Photochemistry of concentrated sulfuric acid in the presence of SO2 and Fe(II), and implications for the cloud chemistry of Venus

Measurements of the conversion of trace amounts of Fe(II) to Fe(III) have been used to monitor the progress of photochemistry in bulk samples of concentrated sulfuric acid under 248 nm laser irradiation. Because sulfuric acid does not undergo single-photon absorption at 248 nm, and because conditions that could promote multiphoton absorption were absent, primary processes in this system must involve the additives or other impurities. Major primary processes in our system are considered to be production of H atoms by photolysis of complex ions containing Fe(II), of SO3− radical ions and atomic hydrogen by photolysis of HSO3−, and of SO4− radical ions by photolysis of complex ions containing Fe(III). Kinetic modeling shows that processes involving Fe(III) are normally less important in our system than processes involving Fe(II). Independently of which primary process is dominant, secondary reactions occur which are similar to those that result in the formation of acid rain in Earth’s troposphere. These secondary processes lead to removal of oxygen in a very fast reaction with SO3− and thence to production of HSO5−, the stable anion of peroxymonosulfuric acid, which can oxidize CO to CO2. Our results imply that photochemical reactions in the sulfuric acid clouds of Venus can affect the composition of the atmosphere at high altitudes, by removing O2 and CO and regenerating CO2, and so help stabilize the 96% CO2 atmosphere of Venus against photolysis by solar ultraviolet radiation.