An Exploration of Polysulfur Chemistry in a Simulated Venus Atmosphere

Venus at ultraviolet (UV) wavelengths exhibits distinct light and dark markings (Rossow et al., JAS ,1980). The discovery of sulfur dioxide (SO₂) using a ground-based high resolution spectrometer explained Venus’ albedo at wavelengths < 320 nm but not these dark markings at 320-500 nm (Esposito et al. GRL, 1979; Pollack et al., JGR , 1980; Pérez-Hoyos et al., JGR , 2018). So at least one other absorber must be important at these wavelengths. Radiative balance simulations suggest this unidentifed absorber is responsible for absorbing about half of the solar energy absorbed by Venus’ atmosphere (Titov et al., Space Sci Rev, 2018). Polysulfur species (S_x) have been suggested but a sufficiently fast pathway to form these polysulfur species hasn’t been identified (Mills et al., Planet Space Sci, 2007). One pathway that has received minimal attention since it was proposed by Halstead and Thrush (Proc. R. Soc. Lond. A Math. Phys. Sci., 1966) is the reaction SO+OCS→CO₂+S₂. This reaction was proposed by Halstead and Thrush (1966) to explain the disagreement between their inferred upper limit rate for 2SO{+M} →SO₂+S {or (SO)₂}  and the rate inferred by Sullivan and Warneck (Ber. Bunsenges. Phys. Chem., 1965). Baulch et al. (Butterworths, 1976) included the Halstead and Thrush (1966) rate for SO+OCS→CO₂+S₂ in their assessment of high temperature gas kinetic data but noted it should be used with caution. This reaction potentially enhances the rates of formation of both CO₂ and S_x, and, thus, it potentially contributes to two long-standing issues in Venus atmospheric chemistry: the overprediction of mesospheric column O₂ and the requirement for faster production of S₂ (if S_x contributes significantly to the unidentified UV absorption). When included in a 1-D photochemical model with the Halstead and Thrush (1966) rate coefficient, this reaction dominates production of S₂ in the upper cloud layer. This occurs for both the Pinto et al. (Nature Comm., 2021) and Francés-Monerris et al. (Nature Comm., 2022) schemes for enhanced S₂ production. The resultant SO₂ profile differs significantly from previous simulations (Zhang et al., Icarus, 2012) but remains marginally compatible with existing observations (Jessup et al., Icarus, 2015). Similar behaviour is found when the volume mixing ratio for OCS at 58 km is increased to 4x10^-6 and SO₂ is reduced to 1.2x10^-6, even if the SO+OCS reaction rate is set to zero. The results from a suite of simulations exploring this new region of parameter space and the potential implications for the Venus upper cloud will be discussed.