  The current Venus climate is largely regulated by globally-covered
concentrated sulfuric acid clouds from binary condensation of sulfuric acid
(H2SO4) and water (H2O). To understand this complicated H2SO4-H2O gas-cloud
system, previous theoretical studies either adopted complicated microphysical
calculations or assumed that both H2SO4 and H2O vapor follow their saturation
vapor pressure. In this study, we developed a simple one-dimensional cloud
condensation model including condensation, diffusion and sedimentation of H2SO4
and H2O but without detailed microphysics. Our model is able to explain the
observed vertical structure of cloud and upper haze mass loading, cloud
acidity, H2SO4, and H2O vapor, and the mode-2 particle size on Venus. We found
that most H2SO4 is stored in the condensed phase above 48 km, while the
partitioning of H2O between the vapor and clouds is complicated. The cloud
cycle is mostly driven by evaporation and condensation of H2SO4 rather than H2O
and is about seven times stronger than the H2SO4 photochemical cycle. Most of
the condensed H2O in the upper clouds is evaporated before the falling
particles reach the middle clouds. The cloud acidity is affected by the
temperature and the condensation-evaporation cycles of both H2SO4 and H2O.
Because of the large chemical production of H2SO4 vapor and relatively
inefficient cloud condensation, the simulated H2SO4 vapor above 60 km is
largely supersaturated by more than two orders of magnitude, which could be
tested by future observations.
