Heterogeneous Formation of HONO Catalyzed by CO₂

Abstract

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.