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.