Formation of HONO from the NH₃-promoted hydrolysis of NO₂ dimers in the atmosphere

One challenging issue in atmospheric chemistry is identifying the source of nitrous acid (HONO), which is believed to be a primary source of atmospheric “detergent” OH radicals. Herein, we show a reaction route for the formation of HONO species from the NH₃promoted hydrolysis of a NO₂ dimer (ONONO₂), which entails a low free-energy barrier of 0.5 kcal/mol at room temperature. Our systematic study of HONO formation based on NH₃ + ONONO₂ + nH₂O and water droplet systems with the metadynamics simulation method and a reaction pathway searching method reveals two distinct mechanisms: (i) In monohydrates (n = 1), tetrahydrates (n = 4), and water droplets, only one water molecule is directly involved in the reaction (denoted the single-water mechanism); and (ii) the splitting of two neighboring water molecules is seen in the dihydrates (n = 2) and trihydrates (n = 3) (denoted the dual-water mechanism). A comparison of the computed free-energy surface for NH₃-free and NH₃-containing systems indicates that gaseous NH₃ can markedly lower the free-energy barrier to HONO formation while stabilizing the product state, producing a more exergonic reaction, in contrast to the endergonic reaction for the NH₃-free system. More importantly, the water droplet reduces the free-energy barrier for HONO formation to 0.5 kcal/mol, which is negligible at room temperature. We show that the entropic contribution is important in the mechanism by which NH₃ promotes HONO formation. This study provides insight into the importance of fundamental HONO chemistry and its broader implication to aerosol and cloud processing chemistry at the air–water interface.