Photocatalytic Oxidation of SO2 by TiO2: Aerosol Formation and the Key Role of Gaseous Reactive Oxygen Species

Photocatalytic materials are proved to effectively eliminate gaseous pollutants and are widely used in the environment. However, as one of the rare experiments focusing on their influence on secondary aerosol formation generated in the gas phase (SA(g)), our study demonstrated the high-yield SA(g) formation in the photocatalysis process. In this study, the photo-degradation of SO2 by TiO2 under various relative humidity (RH) conditions was deeply explored with multiple methods. Unexpectedly, H2SO4 aerosols (SA(g-H2SO4)) in yields of 10.10-32.64% were observed under the studied RH conditions for the first time. Gaseous (OH)-O-center dot and H2O2 generated from the oxidation of H2O and reduction of O-2 by TiO2 were directly detected in the photocatalysis process, and they were identified as the determining factor for SA(g-H2SO4) formation. The formation of SA(g-H2SO4) was also influenced by RH, the heterogeneous reaction of SO2, and the uptake of H2SO4. The role of the released gaseous (OH)-O-center dot and H2O2 on atmospheric chemistry was proved to be unignorable by adopting the obtained parameters into the real environment. These findings provided direct experimental evidence of secondary pollution in the photocatalysis process and are of great significance to the field of atmospheric environment and photocatalytic materials.