Prediction of sulphate hazes in the lower Venus atmosphere

The physical origin and chemical nature of haze particles below the main sulphuric acid clouds in the Venus atmosphere is investigated. We make a number of predictions based on our theoretical models concerning the chemical state of the gas and the properties and material composition of μm-sized particles in the lower Venus atmosphere, from ground level to a height of about 50 km. Our GGchem phase-equilibrium model (Woitke et al. 2018) for the Venus surface predicts a number of metal-chloride and metal-fluoride molecules to be present in the gas over the surface in trace concentrations < 2×10⁻¹², in particular FeCl₂, NaCl, KCl and SiF₄. Using an improved version of the DiffuDrift model developed by Woitke et al. (2020) we find that these molecules can deposit to form solid potassium sulphate K₂SO₄, sodium sulphate Na₂SO₄, and pyrite FeS₂, at heights larger than about 15.5 km, 9.5 km and 2.4 km, respectively. We call these condensations sulphate hazes, because their opacity is insufficient to make the lower Venus atmosphere optically thick. The most prominent material is found to be Na₂SO₄, which is expected to deposit on the surfaces of chemically passive aerosol particles in form of a mantle with a thickness of a few 100 mono-layers. Our models predict that such haze particles, with sizes between about 0.1 to 0.3 μm, can be dredged up from the ground to reach the sulphuric acid cloud base from below by diffusion in concentrations of about 300-1500 particles per gram of gas, depending on the efficiency of coagulation. Only these sub-micron particles can reach the main cloud layer from below. Particles larger than about 2 μm are found to stay more concentrated to the ground < 10 km.

References:

Woitke, P., Helling, C., Hunter, G. H., et al. (2018), “Equilibrium chemistry down to 100 K. Impact of silicates and phyllosilicates on the carbon to oxygen ratio“, A&A 614, A1

Woitke, P., Helling, C., & Gunn, O. (2020), “Dust in brown dwarfs and extra-solar planets. VII. Cloud formation in diffusive atmospheres“, A&A, 634, A23

Rimmer P., Jordan S., Constantinou T., Woitke P., Shorttle O., Hobbs R., Paschodimas A. (2021), “Hydroxide Salts in the Clouds of Venus: Their Effect on the Sulfur Cycle and Cloud Droplet pH”, PSJ 2, 4, id133.