1,1,1,2,1- 1The Open University, School of Physical Sciences, United Kingdom of Great Britain – England, Scotland, Wales (jo.egan@open.ac.uk)
- 2School of Chemistry, University of Leeds, UK
The atmosphere of the planet Venus contains an extensive sulfur cycle, with permanent sulfuric acid (H2SO4) clouds and haze layers spanning altitudes from 48 km to ~90 km. H2SO4 is formed from SO2 near the cloud top. Observations of SO2 in the Venusian atmosphere show concentrations of ~100 ppm below the sulfuric acid clouds and 0.1 – 1 ppb at the cloud top. Known chemistry is insufficient to produce the observed decrease in SO2 and no chemical models have successfully reproduced the three orders-of-magnitude decrease. The precise cause of the SO2 decrease remains unknown. Many chemical processes have been proposed that could contribute to the removal of SO2, including conversion to other sulfur reservoir species such as polysulfur (S2, S3, S4, S8, etc.) or polysulfur oxides (SnO); oxidative uptake to droplets by H2O2 or O3; increased uptake due to buffering of the pH of the cloud droplets by dissolved salts, and reaction with NOx species.
Two possible routes for SO2 + NOx chemistry have been postulated: reactive uptake [1] and the lead chamber process [2]. [1] reported single droplet levitation experiments which showed droplet growth only when SO2 and NO2 were both present. [2] proposed that NO and NO2 could catalyse SO2 conversion to SO3, leading to conversion to sulfuric acid by reaction with water, in a process analogous to the Lead Chamber process previously used to produce sulfuric acid for industrial use on Earth.
We present a newly developed droplet chemistry module in the Venus Planetary Climate Model, which extends the gas phase chemistry scheme to permit aqueous and droplet surface reactions. Gases condense to the cloud droplets according to their Henry’s law solubilities, and react with other species, which are then released from the droplets when they exceed their solubility limits. We use this new scheme to test both reactive uptake of SO2 and NO2 and the “lead chamber” process. In both of these cases, we predict the increases required to the reaction rates or the NOx reactant concentrations to fully explain the observed SO2 decrease through the clouds.
References:
- Ubukata, S., et al., Uptake of SO2 into sulfuric acid droplets through the oxidation by NO2 under Venus-analogous conditions. ACS Earth and Space Chemistry, 2025. 9(6): p. 1525–1533.
- Sill, G.T., The clouds of Venus: Sulfuric acid by the lead chamber process. Icarus, 1983. 53(1): p. 10–17.
How to cite: Egan, J., Ceragioli, B., Cohen, M., Holmes, J., Plane, J., and Patel, M.: Depletion of Venusian SO2 by liquid phase NOx chemistry, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-491, https://doi.org/10.5194/epsc2026-491, 2026.
