Trace Species Variations Enlightened by Chemical-Transport Models in the Middle Atmosphere of Venus

Abstract:

Photochemistry and dynamics are two important factors in determining trace species variations in planetary atmospheres. On Earth, there have been many studies working on discriminating effects from the two competing processes by using the Chemical-Transport Model (CTM). However, similar studies on Venus are still rare. To better understand the effects of photochemistry and dynamics, here we investigate species variations in the middle atmosphere of Venus through comparisons of observations with both one-dimensional and two-dimensional CTMs.

Recent ground-based observations found that simultaneously observed SO₂ and H₂O at 64 km vary with time and are temporally anti-correlated. To understand the observations, we explore the sulfur-water chemistry using a one-dimensional CTM [1]. 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) [2]. Varying mixing ratios at 58 km can explain the observed variability of SO₂ and H₂O. The sulfur-water chemistry is responsible for the H₂O-SO₂ temporal anti-correlation. Eddy transport change alone, however, cannot explain the simultaneous variations of both species. The results imply that species abundance variations in the middle atmosphere are significantly influenced by the lower atmospheric processes.

Species like CO and SO₂ may have specific diurnal patterns in the middle atmosphere of Venus, indicated by spaceborne and ground-based measurements. The super-rotation and sub-solar-to-anti-solar circulation coupled with photochemistry could cause the diurnal patterns. We are developing a two-dimensional CTM with full complex chemistry and couple it with Venus GCM outputs to uncover the diurnal variations of these species and the controlling mechanisms. We will also develop a three-dimensional CTM to comprehensively understand both spatial and temporal variations of multiple species in the Venus atmosphere. Furthermore, we emphasize the importance of continuous ground-based observations and new spacecraft missions for understanding the interactions between photochemistry and atmospheric dynamics and their effects on trace species variations.

References: 1. Zhang, X. et al. Icarus 217, 714–739 (2012). 2. Shao, W. D. et al. JGR: Planets, e2019JE006195 (2020).