New CO2-collisional parameters for H2O: their impact on water vapor retrievals in Venus’ atmosphere

Upcoming missions to Venus will be equipped with advanced spectrometers capable of resolving fine atmospheric spectral features. To fully exploit this observational potential, accurate spectroscopic parameters adapted to Venus’ CO₂-rich atmosphere are required. In particular, the retrieval of water vapor concentrations relies heavily on appropriate collisional parameters. However, current spectroscopic databases lack CO₂-broadening parameters for H₂O, and air-broadening parameters are often used as substitutes (Jorge et al., 2024), which can introduce significant biases in retrieved H₂O concentrations.

To address this issue and to follow up the work of Régalia et al. (2019), new high-resolution infrared spectra of H₂O broadened by CO₂ were recorded in spectral regions of planetary interest (1.18, 2.34, and 2.7 µm), using the Fourier Transform Spectrometer at the GSMA laboratory in Reims, France. For 187 isolated H₂O transitions, CO₂-broadened half-widths and pressure-shift coefficients were determined through a multi-spectrum fitting procedure using the Voigt profile (Plateaux et al., 2001; Lyulin, 2015). To take into account finer physical effects, a quadratic speed-dependent Voigt profile (Ngo et al., 2012, 2013) was also applied, allowing the measurement of speed-dependence coefficients for 106 transitions.

The Voigt experimental results were used to improve the molecular interaction potential of the H₂O–CO₂ collision system. Since experimental data alone are not sufficient to entirely model an atmospheric spectrum, new calculations were performed using the semi-classical Complex Robert-Bonamy-Ma (CRBM) formalism (Robert & Bonamy, 1979; Ma et al., 2007). This allowed the determination of half-widths and pressure shifts for thousands of H₂O transitions. Several key atmospheric windows of Venus were considered: 1.18, 1.38, 1.74, and 2.34 µm. The resulting calculated linelist can now be directly used in radiative transfer applications (Ducreux et al., 2024, 2025 submitted).

The impact of using these new CO₂-collisional parameters for water vapor retrievals was evaluated through simulations using the ASIMUT-ALVL radiative transfer code (Vandaele et al. 2006; Spurr, 2008). Venusian spectra were simulated in different infrared windows probing various altitude ranges on both the nightside and dayside of the planet. For each case, 1000 spectra were generated using the new CO₂-specific linelist, with added Gaussian noise based on a selected signal-to-noise ratio. Retrievals were then performed using the standard air-broadening linelist from HITRAN2020 (Gordon et al., 2022) for the entire statistical sample. The dependence on spectral resolution and signal-to-noise ratio were also investigated. The results demonstrate that relying on spectroscopic parameters not suited for a CO₂-rich atmosphere can lead to significant errors in retrieving water vapor concentrations on Venus. This highlights the inadequacy of air-broadening parameters for a CO₂-dominated environment and highlights the necessity of dedicated spectroscopic data for accurate retrievals.

Moreover, the need for accurate spectroscopic parameters to study CO₂-rich atmospheres also applies to the main deuterated isotopologue of water vapor, HDO. New spectra of HDO broadened by CO₂ were therefore recorded in the same spectral regions as H₂O and CO₂-collisional parameters for HDO have been derived to build a measured linelist, which will soon be augmented with CRBM calculations. The final objective is to provide the planetary community with a CO₂-specific linelist for HDO, suitable for radiative transfer modelling in CO₂-rich planetary atmospheres.