In-droplet sulphur chemistry in the Venus clouds with the Venus Planetary Climate Model

Introduction
Forthcoming missions to Venus are designed to provide new chemical data that will enhance our knowledge of Venus’ middle atmosphere chemistry. Central to this effort is ESA’s EnVision mission, which aims, among other objectives, to characterize sulphur-bearing trace species. Particularly SO₂, which serves as the primary photochemical precursors for the formation of the Venusian clouds, predominantly composed of a H₂O-H₂SO₄ binary solution.

Atmospheric modelling is essential to contextualize both past and future observations. The Venus Planetary Climate Model (Venus PCM) is a state-of-the-art 3D General Circulation Model (GCM) developed over the last 15 years that is capable of self-consistently simulating key dynamical and chemical features of Venus’ atmosphere, from the ground to the thermosphere [1,2,3,4]. Nonetheless, modelling the observed mesospheric SO₂ vertical profile remains a significant challenge [4].  We investigate the mechanisms proposed by [5] driving the observed three order of magnitude depletion within the cloud layer, from 130 ppmv below the clouds to only 0.1 ppmv above the clouds.

 

Methods and results
We have implemented an in-droplet chemical set of reactions within the Venus PCM following [5]. This approach analyses how SO₂dissolution into the cloud droplets and subsequent dissociation into ionic species successfully account for the steep vertical depletion of SO₂. The dissolution of SO₂ into the droplets is driven under the assumption of the presence of salts (e.g. NaOH, which is used as a proxy) in the clouds. The salts act as a pH buffer and enhance SO₂ dissolution into the droplets. SO₂ depletion is then governed by the cloud’s uptake capacity, itself determined by the salt concentration. While a reduced set of reactions was previously tested in the Venus PCM as an initial approach [6], this work implements the complete chemical processing following Henry’s Law and in-droplets’ ionic chemical reactions. Furthermore, we also evaluate the droplet sedimentation and the thermal evaporation of species at the cloud base.

We will show preliminary results from the first-time implementation of the full in-droplet chemistry model in the Venus PCM.  We will analyse not only 1D vertical profiles, but also latitude-local time maps at different altitudes, discussing day and night differences and the well-known anticorrelation of SO₂ and H₂O. Finally, we will evaluate the implications of these results in the interpretation of future observations by the EnVision mission.

 

Acknowledgments: grant PRE2022-104364, funded by MCIN/AEI/10.13039/501100011033 and by FSE+; Severo Ochoa grant CEX2021-001131-S funded by MICIU/AEI/ 10.13039/501100011033; Program EMERGIA 2021 (EMC21 00249); Spanish MCIU, the AEI and EC-FEDER funds under project PID2021-126365NB-C21.

References:
[1] Lebonnois et al. 2010, JGR, Vol.115, Issue E6
[2] Gilli et al. 2021, Icarus, Vol. 366, 114432
[3] Martinez et al. 2024, Icarus, Vol. 415, 116035
[4] Stolzenbach et al. 2023, Icarus, Vol. 395, 115447
[5] Rimmer et al. 2021, Planet. Sci. J., Vol. 2, 133
[6] Mendi-Martos et al., Vol. 18, EPSC-DPS2025-1368, 2025