  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 $20\,{\rm ppm}$. 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
$\approx 10^{-13} \, {\rm mol \, cm^{-2} \, s^{-1}}$ 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
SO$_2$ below the clouds.
