Photochemistry of Imidazole-2-carbaldehyde in Droplets as a Potential Source of H 2 O 2 and Its Oxidation of SO 2 .

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.