The pressure dependence of the experimentally-determined line intensity and continuum absorption of pure CO2 in the 1.6µm region

Abstract

Fourier-transform measurements of pure CO2 spectra in the 1.6 µm region covering bands from the ground state to the 30011, 30012, 30013, and 30014 states at ambient temperature and 212 K with pressures up to 1 atm have been recorded. The measured spectra have been multispectrum-fitted with the Hartmann-Tran model with first order line-mixing taken into account. In the fits, a linear decrease of the line intensities with increasing pressure was introduced. From the fitted baselines, the self-continuum, which complements the contribution of the local lines within +/- 25 cm-1, was determined for the two strongest bands, 30012-00001 and 30013-00001. The depleted intensity was found to be transferred to the continuum for both temperatures. The intensity in the continuum at 1 atm was about 1 % of the total band intensity for ambient temperature and about 3 % at 212 K. For both temperatures, the average depleted intensity/continuum area was found in excellent agreement with values estimated from the second virial coefficient. From these results, a new spectroscopic database with pure CO2 line profile parameters was produced, with systematic line intensity uncertainties well below 0.1 %. Retrieval simulations for the CO2 1.6 µm and 2 µm bands measurements, planned with the future CO2M mission, similar to the OCO mission, were carried out to assess the impact of intensity depletion and continuum. The impact of the continuum was larger than that of the depletion and larger for the 2 µm band than for the 1.6 µm band, with a substantial column error of 2 % for the 2 µm band when omitting both, depletion and continuum. The findings regarding pressure-dependent intensity and continuum may be applicable to other in-band continua showing similarity to the monomer band in shape and position. The new results may increase the knowledge on the physical nature of continua.