Impact of broadening line parameters on H2O retrievals in CO2 dense atmospheres

For decades, the remote sensing measurements have been made in planetary atmospheres in the Solar System and beyond. As the performance of the space instruments improves, the atmospheric science community is more and more in need of accurate spectroscopic data. The current databases offer some parameters for non-Earth atmospheres but are far from complete for all situations. For example, measured H₂O line parameters in CO₂-rich atmospheres such as Mars and Venus are missing while they are of prime importance to learn about the evolution of the atmospheres.

After the study published in 2019 by Régalia et al., we measured new Fourier Transform Spectrometer spectra in the 1.88 micron range using a Connes’ type FT spectrometer built in Reims. The spectra were analysed using a multispectrum fitting procedure to obtain the line-shape parameters of H₂O broadened by CO₂. These results were used to constrain the intermolecular potential and to calculate the half-width, line shift, and their temperature dependence using the Modified Complex Robert-Bonamy formalism.

The impact of these new parameters on the spectral retrievals in the atmospheres of Mars and Venus will be assessed by calculating the equivalent widths in different cases. This exercise may highlight once more that using the correct line shape and line parameters is of utmost importance now that our space instruments have high spectral resolution. More especially the impact of the Double Power law (Gamache and Vispoel, 2018) for the temperature dependence parameter is tested in the context of the scientific preparation of VenSpec-H, spectrometer part of the EnVision payload.