Photochemical modelling of Venus clouds using Pioneer Venus data

In order to understand the evolution of water on Venus, we must know the hydrogen escape flux as a function of the tropospheric water abundance. We have studied the connection between total stratospheric hydrogen and exobase hydrogen available to non-thermal escape processes and examined the details of the photochemical trap for water at the Venus cloud tops. Our immediate goal is to calculate the stratospheric water abundance as a function of the tropospheric water abundance. Photochemical production of H2SO4 acts as a sink for both water and sulfur and is capable of keeping stratospheric abundances low if a proper balance exists between the tropospheric abundances. If production of H2SO4 were the only sink for H2O and SO2, the excess in tropospheric abundance of one over the other would reach the stratosphere, and the functional dependence of stratospheric H2O on tropospheric H2O would be linear near the present state. On Venus, however, sulfuric acid condenses at cloud top temperatures and the resulting aerosols can absorb additional water of hydration. This complicates the water budget, increasing the efficiency of sulfur as a sink for water. We have investigated the balance between tropospheric H2O and SO2 and how delicate the balance is. Our major conclusions from this work are the following: (1) H2O and SO2 are mutually limiting if proper tropospheric balance is maintained; (2) changes in tropospheric abundances on the order of 5 ppm are significant; and (3) changes in mixing rates near the cloud tops can cause dramatic changes in SO2 without causing dramatic changes in H2O