Heterogeneous Formation of HONO Catalyzed by CO2

Gas-phase nitrous acid (HONO) is a major precursor of hydroxyl radicals that dominate atmospheric oxidizing capacity. Nevertheless, pathways of HONO formation remain to be explored. This study unveiled an important CO2-catalysis mechanism of HONO formation, using Born–Oppenheimer molecular dynamics simulations and free-energy samplings. In the mechanism, HCO3 – formed from CO2 hydrolysis reacts with NO2 dimers to produce HONO at water surfaces, and simultaneously, itself reconverts back to CO2 via intermediates OC­(O)­ONO– and HOC­(O)­ONO. A flow system experiment was performed to confirm the new mechanism, which indicated that HONO concentrations with CO2 injections were increased by 29.4–68.5%. The new mechanism can be extended to other humid surfaces. Therefore, this study unveiled a previously overlooked vital role of CO2 that catalyzes formation of HONO and affects atmospheric oxidizing capacity.