Oxidation mechanism of ammonia in water clusters

Oxidation of ammonia is an important atmospheric process, but detailed mechanisms are for the most part yet unknown. Here the later steps in a mechanism forming nitric oxide and water from NH₂ and O₂ are studied. The transition states for isomerisation of NH₂OO to HNOOH and for dissociation of HNOOH to HNO and OH are identified and the subsequent formation of nitric oxide and water is discussed. This process is studied in small water clusters to model the effect of the nearest neighbouring water molecules in larger clusters or at extended surfaces, demonstrating the effect of solvation in lowering the barrier height for NH₂OO to HNOOH isomerisation. Proton transfer through the surrounding water provides a feasible reaction path in this system. As a result, it is shown that the high barrier for isomerisation between intermediates in the gas phase is not likely to prevent ammonia oxidation in small water clusters or aerosols.