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

Introduction

Venus is set to be a key part of the next decades of planetary exploration with several missions planned by NASA and ESA (DAVINCI, VERITAS, EnVision). These missions aim to better understand how Venus and the Earth started being so similar but evolved into such different worlds. Modelling the Venus atmosphere is essential to support past and future observations. The Venus Planetary Climate Model (Venus PCM) used in this study is a 3D model that has been developed by the Laboratoire de Météorologie Dynamique (LMD) for more than 15 years in collaboration with other institutions, including the Instituto de Astrofísica de Andalucía (IAA-CSIC) [1,2,3].

The Venus PCM has been used to simulate the Venus photochemistry and clouds from the surface to the bottom of the thermosphere [4], and here we use the last version of the model with the goal of shedding a light on the mesospheric SO₂ issue. Not only is this species highly variable in Venus mesosphere, but reproducing the observed SO₂ depletion in the cloud layer also remains challenging. In particular, the three orders of magnitude depletion is not well reproduced by models, which can only simulate a decrease from 10 ppmv at 40 km to 0.1 ppmv at 80 km [4].

The interest of studying SO₂ lies in its fundamental role in the cloud formation in Venus. In particular, SO₂ is the source to form the sulfuric acid H₂SO₄ that is part of the binary mixture (H₂SO₄-H₂O) that defines the droplets in the clouds of Venus.

Implementation of in-droplets sulphur chemistry

In this work, we aim to reproduce the observed variation of SO₂ with altitude by using in-droplet sulphur chemistry for the first time in the Venus PCM, following the work by [5], in which they consider that clouds contain hydroxide salts like NaOH, which drive the droplet chemistry. NaOH is a proxy used in that study to represent other plausible sources of delivered hydrogen. They presented the following set of chemical reactions, whose rates are tuned to emulate the dissolution of SO₂ into the cloud droplet, and its release into gas phase when the droplet rains out and evaporates

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As a first step, we implemented this set of chemical reactions in the Venus PCM. Preliminary results show that we can dissolve SO₂into the clouds using the chemical reactions from [5] with SO₂ initialized to a value of 150 ppmv below 40 km, in line with available observations.

We will present on-going work aimed to implement the full set of reactions proposed by [5] and we will discuss the implication of model results in the interpretation of future Envision observations.

 

Keywords: Venus atmosphere, General Circulation Model, cloud, chemistry.

 

Acknowledgments: 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