A New Model of Sulfur Species in the Upper Atmosphere of Venus

The photochemistry of sulfur species in the upper atmosphere of Venus is not well understood. The parent molecules originating in the deep atmosphere are most likely SO 2 and OCS. At and above the cloud tops, these molecules are photolyzed, leading to the production of S, SO, SO 3 and H 2 SO 4 . In addition, OS-SO, the SO dimer, could also be present.

We explore the effects of reactions involving SO, OSSO, and S 2 O, and of new reactions involving other S_xO_yspecies in the upper atmosphere of Venus. Some of these reactions could provide important new sources of polysulfur (S x ) which could be much larger than in earlier models of sulfur chemistry. In our current model, we examine the implications of laboratory experiments of Wu et al. (Chem. Comm., 2018, 54, 4517) that have found significant yields of S 2 following irradiation of the SO dimer. In our current model, production of S 2 does not directly involve S atoms, as it is derived from alternative paths involving SO dimer photochemistry, thus bypassing the rapid recycling of S to SO by O₂.

We also explore the production of S_xO_y species in addition to S_x. Several studies have shown that the formation of H 2 SO 4 , does not occur through a simple bimolecular reaction involving SO 3 and H₂O. Instead, a more complex mechanism is involved. We examine the implications of these more complex reaction mechanisms for the stability of SO 2 , SO 3 and H 2 SO 4 above the cloud tops on Venus.

The implications of our new model are that more complex sulfur compounds, beyond OCS, SO, SO2, SO3 and H2SO4, could contribute to the absorption of UV in the enigmatic 320-400 nm range. In addition, new insights may be gained into the stability of H2SO4 at high elevations. These results may be useful for interpreting new observations obtained from Japan's Venus Climate Orbiter, Akatsuki .