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

Hydrogen peroxide (HO) 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 HO are often observed in heavy haze events and polluted air. However, the source of HO remains unclear. Here, using the droplets formed by the deliquescence of hygroscopic compounds under a high relative humidity (RH), the formation of HO 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 HO and organic peroxides in the presence of O, which has an evident oxidizing effect on SO, suggesting the potential involvement of this pathway in the formation of atmospheric sulfate. HO formation is limited in acidic droplets or droplets containing ammonium ions, and no HO is detected in droplets containing nitrate, whereas droplets containing citric acid have an obvious promotion effect on HO formation. These findings provide valuable insights into the behaviors of atmospheric photosensitizers, the source of HO, and the formation of sulfate in atmospheric droplets.