A Simple Condensation Model for the H2SO4-H2O Gas-cloud System on Venus

TL;DR

This paper presents a simplified one-dimensional model of Venus's sulfuric acid cloud system that explains observed cloud structures and dynamics without complex microphysics, highlighting the dominant role of H2SO4 condensation and evaporation processes.

Contribution

The study introduces a simple, microphysics-free model that successfully reproduces key features of Venus's cloud system and elucidates the dominant processes driving cloud formation and maintenance.

Findings

Most H2SO4 is stored above 48 km in condensed form.

Cloud cycle driven mainly by H2SO4 condensation and evaporation.

H2SO4 vapor above 60 km is highly supersaturated, exceeding equilibrium levels.

Abstract

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.