The primary sulfur species in Venus’ atmosphere, sulfur dioxide (SO2) may be a precursor for the unidentified UV-blue absorber(s), which, along with CO2 near the tops of the clouds, appears to be responsible for absorbing about half of the solar energy deposited in Venus’ atmosphere . Published simulations using standard photochemistry [2,3] indicate the mixing ratio of SO2 should decrease roughly monotonically with increasing altitude, although a small inversion is evident in one set of simulations . Observations, however, despite disagreeing on the magnitude of the phenomenon, have consistently found an inversion layer in the upper mesosphere (above about 85 km altitude) where the mixing ratio of SO2 increases with increasing altitude [4,5,6]. Simulations using H2SO4 as the medium for transporting sulfur from the lower mesosphere to the upper mesosphere that replicated the upper mesosphere SO2 inversion layer  either required assumptions that stretch the boundaries of known laboratory data or had a calculated H2SO4 abundance that exceeds the observational upper limit on upper mesospheric gaseous H2SO4 . A possible alternative is transport via a combination of sulfur-chlorine-oxides .
The Caltech/JPL photochemical model  was used for the numerical simulations. Preliminary results using a simplified model suggest the inclusion of both ClSO2 and SO2Cl2 in the model and adjustment of selected reaction rate coefficients within their standard uncertainties can produce at least a factor of two upper mesosphere inversion for SO2. The results from simulations using a more comprehensive photochemical model will be presented.
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