Characterization of the H2O+CO2 continuum within the infrared transparency windows

Abstract

Absorption spectra of humidified CO2 have been recorded at room temperature by cavity enhanced absorption techniques (CRDS and OFCEAS): (i) in three spectral ranges of the 1.6 µm window (5720–6045 cm−1; 6390–6460 cm−1 and 6570–6665 cm−1), (ii) in four narrow spectral intervals of the 2.3 µm window (4243–4255 cm−1; 4301.3–4302 cm−1; 4421.5–4440 cm−1 and 4518–4535 cm−1), and (iii) around 2853 cm−1. All these spectral ranges are situated in transparency windows of both H2O and CO2. The binary absorption coefficients (BCO2−H2O+BH2O−CO2)are retrieved from low pressure spectra (<1 atm) recorded with different molar fractions of water vapor in CO2 after subtracting the H2O and CO2 local monomer contributions and the self-continuum contribution of each species (i.e. H2OH2O and CO2CO2). Experimental room temperature binary coefficients are then compared to the only available empirical model based on line shape profiles with χ-factors. This model well reproduces our experimental values on the low- and high-frequency edges of the 1.6 µm window and gives a relatively good agreement for the 2853 cm−1 data point. Larger differences are observed in the 2.3 µm window where the calculated values are underestimated by a factor of 3. Around 6000 cm−1, an additional absorption peak is observed which is tentatively interpreted as a collision induced absorption band due to the simultaneous excitation of the H2O and CO2 molecules.