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Observed chemical species in the Venusian mesosphere show local-time variabilities. SO₂at the cloud top exhibits two local maxima over local time, H₂O at the cloud top is uniformly distributed, and CO in the upper atmosphere shows a statistical difference between the two terminators. In this study, we investigated these local-time variabilities using a three-dimensional (3D) general circulation model (GCM) in combination with a two-dimensional (2D) chemical transport model (CTM). Our simulation results agree with the observed local-time patterns of SO₂, H₂O, and CO. The two-maximum pattern of SO₂at the cloud top is caused by the superposition of the semidiurnal thermal tide and the retrograde superrotating zonal (RSZ) flow. SO₂above 85 km shows a large day–night difference resulting from both photochemistry and the subsolar-to-antisolar (SS-AS) circulation. The transition from the RSZ flows to SS-AS circulation can explain the CO difference between two terminators and the displacement of the CO local-time maximum with respect to the antisolar point. H₂O is long-lived and exhibits very uniform distribution over space. We also present the local-time variations of HCl, ClO, OCS, and SO simulated by our model and compare to the sparse observations of these species. This study highlights the importance of multidimensional CTMs for understanding the interaction between chemistry and dynamics in the Venusian mesosphere.

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 H₂SO₄and 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 H₂SO₄and 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 H₂SO₄is 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 H₂SO₄rather than H₂O and is about seven times stronger than the H₂SO₄photochemical 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 H₂SO₄and H₂O. Because of the large chemical production of H₂SO₄vapor and relatively inefficient cloud condensation, the simulated H₂SO₄vapor above 60 km is largely supersaturated by more than two orders of magnitude, which could be tested by future observations.

Sulfur‐water chemistry plays an important role in the middle atmosphere of Venus. Ground‐based observations have found that simultaneously observed SO₂and 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 SO₂and 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 SO₂self‐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 SO₂and H₂O. The sulfur‐water chemistry in the middle atmosphere is responsible for the H₂O‐SO₂anticorrelation 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 SO₂and 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.