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

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. Plain Language Summary: Sulfur chemistry composes one important chemical cycle in the Venusian atmosphere. Sulfur dioxide, the most abundant sulfur species, is transported from the lower atmosphere (below the clouds) to the middle atmosphere. On the dayside, sulfur dioxide is dissociated by ultraviolet light and forms various sulfur‐bearing species. These species, like polysulfur and sulfuric acid, are critical for the formation of Venus' haze and the sulfuric acid clouds. Sulfur dioxide is observed to vary by orders of magnitude, though mechanisms underlying those variations remain elusive. In this work, we use a one‐dimensional photochemical model to explain the coevolution of sulfur dioxide and water from ground‐based observations. We find that sulfur chemistry and variations inside the clouds are two important factors affecting temporal variations of sulfur dioxide and water. Our study highlights the importance of the interaction among the lower atmosphere, the clouds, and the middle atmosphere on Venus. Key Points: We found that there is no bifurcation behavior in the sulfur‐water chemical system as previously claimedThe observed SO2‐H2O anticorrelation can be explained by the sulfur‐water chemistry with mixing ratio variations at the middle cloud topThis suggests that the observed temporal variations of SO2 and H2O are linked to the lower atmospheric processes [ABSTRACT FROM AUTHOR]