Chemical kinetic model for the lower atmosphere of Venus

Abstract

A self-consistent chemical kinetic model of the Venus atmosphere at 0–47 km has been calculated for the first time. The model involves 82 reactions of 26 species. Chemical processes in the atmosphere below the clouds are initiated by photochemical products from the middle atmosphere (H 2SO 4, CO, S x ), thermochemistry in the lowest 10 km, and photolysis of S 3. The sulfur bonds in OCS and S x are weaker than the bonds of other elements in the basic atmospheric species on Venus; therefore the chemistry is mostly 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 values 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 abundance and vertical profile of gaseous H 2SO 4 are similar to those observed by the Mariner 10 and Magellan radio occultations and ground-based microwave telescopes. While the calculated mean S 3 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 ClSO 2 and SO 2Cl 2 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 CO 2 and SO 2 into SO 3, CO, and S x , and a minor photochemical cycle which forms CO and S x from OCS. The net effect of thermochemistry in the lowest 10 km is formation of OCS from CO and S x . Chemistry at 30–40 km removes the downward flux of SO 3 and the upward flux of OCS and increases the downward fluxes of CO and S x . The geological cycle of sulfur remains unchanged.