Chemical Cycling in the Venusian Atmosphere: A Full Photochemical Model Fromthe Surface to 110 km

Venus is an exceptional natural experiment to test our understanding of atmospheric sulfur chemistry. Previous modeling efforts have focused on understanding either the middle or lower atmosphere. In this work, we performed the first full atmosphere analysis of the chemical transport processes on Venus from the surface to 110 km using a 1-D diffusion model with photochemistry. We focused on the cycling of chemical species between the upper and lower atmospheres and interactions between distinct species groups including SOx, COx + OCS, chlorides, NOx, O-x, and S-x. We tested different eddy diffusivity profiles and investigated their influences on the vertical profiles of important species. We find that the assumed boundary conditions in previous models strongly impacted their simulation results. This has a particularly large effect for SO2. We find the high SO2 abundance in the lower atmosphere is readily transported into the middle atmosphere, far exceeding observed values. This implies some yet unknown chemistry or process limiting SO2 mixing. We summarize outstanding questions raised by this work and note chemical reactions that should be the highest priority for future laboratory studies and ab initio calculations. Plain Language Summary Venus's atmosphere can be broadly separated into lower and middle regions, separated by a thick cloud deck. Chemistry in the lower atmosphere is controlled by the high temperatures below the clouds. In the middle atmosphere, photochemistry stimulated by solar UV radiation is dominant. Previous works have modeled either the lower or middle atmosphere to understand these chemical processes. In this work, we create a single model that encompasses both regions to understand how chemical species are cycled. We find that the large abundance of SO2 in the lower atmosphere is transported into the middle atmosphere, far exceeding what is observed. We argue that this suggests some as of yet unknown chemistry or process that is limiting the SO2 flux from the lower atmosphere.