New Insights in the Atmospheric HONO Formation: New Pathways for N2O4 Isomerization and NO2 Dimerization in the Presence of Water

Isomerization of N2O4 and dimerization of NO2 in thin water films on surfaces are believed to be key steps in the hydrolysis of NO2, which generates HONO, a significant precursor to the OH free radical in lower atmosphere and high-energy materials. Born–Oppenheimer molecular dynamics simulations using the density functional theory are carried out for NO2(H2O)m, m ≤ 4, and N2O4(H2O)n clusters, n ≤ 7, used to mimic the surface reaction, to investigate the mechanism around room temperature. The results are (i) the NO2 dimerization and N2O4 isomerization reactions occur via two possible pathways, the non-water-assisted and water-assisted mechanisms; (ii) the NO2 dimerization in the presence of water yields either ONONO2(H2O)m or NO3–NO+(H2O)m clusters, but it is also possible to form the HNO3(NO2–)(H3O+)(H2O)m−2 transition state to form HONO and HNO3, directly; (iii) the N2O4 isomerization yields the NO3–NO+(H2O)n cluster, but it does not hydrolyze faster than the NO2+NO2–(H2O)n hydrolysis to directly form the HONO and HNO3. New insights for hydrolysis of oxides of nitrogen in and on thin water films on surfaces in the atmosphere are discussed.