Atmospheric Oxidation of Imidazole by Hydroxyl Radicals: Fate of Peroxyl Radical Products

In recent years, imidazole and its derivatives have received attention due to their role as components of brown carbon in atmospheric aerosol particles. These compounds absorb solar radiation in the UV-visible range, altering the aerosol optical properties, and acting as photosensitizers, prompting accelerated aerosol particle growth. Although atmospheric imidazoles are thought to be mainly produced in the particle-phase (e.g. via reaction of glyoxal with amines), these compounds were recently observed also in the gas-phase. Studies investigating the fate of imidazole in the gas-phase explored the initial steps of its oxidation by OH radical, identifying the major outcome to be the formation of an OH-addition product. However, this product is an alkyl (C-centered) radical, and its fate following reaction with O₂ is still unexplored. In this work, we employed computational methods to investigate the reaction channels available to the first-generation peroxyl radicals produced from the reaction of imidazole with OH radical and O₂. The unimolecular reaction pathways were explored with the automated reaction search and kinetics code KinBot. Product distributions under a range of temperatures and NO_x concentrations were subsequently obtained by assembling and solving a master equation. Our findings predict that under most conditions considered, the formation of two major closed-shell products is expected: the cyclic diimine 4H-imidazole-4-ol, and the ring-opened species N,N’-diformylformamidine (FMF). The relative yields of these two products is, however, sensitive to NO_x levels. While both compounds may be produced under pristine conditions (low NO_x), the yield of FMF is predicted to be above 95 % under more polluted conditions (high NO_x). These compounds may be further oxidized in the gas-phase, or they may partition into aerosol particles to participate in aqueous-phase reactions.