paper

Hydroxide salts in the clouds of Venus: their effect on the sulfur cycle and cloud droplet pH

arXiv:2101.08582

Abstract

The depletion of SO $_{2}$ and H $_{2}$ O in and above the clouds of Venus (45 -- 65 km) cannot be explained by known gas-phase chemistry and the observed composition of the atmosphere. We apply a full-atmosphere model of Venus to investigate three potential explanations for the SO $_{2}$ and H $_{2}$ O depletion: (1) varying the below-cloud water vapor (H $_{2}$ O), (2) varying the below-cloud sulfur dioxide (SO $_{2}$ ), and (3) the incorporation of chemical reactions inside the sulfuric acid cloud droplets. We find that increasing the below-cloud H $_{2}$ O to explain the SO $_{2}$ depletion results in a cloud top that is 20 km too high, above-cloud O $_{2}$ three orders of magnitude greater than observational upper limits and no SO above 80 km. The SO $_{2}$ depletion can be explained by decreasing the below-cloud SO $_{2}$ to $20 ppm$ . The depletion of SO $_{2}$ in the clouds can also be explained by the SO $_{2}$ dissolving into the clouds, if the droplets contain hydroxide salts. These salts buffer the cloud pH. The amount of salts sufficient to explain the SO $_{2}$ depletion entail a droplet pH of $\sim 1$ at 50 km. Since sulfuric acid is constantly condensing out into the cloud droplets, there must be a continuous and pervasive flux of salts of $\approx 1 0^{- 13} mol c m^{- 2} s^{- 1}$ driving the cloud droplet chemistry. An atmospheric probe can test both of these explanations by measuring the pH of the cloud droplets and the concentrations of gas-phase SO $_{2}$ below the clouds.

Accepted for publication in The Planetary Science Journal. We kindly ask the planetary science community for feedback and comments. (67 pages, 8 Figures, 7 Tables, Full Model Output Data: https://doi.org/10.7910/DVN/AKUTME)