Photochemistry of Airborne Dust Produces Nucleation Events in the Troposphere
Résumé
Dust particles acts as a sink for many gases, such as sulfur dioxide. It is well known that SO2 reacts on dust particle surfaces leading to the production of SO4--. It is known that light-driven reactions on dust particles containing TiO2 or Fe(III) oxides produce OH radicals from water. These radicals can convert SO2 adsorbed onto the particles into H2SO4, which stays on the particles. Here, we show that OH radicals may leave the dust particles and go on to initiate gas-phase chemistry with gaseous SO2. To simulate such conditions, we shone light into a flow tube containing low concentrations of dust, along with water vapor and SO2, and monitored the particle concentrations inside the tube. Low dust concentrations are necessary for new particle nucleation; high dust concentrations would provide enough surface area for sulfuric acid to adsorb onto the dust, instead of nucleating aerosol. These findings are supported by recent field observations near Beijing and Lyon.