Revisiting the Sulfur‐Water Chemical System in the Middle Atmosphere of Venus

Abstract

Sulfur‐water chemistry plays an important role in the middle atmosphere of Venus. Ground‐based observations have found that simultaneously observed SO2 and H2O at ~64 km vary with time and are temporally anticorrelated. To understand these observations, we explore the sulfur‐water chemical system using a one‐dimensional chemistry‐diffusion model. We find that SO2 and H2O mixing ratios above the clouds are highly dependent on mixing ratios of the two species at the middle cloud top (58 km). The behavior of sulfur‐water chemical system can be classified into three regimes, but there is no abrupt transition among these regimes. In particular, there is no bifurcation behavior as previously claimed. We also find that the SO2 self‐shielding effect causes H2O above the clouds to respond to the middle cloud top in a nonmonotonic fashion. Through comparison with observations, we find that mixing ratio variations at the middle cloud top can explain the observed variability of SO2 and H2O. The sulfur‐water chemistry in the middle atmosphere is responsible for the H2O‐SO2 anticorrelation at 64 km. Eddy transport change alone cannot explain the variations of both species. These results imply that variations of species abundance in the middle atmosphere are significantly influenced by the lower atmospheric processes. Continued ground‐based measurements of the coevolution of SO2 and H2O above the clouds and new spacecraft missions will be crucial for uncovering the complicated processes underlying the interaction among the lower atmosphere, the clouds, and the middle atmosphere of Venus.

HDO and SO2thermal mapping on Venus

“…Photochemical models have been developed to model the behavior of water vapor and sulfur dioxide at the cloud top and within the clouds of Venus (Parkinson et al 2015). Using a 1D chemistry-diffusion model (Zhang et al 2012), Shao et al (2020) have found that the anti-correlation of SO 2 and H 2 O at 64 km can be generally explained by the sulfur chemistry in the middle atmosphere (58-100 km) of Venus. Even when SO 2 and H 2 O vary randomly at the middle cloud top (58 km), their model finds that the two species at 64 km are mostly anti-correlated, because SO 2 and H 2 O modulate each other through the sulfuric acid formation and intermediate SO 3 reactions at 64 km.…”

A Recharge Oscillator Model for Interannual Variability in Venus’ Clouds

“…The sulfur dioxide abundances at the cloud tops having a strong relationship to the convective mixing strength (as well as sulfur dioxide and water abundances at the cloud‐base) has been established in multiple chemical modeling studies (Bierson & Zhang, 2019; Krasnopolsky, 2012, 2018; Parkinson et al., 2015). The dependence of cloud top sulfur dioxide on water abundance is complex, with correlated and anticorrelated behavior manifesting depending on whether water or sulfur dioxide is the relatively more abundant species at the convective cloud top (Shao et al., 2020).…”

“…Chemical modeling of Venus has thus far focused primarily on calculating steady state abundances, since models which fully couple chemistry and dynamics still do not exist for Venus (Marcq et al., 2018; Shao et al., 2020). Thus, in this section we start with results from such steady state chemical calculations and then couple the shifts between steady states and dynamical variability.…”

Instrumental requirements for the study of Venus’ cloud top using the UV imaging spectrometer VeSUV

“…Incidentally, other interesting measurements could be obtained during the occultations of calibration stars through the Venusian mesosphere to derive constraints of the vertical profiles of gaseous and particulate species above the clouds. Also, glory observations would allow us to retrieve microphysical properties of cloud aerosols, so that the complex interactive process of photochemistry (Shao et al, 2020) and cloud aerosol formation (McGouldrick, 2017) could be analyzed comprehensively.…”