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 NH3-promoted 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.