Fourier transform infrared spectroscopy measurements of H₂O-broadened half-widths of CO₂ at 4.3 μmThis article is part of a Special Issue on Spectroscopy at the University of New Brunswick in honour of Colan Linton and Ron Lees.

To support remote sensing of carbon dioxide in the troposphere, H 2 O pressure-broadened half-widths were obtained for 182 lines of CO 2 in the 2250–2390 cm –1 region. For this, six spectra of CO 2 were recorded at 0.003 89 cm –1 resolution using a Bruker IFS-125HR at the Jet Propulsion Laboratory. The absorption cell length was 6.14 cm, 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 v 3 band of CO 2 and the v 2 band of H 2 O. Sample temperatures were validated by deriving rotational temperature from the v 3 and v 2 + v 3 – v 2 intensities of 12 CO 2 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 12 CO 2 and the fundamental v 3 band of 13 CO 2 had similar values. While half-widths of CO 2 broadened by other atmospheric gases (such as N 2 , O 2 , CO 2 , and air) tend to decrease with increasing rotational quantum number J″, the H 2 O-broadened half-widths were observed to increase for intermediate J′′ (8 ≤ J′′ ≤ 42): ∼0.127 cm –1 atm –1 near J′′ = 8, and increasing to ∼0.143 cm –1 atm –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 CO 2 , particularly for high-precision remote sensing, to achieve a sub-percent precision in the measurements of CO 2 column averaged mixing ratio. Since little vibrational dependence in line broadening has been seen, these results at 4.3 µm can be used for other bands of CO 2 .

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