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, H2O pressure-broadened half-widths were obtained for 182 lines of CO2 in the 2250–2390 cm–1 region. For this, six spectra of CO2 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 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): ∼0.127 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 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 4.3 µm can be used for other bands of CO2.

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