250-2390cm-1 region. For this, six spectra of CO₂ were recorded at 0.00389cm-1 resolution using a Bruker IFS-125HR at the Jet Propulsion Laboratory. The absorption cell length was 6.14cm, and the water pressures ranged from 20.1 to 26.5 torr (1 torr = 133.322 4 Pa) near room temperatures. Partial pressures of the species in the mixtures were determined by measuring selected line intensities in the v₃ band of CO₂ and the v₂ band of H₂O. Sample temperatures were validated by deriving rotational temperature from the v₃ and v₂+ v₃- v₂ intensities of 12CO₂ and those of CO (1-0). Positions, intensities, and half-widths were retrieved spectrum by spectrum using a nonlinear least-squares line-fitting algorithm, employing a standard Voigt line shape profile and an instrumental line shape consisting of a sinc function with aperture correction. Half-widths obtained for both the fundamental and the hot band of 12CO₂ and the fundamental v₃ band of 13CO₂ had similar values. While half-widths of CO₂ broadened by other atmospheric gases (such as N₂, O₂, CO₂, and air) tend to decrease with increasing rotational quantum number J″, the H₂O-broadened half-widths were observed to increase for intermediate J′′ (8≤ J′′≤ 42): ~0.127cm-1atm-1 near J′′ = 8, and increasing to ~0.143cm-1atm-1 (1 atm = 101.325 kPa) towards J″ = 42. Moreover, for 10≤ J″≤ 40, the empirical widths were within ~2%−3% of theoretical calculations. Since water vapor could reach up to 5% of ambient atmospheric surface pressure in the tropical regions, water broadened half-widths are required to model tropospheric CO₂, particularly for high-precision remote sensing, to achieve a sub-percent precision in the measurements of CO₂ column averaged mixing ratio. Since little vibrational dependence in line broadening has been seen, these results at 4.3μm can be used for other bands of CO₂.Pour réussir la mesure à distance du CO₂ dans la troposphère, nous avons obtenu les valeurs de demi-largeurs, étalées par pression partielle de H₂O, pour 182 lignes dans la région 2250-2390cm-1. À cette fin, nous avons enregistré six spectres de CO₂ avec une résolution de 0,00389cm-1 en utilisant un spectromètre Bruker IFS-125HR au Jet Propulsion Laboratory. La longueur de la cellule d'absorption était 6,14cm et la pression de H₂O a varié de 20,1 à 26,5 torr (1 torr = 133.322 4 Pa) à la température de la pièce. Les pressions partielles des composantes du mélange ont été déterminées en mesurant l'intensité de lignes choisies dans la bande v₃ du CO₂ et dans la bande v₂ de H₂O. Nous avons validé la température des échantillons en obtenant leur température rotationnelle à partir des intensités v₃ et v₂+ v₃- v₂ du CO₂ et celles du CO (1,0). Les positions, intensités et demi-largeurs des lignes ont été extraites spectre par spectre via un algorithme aux moindres carrés non linéaire utilisant un profil de Voigt pour la forme de la ligne et une forme expérimentale faite d'une fonction sinc avec une correction d'ouverture.