One-dimensional Microphysics Model of Venusian Clouds from 40 to 100 km: Impact of the Middle-atmosphere Eddy Transport and SOIR Temperature Profile on the Cloud Structure

Venus is globally covered by thick hydrated sulfuric acid (H2SO4) aerosols, which play a pivotal role in the Venusian atmospheric system. The condensation and evaporation processes affect the abundance of H2SO4 vapor and H2O vapor and the chemistry related to these species. Additionally, the aerosol distribution serves as a key indicator for atmospheric dynamics. Thus, studying aerosol processes is vital for understanding various atmospheric phenomena.

The Solar Occultation in the Infrared (SOIR) instrument aboard the Venus Express (VEx) spacecraft measured the mesospheric aerosol and gas vertical profiles closely linked to these condensation and evaporation processes. These observations showed that the upper haze layers can reach altitudes up to 100 km, with the water vapor concentration increasing with altitude. However, the dynamics that maintain these aerosol layers at such altitudes and the interactions between aerosols and gases remain poorly understood.

To investigate these interactions, we developed a one-dimensional cloud microphysics model that simulates the distribution of H2SO4 vapor, H2O vapor, and H2SO4-H2O aerosols from 40 to 100 km (Karyu et al. 2024). To examine how gases and aerosols are transported, we conducted case studies with four distinct patterns of eddy diffusion coefficients between altitudes of 60–70 km and 85–100 km. In addition, we performed an additional case study using a temperature profile obtained by VEx/SOIR (Mahieux et al. 2015, 2023) to examine the impact of the temperature on the gas species and aerosol distributions.

We found that the high eddy diffusion coefficients derived by Mahieux et al. (2021) significantly enhance the model's ability to replicate the observed distribution of upper haze. This indicates that efficient eddy transport is critical to defining the microphysical properties of the haze layer. Variations in these coefficients also influenced the vertical distribution of water vapor, affecting its overall presence within and above the cloud layers. The H2SO4 VMR in the upper haze layer is also highly sensitive to the eddy diffusion coefficient above 85 km, ranging from ~5 pptv to ~0.5 ppbv. However, the simulated values are orders of magnitude lower than the observational upper limit suggested by Sandor et al. (2012). Finally, the present study identifies the best-fit eddy diffusion coefficients as ∼360 m² s⁻¹ above 85 km and ∼2 m² s⁻¹ between 60 and 70 km.

Moreover, the temperature profile, especially when updated with the recent SOIR data, has a marked effect on the modeled concentrations of H2O and H2SO4 vapors. Higher temperatures in the SOIR profile resulted in greater saturation vapor pressures, leading to the evaporation of upper haze particles and increased vapor concentrations. As a result, the H2O VMR profile aligns well with SOIR observations. This underlines the critical role of aerosol evaporation in the transport of H2O vapor in the Venusian mesosphere.

 

Karyu, H., Kuroda, T., Imamura, T., et al. 2024, PSJ, 5, 57

Mahieux, A., Vandaele, A. C., Bougher, S. W., et al. 2015, P&SS, 113, 309

Mahieux, A., Yelle, R. V., Yoshida, N., et al. 2021, Icar, 361, 114388

Mahieux, A., Robert, S., Piccialli, A., et al. 2023, Icar, 405, 115713

Sandor, B. J., Clancy, R. T., & Moriarty-Schieven, G. 2012, Icar, 217, 839