Photolysis of sulphuric acid as the source of sulphur oxides in the mesosphere of Venus

The sulphur cycle plays fundamental roles in the chemistry1–3 and climate4,5 of Venus. Thermodynamic equilibrium chemistry at the surface of Venus favours the production of carbonyl sulphide6 and to a lesser extent sulphur dioxide. These gases are transported to the middle atmosphere by the Hadley circulation cell7,8. Above the cloud top, a sulphur oxidation cycle involves conversion of carbonyl sulphide into sulphur dioxide, which is then transported further upwards. A significant fraction of this sulphur dioxide is subsequently oxidized to sulphur trioxide and eventually reacts with water to form sulphuric acid3. Because the vapour pressure of sulphuric acid is low, it readily condenses and forms an upper cloud layer at altitudes of 60–70 km, and an upper haze layer above 70 km (ref. 9), which effectively sequesters sulphur oxides from photochemical reactions. Here we present simulations of the fate of sulphuric acid in the Venusian mesosphere based on the Caltech/JPL kinetics model3,10, but including the photolysis of sulphuric acid. Our model suggests that the mixing ratios of sulphur oxides are at least five times higher above 90 km when the photolysis of sulphuric acid is included. Our results are inconsistent with the previous model results but in agreement with the recent observations using ground-based microwave spectroscopy11 and by Venus Express12. A model SO2 profile computed by the Caltech/JPL kinetics model with standard chemistry3,10 without the H2SO4 photolysis (henceforth model A) is shown in Fig. 1a (black solid curve). The rapid decline of SO2 mixing ratio with height in the upper cloud region (60–70 km) is in agreement with the recent observations by Venus Express (blue data point)13. However, the high SO2 mixing ratios observed above 90 km from ground-basedmicrowave measurements11 (black dashed line) and from the Spectroscopy for Investigation of Characteristics of the Atmosphere of Venus (SPICAV) solar occultation on board Venus Express12 (purple curve) clearly exceed the model prediction by orders of magnitude. More information on the SPICAV measurements is available in Supplementary Information. Although the 90–100 km region is generally considered to be the transition zone between the retrograde super-rotating zonal flow and the global subsolar-to-antisolar circulation14, it is difficult for any dynamical process such as advection or eddy mixing to transport large amounts of SO2 from below and maintain a vertical profile that is increasing with altitude. Such a profile is also not likely to be the result of an ephemeral injection event induced by