HR: 14:15h
AN: P23A-02    [Abstracts]
TI: Chemical Kinetic Model for the Lower Atmosphere of Venus
AU: * Krasnopolsky, V A
EM: vkrasn@verizon.net
AF: Department of Physics, Catholic University of America, 620 Michigan Avenue, N.E., Washington, DC 20740, United States
AB: A self-consistent chemical kinetic model of the Venus atmosphere at 0-47 km has been calculated for the first time. The model involves 76 reactions of 27 species. Chemical processes in the atmosphere below the clouds are initiated by photochemical products from the middle atmosphere (SO₃, CO, S_n), thermochemistry in the lowest 10 km, and photolysis of S₃. The sulfur bonds in OCS and S_n are weaker than the bonds of other elements in the basic atmospheric species on Venus; therefore the chemistry is sulfur-driven. Sulfur chemistry activates some H and Cl atoms and radicals, though their effect on the chemical composition is weak. The lack of kinetic data for many reactions presents a problem that has been solved using some similar reactions and thermodynamic calculations of inverse processes. Column rates of some reactions in the lower atmosphere exceed the highest rates in the middle atmosphere by two orders of magnitude. However, many reactions are balanced by the inverse processes, and their net rates are comparable to those in the middle atmosphere. The calculated profile of CO is in excellent agreement with the Pioneer Venus and Venera 12 gas chromatographic measurements and slightly above the value from the nightside spectroscopy at 2.3 μm. The OCS profile also agrees with the nightside spectroscopy which is the only source of data for this species. The model predicts 1.5 ppm of H₂S confirming the questionable detection by the Pioneer Venus mass spectrometer. While the calculated mean S₃ abundance agrees with the Venera 11-14 observations, a steep decrease in S_3 from the surface to 20 km is not expected from the observations. The ClSO₂and SO₂Cl₂ mixing ratios are ~10^-11 in the lowest scale height. The existing concept of the atmospheric sulfur cycles is incompatible with the observations of the OCS profile. A scheme suggested in the current work involves the basic photochemical cycle that transforms CO₂and SO₂into SO₃, CO, and S_n and a minor photochemical cycle which forms SO₂and S_n from OCS. The net effect of thermochemistry in the lowest 10 km is formation of OCS from CO and S_n. Chemistry at 30-40 km removes the downward flux of SO₃ and the upward flux of OCS and increases the downward fluxes of CO and S_n. The geological cycle of sulfur remains unchanged.
DE: 0317 Chemical kinetic and photochemical properties
DE: 5405 Atmospheres (0343, 1060)
DE: 5410 Composition (1060, 3672)
DE: 5455 Origin and evolution
DE: 6295 Venus
SC: Planetary Sciences [P]
MN: 2007 Joint Assembly