Photochemical and thermochemical pathways to S2 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)(2) isomers, cis-OSSO, directly yields S-2 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-3, S-4, and S-8. We report here the results of high-level ab-initio quantum-chemistry computations that demonstrate that S-2 is not a product in cis-OSSO photolysis. Instead, we establish a novel mechanism in which S-2 is formed in a two-step process. Firstly, the intermediate S2O 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, S2O reacts with SO. This modified chemistry yields S-2 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. Polysulfur compounds have been ascribed as the unknown near-UV absorbers in Venusian atmosphere and play a key role in the sulfur chemical cycle of this planet. Here, authors establish their production from (SO)(2) on the grounds of quantifications of photochemical and thermal pathways involved in the sulfur chemical cycle of the planet.