Photochemical and thermochemical pathways to S 2 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 S 2 O 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, S 2 O 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.