Photochemical and thermochemical pathways to S and polysulfur formation in the atmosphere of Venus.

Polysulfur species have been proposed to be the unknown near-UV absorber in the atmosphere of Venus. Recent work argues that photolysis of one of the (SO) isomers, cis-OSSO, directly yields S with a branching ratio of about 10%. If correct, this pathway dominates polysulfur formation by several orders of magnitude, and by addition reactions yields significant quantities of S, S, and S. We report here the results of high-level ab-initio quantum-chemistry computations that demonstrate that S is not a product in cis-OSSO photolysis. Instead, we establish a novel mechanism in which S is formed in a two-step process. Firstly, the intermediate SO is produced by the coupling between the S and Cl atmospheric chemistries (in particular, SO reaction with ClS) and in a lesser extension by O-abstraction reactions from cis-OSSO. Secondly, SO reacts with SO. This modified chemistry yields S and subsequent polysulfur abundances comparable to the photolytic cis-OSSO mechanism through a more plausible pathway. Ab initio quantification of the photodissociations at play fills a critical data void in current atmospheric models of Venus.