Synergistic effect of nitrate-doped TiO 2 aerosols on the fast photochemical oxidation of formaldehyde.

The uptake of formaldehyde (HCHO) on mineral dust affects its budget as well as particle properties, yet the process has not yet been fully investigate. Here, TiO2 and nitrate-doped TiO2 aerosols were used as proxies for mineral dust, and the uptake of HCHO was explored in a chamber under both dark and illuminated conditions. The uptake loss of HCHO on UV-illuminated aerosols is 2-9 times faster than its gaseous photolysis in our experimental system. The uptake coefficient in the range of 0.43-1.68 × 10-7 is 1-2 orders of magnitude higher than previous reports on model mineral dust particles. The reaction rate exhibits a Langmuir-Hinshelwood-type dependence on nitrate content and relative humidity, suggesting the competitive role of nitrate salts, water vapor and HCHO on the TiO2 surface. The reaction produces carbon dioxide as the main product and gaseous formic acid as an important intermediate. The hydroxyl radical produced on illuminated TiO2 primarily drives the fast oxidation of HCHO. The nitrate radical arising from the TiO2 -catalyzed photoreaction of nitrate synergistically promotes the oxidation process. This study suggests a novel oxidation route for HCHO in the atmosphere, taking into account high abundance of both mineral dust and anthropogenic TiO2 aerosols.