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

The depletion of SO₂ and H₂O in and above the clouds of Venus (45-65 km) cannot be explained by known gasphase chemistry and the observed composition of the atmosphere. We apply a full-atmosphere model of Venus to investigate three potential explanations for the SO₂ and H₂O depletion: (1) varying the below-cloud water vapor (H₂O), (2) varying the below-cloud sulfur dioxide (SO₂), and (3) the incorporation of chemical reactions inside the sulfuric acid cloud droplets. We find that increasing the below-cloud H₂O to explain the SO₂ depletion results in a cloud top that is 20 km too high, above-cloud O₂ three orders of magnitude greater than observational upper limits, and no SO above 80 km. The SO₂ depletion can be explained by decreasing the below-cloud SO₂ to 20 ppm. The depletion of SO₂ in the clouds can also be explained by the SO₂ dissolving into the clouds, if the droplets contain hydroxide salts. These salts buffer the cloud pH. The amount of salts sufficient to explain the SO₂ depletion entails a droplet pH of ∼1 at 50 km. Because sulfuric acid is constantly condensing out into the cloud droplets, there must be a continuous and pervasive flux of salts of ≈10^-13 mol cm^-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 SO₂ below the clouds.