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

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The sulphur cycle plays fundamental roles in the chemistry1,2,3 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.

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Figure 1: Model results.
Figure 2
Figure 3: Parameter space for possible solutions.
Figure 4: Production/loss-rate profiles of SO2.


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We thank V. Vaida, F. W. Taylor, S. E. Smrekar, F. W. DeMore and O. B. Toon for comments and M. Gerstell, N. Heavens, R. L. Shia and M. Line for reading the manuscipt. This research was supported by NASA grant NNX07AI63G to the California Institute of Technology. M-C.L. was supported by NSC grant 98-2111-M-001-014-MY3 to Academia Sinica.

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X.Z., Y.L.Y., C.P. and F.M. contributed to the paper writing. X.Z. carried out the modelling work; M-C.L. helped with the modelling; F.M. and J-L.B. provided the data from the SPICAV instrument on board Venus Express; C.P. provided critical evaluation of the H2SO4 photolysis data; Y.L.Y. conceived and supervised the research.

Correspondence to Xi Zhang.

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