Formation of the aminoperoxyl radical in the atmospheric oxidation of ammonia†

Atmospheric oxidation of ammonia is initiated by its reaction with the hydroxyl radical, producing the aminyl radical (NH₂). Thus far, it has been believed that the subsequent fate of NH₂ is to react bimolecularly with other atmospheric trace gases like NO, NO₂, or O₃. Its reaction with O₂ has been considered insignificant under atmospheric conditions. However, this is based on a rate coefficient that is orders of magnitude smaller than those known for analogous reactions of O₂ with carbon-, sulfur-, and other nitrogen-centered radicals. We demonstrate by multireference calculations and kinetic modelling that the reaction of NH₂ and O₂ leading to the formation of the aminoperoxyl radical (NH₂O₂) occurs with a rate coefficient similar to those of the aforementioned analogous radicals. We show that the previously estimated small rate coefficient is due to an unimolecular rate limiting step in the formation of measured products rather than the initial NH₂ + O₂ reaction. The lack of experimental detection of NH₂O₂ in the existing literature is likely due to the experiments being conducted at either high temperature or low pressure. We show that the atmospheric presence of NH₂O₂ depends greatly on atmospheric conditions. Its formation is an important, yet previously overlooked pathway in atmospheric ammonia oxidation, especially at low temperatures.