OH-Initiated Oxidation of Imidazoles in Tropospheric Aqueous-Phase Chemistry.

Imidazoles formed via the reaction of dicarbonyls with nitrogen containing compounds in the atmospheric particle phase can be expected to initiate secondary organic aerosol (SOA) growth due to their potential to act as photosensitizers. Recent field studies quantified imidazoles for the first time in ambient aerosol samples from Europe and China. However, knowledge about the kinetics and mechanisms of particle-phase reactions involving imidazoles is still very limited. In the present study, the radical-driven aqueous-phase oxidative degradation reactions of the hydroxyl radical (·OH) with imidazoles were investigated. For the imidazoles, the following rate constants at 298 K and acidic conditions are obtained as k(imidazole-2-carboxaldehyde) = (1.8 ± 0.1) × 109 L mol-1 s-1, k(1-methylimidazole) = (2.3 ± 0.1) × 109 L mol-1 s-1, k(2-methylimidazole) = (3.9 ± 0.1) × 109 L mol-1 s-1, k(4-methylimidazole) = (3.2 ± 0.2) × 109 L mol-1 s-1, k(1-ethylimidazole) = (2.6 ± 0.1) × 109 L mol-1 s1, and k(2-ethylimidazole) = (3.9 ± 0.1) × 109 L mol-1 s-1. Temperature and pH dependencies of the reactions as well as the activation parameters have been determined and discussed. The possible implications and restrictions for imidazoles acting as photosensitizers in tropospheric particles have been considered.