Photochemistry of Imidazole-2-carbaldehyde in Droplets as a Potential Source of H<sub>2</sub>O<sub>2</sub> and Its Oxidation of SO<sub>2</sub>
Yuwen Wang, Lingdong Kong, Jie Tan, Beibei Liu, Yixuan An, Lianghai Xia, Lu Yu, Qing Li, Lin Wang
Abstract
Hydrogen peroxide (H 2 O 2 ) plays a crucial role as an oxidizing agent within the tropospheric environment, making a substantial contribution to sulfate formation in hydrated aerosols and cloud and fog droplets. Field observations show that high levels of H 2 O 2 are often observed in heavy haze events and polluted air. However, the source of H 2 O 2 remains unclear. Here, using the droplets formed in situ by the deliquescence of hygroscopic compounds under a high relative humidity (RH), the formation of H 2 O 2 by the photochemistry of imidazole-2-carbaldehyde (2-IC) under ultraviolet irradiation was explored. The results indicate that 2-IC produces IM–C • –OH and IM–C • ═O radicals via H transfer itself to its excited triplet state and generates H 2 O 2 and organic peroxides in the presence of O 2, which has an evident oxidizing effect on SO 2, suggesting the potential involvement of this pathway in the formation of atmospheric sulfate. H 2 O 2 formation is limited in acidic droplets or droplets containing ammonium ions, and no H 2 O 2 is detected in droplets containing nitrate, whereas droplets containing citric acid have an obvious promotion effect on H 2 O 2 formation. These findings provide valuable insights into the behaviors of atmospheric photosensitizers, the source of H 2 O 2, and the formation of sulfate in atmospheric droplets.