Chemistry of Venus’ Atmosphere Vladimir A. Krasnopolsky Photochemical model for km Photochemical model for km Chemical kinetic model for the lower atmosphere (0-47 km) Chemical kinetic model for the lower atmosphere (0-47 km) Nighttime atmosphere and night airglow Nighttime atmosphere and night airglow

Modeling of H 2 SO 4 vapor and its photolysis rate: Initial data (Icarus 215, 197, 2011)

Calculated H 2 SO 4 is at 96 km, smaller than adopted by Zhang et al. (2012) by a factor of 2x10 6. This source of SO X may be neglected.

Photochemical model at km: Main features (Icarus 218, 230, 2012) Improved numerical accuracy: step = 0.5 km instead of 2 km that is comparable with H ≈ 5 km Improved numerical accuracy: step = 0.5 km instead of 2 km that is comparable with H ≈ 5 km NUV absorption is based on the Venera 14 data NUV absorption is based on the Venera 14 data H 2 O is calculated and not adopted in the model H 2 O is calculated and not adopted in the model Standard ClCO cycle, not scaled by a factor of ≈40 Standard ClCO cycle, not scaled by a factor of ≈40 NO and OCS chemistries NO and OCS chemistries Column rates are given for all reactions Column rates are given for all reactions

CO: Model and observations

Main feature of Venus’ photochemistry is formation of sulfuric acid in a narrow layer at 66 km that greatly reduces SO 2 and H 2 O above the layer. Minor variations of eddy diffusion and/or SO 2 /H 2 O can greatly change the delivery of SO 2 and H 2 O through this bottleneck and chemistry above the clouds

SO 2, OCS, SO, and S a

H 2 O: variations of SO 2 = ±5% at 47 km

Oxygen species O 2 column is similar to that in MA07 and both exceed the observed upper limit by a factor of 10 Ozone is similar to that observed by SPICAV at night (Montmessin et al. 2011)

Conclusions to Photochemistry at km Formation of sulfuric acid in a narrow layer near 66 km is a key feature that greatly reduces SO 2 and H 2 O above the clouds Delivery of SO 2 and H 2 O through this bottleneck is controlled by eddy diffusion and SO 2 /H 2 O ratio. Minor variations of atmospheric dynamics in the cloud layer induce strong variations in chemistry above the clouds H 2 SO 4, CO, and SO 2 Cl 2 are photochemical products delivered into the lower atmosphere and processed by thermochemistry there. While the overall agreement with the observational data is very good, some aspects deserve discussion: – O 2 column significantly exceeds the observed upper limit, and I do not have ideas how to solve the problem; – The model does not provide a source of SO X above 90 km. The interpretation of the SO X observations may be not unique; – SO 2 = 9.7 ppm at 47 km disagrees with SO 2 = 130 ppm at 35 km.

S 3 and S 4 Abundances and Improved Chemical Kinetic Model for the Lower Atmosphere of Venus (Icarus, submitted) Improved retrieval of S 3 and S 4 from analysis of Venera 11 by Maiorov et al. (2005) S 4 cycle by Yung et al. (2009) Reduction of the H 2 SO 4 and CO fluxes from the middle atmosphere by a factor of 4 relative to Kr07 OCS is completely calculated by the model (its abundance at the surface was a free parameter in Kr07) Some minor improvements

Absorption spectra of S 3 and S 4

Χ 2 -fitting of the true absorption spectra (Maiorov et al. 2005) by sums of S 3 and S 4 : S 3 = 11±3 ppt at 3-10 km and 18±3 ppt at km S 4 = 4±4 ppt at 3-10 km and 6±2 ppt at km

Main reactions in KP94 and Kr07: SO 3 + OCS → CO 2 + (SO) 2 (SO) 2 + OCS → CO + SO 2 + S 2 Net SO OCS → CO 2 + CO + SO 2 + S 2 S 4 cycle (Yung et al. 2009): S 2 + S 2 + M → S 4 + M S 4 + hv → S 3 + S S 3 + hv → S 2 + S 2(S + OCS → CO + S 2 ) Net 2 OCS → 2 CO + S 2

Model: 89 reactions of 28 species, some improvements to Kr07 S 3 + hν → S 2 + S S 3 + hν → S 2 + S I=0.017*10 -3 * ( h+0.063h 2 ) S 4 + hν → S 2 + S 2 S 4 + hν → S 2 + S 2 I = 0.01*( h– h 2 ) S 4 + hv → S + S 3 S 4 + hv → S + S 3 I=1*10 -5 *( h+0.15h 2 )

Models with (solid) and without (thin) S 4 cycle

Basic species in the model

Model for nighttime atmosphere and nightglow at km (Icarus 207, 17, 2010) Involves 61 reactions of 24 species Involves 61 reactions of 24 species Odd hydrogen and chlorine chemistries Odd hydrogen and chlorine chemistries Fluxes of O, N, and H at 130 km as input parameters Fluxes of O, N, and H at 130 km as input parameters Requires 45% of the dayside oxygen production above 80 km to fit the observed mean O μm emission of 0.5 MR Requires 45% of the dayside oxygen production above 80 km to fit the observed mean O μm emission of 0.5 MR Comparison with GCMs by Bougher et al. (1990) and Brecht et al. (2011) Comparison with GCMs by Bougher et al. (1990) and Brecht et al. (2011)

Calculated vertical profiles

Nightglow profiles 4πI O2 = 0.158(Φ O /10 12 ) 1.14 MR 4πI NO = 224(Φ N /10 9 )(Φ O /10 12 ) 0.38 R 4πI OH (1-0) = 1.2(Φ O /10 12 ) 1.46 X ln X kR, X=Φ H /10 8

Problems SPICAV stellar occultations result O 3 ≈ 5x10 7 cm -3 at km that agrees with the global-mean model but much smaller than that in the nighttime model SPICAV stellar occultations result O 3 ≈ 5x10 7 cm -3 at km that agrees with the global-mean model but much smaller than that in the nighttime model Is the SPICAV low ozone compatible with the observed OH nightglow that is excited mostly by Is the SPICAV low ozone compatible with the observed OH nightglow that is excited mostly by H + O 3 → OH* + O 2 ?