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.