Mechanism of nitrous oxide (HONO) formation in D-layer of ionosphere

Nitrous oxide (HONO) is an active oxidant and a major source of hydroxyl radicals in the D-layer of the ionosphere (60-90 km above the Earth's surface). However, the mechanism underlying the formation of HONO remains unclear. To elucidate the mechanism of HONO formation, sequential (stepwise) reactions of H2O with NO+ were investigated using direct ab initio molecular dynamics calculations. The target reactions were NO+(H2O) + H2O and NO+(H2O)2 + H2O, i.e., NO+(H2O)n-1 + H2O → HONO-H+(H2O)n-1 (HONO product) (n = 2-5). In the case of n = 2, only the solvation of NO+ by H2O was found: NO+(H2O) + H2O → NO+(H2O)n (solvation product) (n = 2). HONO was obtained as the product at n = 3, although the reaction efficiency was low. The HONO product was efficiently formed when n = 4-5. The mechanism of HONO formation and the role of H2O in the reactions are discussed based on theoretical analysis.