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.