High Resolution FTIR Measurements of Voigt Half-Widths of 12CO2 Broadened by H2O at 4.3 μm

To support remote sensing of carbon dioxide in the tropics, the half-widths were obtained for nearly 150 lines of 12CO2 in the 2290 - 2390 cm-1 region. For this, six spectra of 12CO2 were recorded at 0.00389 cm-1 resolution using a Bruker IFS-125HR at Jet Propulsion Laboratory. The absorption cell length was 6.1 cm, and the water pressures ranged from 20.1 to 26.5 torr near 300 K. Partial pressures of CO2 and H2O in the mixtures were determined, respectively, by measuring line intensities of CO2 in the ν3 band and those of H2O in the ν2 band. Cell temperatures were validated by deriving rotational temperature from the ν3 and ν2+ν3- ν2 intensities. Positions, intensities and broadening values were retrieved spectrum by spectrum using a non-linear least squares line fitting algorithm employing a Voigt line shape profile and an instrumental line shape consisting of a sinc function with aperture corrections. Widths were obtained for both the fundamental and the hot band of 12CO2. Half-widths of carbon dioxide broadened by other atmospheric gases (such as O2, N2, CO2) decrease with increasing rotational quantum number J, but the H2O-broadened half-widths follow behavior predicted from theory1, with values of ~ 0.125 cm-1/atm at low {J}, increasing to ~ 0.145 cm-1/atm near J equal 50 to 60. Therefore, the contribution from water broadened carbon dioxide should be taken into account in the modeling of tropospheric CO2, particularly for experiments such as the Orbiting Carbon Observatory, in order to achieve a sub-percent precision in the measurements of the CO2 column averaged mixing ratio.2 1 Rosenmann, L., J. M. Hartmann, M.Y. Perrin, and J. Taine (1988), Appl. Opt., 27, 3902 - 3906. 2 The research described in this paper was performed at the Jet Propulsion Laboratory, California Institute of Technology, under contract with The National Aeronautics and Space Administration.