Local-time Dependence of Chemical Species in the Venusian Mesosphere

TL;DR

This study uses advanced 3D and 2D models to explain local-time variabilities of chemical species in Venus's mesosphere, aligning well with observations and revealing the influence of atmospheric dynamics and photochemistry.

Contribution

It introduces a combined 3D GCM and 2D CTM approach to accurately simulate and interpret chemical distributions and their variabilities in Venus's mesosphere.

Findings

SO2 exhibits two local maxima due to thermal tide and RSZ flow.

H2O remains uniformly distributed due to its long lifetime.

CO distribution differences are explained by circulation patterns.

Abstract

Observed chemical species in the Venusian mesosphere show local-time variabilities. SO2 at 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 SO2 at the cloud top is caused by the superposition of the semidiurnal thermal tide and the retrograde superrotating zonal (RSZ) flow. SO2 above 85 km shows a large day-night difference resulting from both photochemistry and the sub-solar to anti-solar (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 anti-solar 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.