Chemistry of Venus’ AtmosphereVladimir A. Krasnopolsky

• Photochemical model for 47-112 km

• Chemical kinetic model for the lower atmosphere (0-47 km)

• Nighttime atmosphere and night airglow

Modeling of H2SO4 vapor and its photolysis rate: Initial data (Icarus 215, 197, 2011)

Calculated H2SO4 is 10-13 at 96 km, smaller than adopted by Zhang et al. (2012) by a factor of 2x106. This source of SOX may be neglected.

Photochemical model at 47-112 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

• NUV absorption is based on the Venera 14 data• H2O is calculated and not adopted in the model• Standard ClCO cycle, not scaled by a factor of

≈40• NO and OCS chemistries• 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 SO2 and H2O above the layer.

Minor variations of eddy diffusion and/or SO2/H2O can greatly change the delivery of SO2 and H2O through this bottleneck and chemistry above the clouds

SO2, OCS, SO, and Sa

H2O: variations of SO2 = ±5% at 47 km

Oxygen species

• O2 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 47-112 km• Formation of sulfuric acid in a narrow layer near 66 km is a key

feature that greatly reduces SO2 and H2O above the clouds• Delivery of SO2 and H2O through this bottleneck is controlled by eddy

diffusion and SO2/H2O ratio. Minor variations of atmospheric dynamics in the cloud layer induce strong variations in chemistry above the clouds

• H2SO4, CO, and SO2Cl2 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:– O2 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 SOX above 90 km. The

interpretation of the SOX observations may be not unique;– SO2 = 9.7 ppm at 47 km disagrees with SO2 = 130 ppm at 35 km.

S3 and S4 Abundances and Improved Chemical Kinetic Model for the Lower Atmosphere of Venus

(Icarus, submitted)• Improved retrieval of S3 and S4 from analysis of

Venera 11 by Maiorov et al. (2005)• S4 cycle by Yung et al. (2009)• Reduction of the H2SO4 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 S3 and S4

Χ2-fitting of the true absorption spectra (Maiorov et al. 2005) by sums of S3 and S4:

S3 = 11±3 ppt at 3-10 km and 18±3 ppt at 10-19 kmS4 = 4±4 ppt at 3-10 km and 6±2 ppt at 10-19 km

• Main reactions in KP94 and Kr07:SO3 + OCS → CO2 + (SO)2

(SO)2 + OCS → CO + SO2 + S2

Net SO3 + 2 OCS → CO2 + CO + SO2 + S2

• S4 cycle (Yung et al. 2009):S2 + S2 + M → S4 + MS4 + hv → S3 + S S3 + hv → S2 + S2(S + OCS → CO + S2)Net 2 OCS → 2 CO + S2

Model: 89 reactions of 28 species, some improvements to Kr07

• S3 + hν → S2 + SI=0.017*10-3 *

(4.4+1.36h+0.063h2)

• S4 + hν → S2 + S2

I = 0.01*(1.4+0.535h–0.0013h2)

• S4 + hv → S + S3

I=1*10-5*(8.5+2.4h+0.15h2)

Models with (solid) and without (thin) S4 cycle

Basic species in the model

Model for nighttime atmosphere and nightglow at 80-130 km

(Icarus 207, 17, 2010)

• Involves 61 reactions of 24 species• Odd hydrogen and chlorine chemistries• 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 O2 1.27 μm emission of 0.5 MR

• Comparison with GCMs by Bougher et al. (1990) and Brecht et al. (2011)

Calculated vertical profiles

Nightglow profiles4πIO2 = 0.158(ΦO/1012)1.14 MR

4πINO = 224(ΦN/109)(ΦO/1012)0.38 R4πIOH (1-0) = 1.2(ΦO/1012)1.46 X0.46-0.048 ln X kR, X=ΦH/108

Problems

• SPICAV stellar occultations result O3 ≈ 5x107 cm-3 at 90-100 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

H + O3 → OH* + O2 ?