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 photodegradation 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 and H₂O₂ generated from the oxidation of H₂O and reduction of O₂ 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 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.