H2O-H2SO4 System in Venus' Clouds and OCS, CO, and H2SO4 Profiles in Venus' Troposphere

Abstract

A coupled problem of diffusion and condensation is solved for the H2SO4-H2O system in Venus' cloud layer. The position of the lower cloud boundary and profiles of the H2O and H2SO4 vapor mixing ratios and of the H2O/H2SO4 ratio of sulfuric acid aerosol and its flux are calculated as functions of the column photochemical production rate of sulfuric acid, ΦH2SO4. Variations of the lower cloud boundary are considered. Our basic model, which is constrained to yield ƒH2O (30 km) = 30 ppm (Pollack et al . 1993), predicts the position of the lower cloud boundary at 48.4 km coinciding with the mean Pioneer Venus value, the peak H2SO4 mixing ratio of 5.4 ppm, and the H2SO4 production rate ΦH2SO4 = 2.2 × 1012 cm-2 sec-1. The sulfur to sulfuric acid mass flux ratio in the clouds is 1:27 in this model, and the mass loading ratio may be larger than this value if sulfur particles are smaller than those of sulfuric acid. The model suggests that the extinction coefficient of sulfuric acid particles with radius 3.7 μm (mode 3) is equal to 0.3 km-1 in the middle cloud layer. The downward flux of CO is equal to 1.7 × 1012 cm-2 sec-1 in this model. Our second model, which is constrained to yield ƒH2SO4 = 10 ppm at the lower cloud boundary, close to the value measured by the Magellan radiooccultations, predicts the position of this boundary to be at 46.5 km, which agrees with the Magellan data; ƒH2O(30 km) = 90 ppm, close to the data of Moroz et al. (1983) at this altitude; ΦH2SO4 = 6.4 × 1012 cm-2 sec-1; and ΦCO = 4.2 × 1012 cm-2 sec-1. The S/H2SO4 flux mass ratio is 1:18, and the extinction coefficient of the mode 3 sulfuric acid particles is equal to 0.9 km-1 in the middle cloud layer. A strong gradient of the H2SO4 vapor mixing ratio near the bottom of the cloud layer drives a large upward flux of H2SO4, which condenses and forms the excessive downward flux of liquid sulfuric acid, which is larger by a factor of 4-7 than the flux in the middle cloud layer. This is the mechanism of formation of the lower cloud layer. Variations of the lower cloud layer are discussed. Our modeling of the OCS and CO profiles in the lower atmosphere measured by Pollack et al. (1993) provides a reasonable explanation of these data and shows that the rate coefficient of the reaction SO3 + CO → CO2 + SO2 is equal to 10-11 exp(-(13,100 ± 1000)/T) cm3/s. The main channel of the reaction between SO3 and OCS is CO2 + (SO)2, and its rate coefficient is equal to 10-11 exp(-(8900 ± 500)/T) cm3/s. In the conditions of Venus' lower atmosphere, (SO)2 is removed by the reaction (SO)2 + OCS → CO + S2 + SO2. The model predicts an OCS mixing ratio of 28 ppm near the surface.