1. Photochemical Control of the Distribution of Venusian Water and Comparison to Venus Express SOIR Observations Christopher D. Parkinson 1, Yuk L. Yung 2, Larry Esposito 3, Peter Gao 2, Stephen W. Bougher 1, Mathieu Hirtzig 4 1 Department of Atmospheric, Oceanic, and Space Sciences, University of Michigan, MI, USA 2 Division of Geological and Planetary Sciences, California Institute of Technology, CA, USA 3 Laboratory for Atmospheric and Space Physics, University of Colorado, CO, USA 4 Foundarion “La main à la pâte”, Montrouge, France DPS 2014, Tucson, Arizona November 12, 2014

2. Important reaction pathways connecting SO, SO 2, SO 3 and H 2 SO 4 H 2 SO 4, Aerosols, Clouds

3. Photochemistry Use Caltech/JPL KINETICS model to solve continuity equation: To obtain mixing ratio for key species using Standard reference lower boundary values, Maximum SO2, minimum H2O values, and Minimum SO2, maximum H2O values

4. Lower Boundary mixing ratio sensitivity study Water profiles with fixed SO 2 lower boundary of 25 ppm: H 2 O varies between 10- 35 ppm SO 2 profiles with fixed H2O lower boundary of 18 ppm: SO 2 varies between 5-100 ppm

5. Water and SO 2 Sensitivity Study SO 2 sensitivity study: Mixing ratio of H 2 O at 80 km Water sensitivity study: Mixing ratio of SO 2 at 80 km Last slide does not explain the whole story, as only the black curves are correlated between the two figures (viz., 18 ppm H 2 O and 25 ppm SO 2 for our std ref case). Figures are “anti-symmetric” and that there is a dramatic chemical bifurcation in the middle of each plot. In regions of high SO 2 abundance, we have a corresponding low H 2 O abundance, and vice versa.

6. Temperature and Eddy Diffusion Profiles (from Zhang et al, 2012) VEx VeRa daytime temperature profile below 100 km from observations of near the polar region (71° N) Above 100 km the temperature is from Seiff (1983) K zz = K o (n(z)/n ref ) -a

7. Eddy Diffusion Sensitivity Study Complicated relationship, so need to consider SO 2 and H 2 O fluxes in order to interpret sensitivity to eddy diffusion

8. Conclusion (1) H 2 O is modeled between 10 – 35 ppm at our 58 km lower boundary using an SO 2 mixing ratio of 25 ppm as our nominal reference value. SO 2 mixing ratio is then varied at lower boundary between 5 and 100 ppm holding the H 2 O mixing ratio of 18 ppm at the lower boundary SO 2 can control the water distribution at higher altitudes. SO 2 and H 2 O can regulate each other via formation of H 2 SO 4.

9. Conclusion (2) In regions of high mixing ratios of SO 2 there exists a “runaway effect” such that SO 2 gets oxidized to SO 3, which quickly soaks up H 2 O causing a major depletion of water between 70 and 100 km. (i.e. a complete sequestration of H2O by H2SO4 aerosols). In addition to explaining some of the observed variability in SO 2 and H 2 O on Venus, our work may also shed light on the observations of dark and bright contrasts at the Venus cloud tops observed in the ultraviolet spectrum.

10. Extra slides past this point… Not for talk…for possible questions asked.

11. H 2 SO 4 Saturation Vapor Pressure Stull (1947) Richardson et al. (1986) Ayers et al. (1980)

12. SO, SO 3 & H 2 SO 4 Profiles

13. Reactions Rates related to SO 2 a. SO 3 → SO 2 + O ClO + SO → Cl + SO 2 O + SO + M → SO 2 + M ClCO 3 + SO → Cl+ SO 2 + CO 2 SO + SO 3 → 2SO 2 b. SO 2 → S + O 2 SO 2 → SO + O O + SO 2 + M → SO 3 + M ClCO 3 + SO 2 → Cl + SO 3 + CO 2

14. Reactions Rates related to SO a. SO 2 → SO + O S + O 2 → SO + O b. SO → S + O ClO + SO → Cl + SO 2 O + SO + M → SO 2 + M ClCO 3 + SO → Cl+ SO 2 + CO 2 SO + SO + M → (SO) 2 + M SO + SO 3 → 2SO 2

15. Reactions Rates related to SO 3 a. H 2 SO 4 → SO 3 + H 2 O O + SO 2 + M → SO 3 + M ClCO 3 + SO 2 → Cl + SO 3 + CO 2 b. SO 3 → SO 2 + O SO 3 + H 2 O → H 2 SO 4 O + SO 3 → SO 2 + O 2 SO + SO 3 → 2SO 2