The CO2 absorption continuum by high pressure CRDS in the 1.74 µm window

Abstract

The very weak absorption continuum of CO2 is studied by Cavity Ring Down Spectroscopy in three 20cm−1 wide spectral intervals near the centre of the 1.74µm window (5693–5795cm−1). For each spectral interval, a set of room temperature spectra is recorded at pressures between 0 and 10bar thanks to a high pressure CRDS spectrometer. The absorption continuum is retrieved after subtraction of the contributions due to Rayleigh scattering and to local lines of CO2 and water (present as an impurity in the sample) from the measured extinction. Due to some deficiencies of the CO2 HITRAN2012 line list, a composite line list had to be built on the basis of the Ames calculated line list with line positions adjusted according to the Carbon Dioxide Spectroscopic Databank and self-broadening and pressure shift coefficients calculated with the Complex Robert Bonamy method. The local line contribution of the CO2 monomer is calculated using this list and a Voigt profile truncated at ±25cm−1 from the line centre. Line mixing effects were taken into account through the use of the impact and Energy Corrected Sudden approximations.The density dependence of the retrieved continuum absorption was found to be purely quadratic in the low frequency interval below 5710cm−1 but a small significant linear contribution was required to reproduce the observations above this value. This linear increase is tentatively attributed to the foreign-continuum of water vapor present in CO2 sample with a relative concentration of some tens ppm.The retrieved binary coefficient is observed to vary smoothly with the wavenumber with a minimum value of 6×10−10cm−1amagat−2. By gathering the present data with the results reported in Kassi et al. J Quant Spectrosc Radiat Transf 2015;167:97, a recommended set of binary coefficients is provided for the 5700–5950cm−1 region.