Resolving the Formation Mechanism of HONO via Ammonia-Promoted Photosensitized Conversion of Monomeric NO 2 on Urban Glass Surfaces.

Understanding the formation processes of nitrous acid (HONO) is crucial due to its role as a primary source of hydroxyl radicals (OH) in the urban atmosphere and its involvement in haze events. In this study, we propose a new pathway for HONO formation via the UVA-light-promoted photosensitized conversion of nitrogen dioxide (NO 2 ) in the presence of ammonia (NH 3 ) and polycyclic aromatic hydrocarbons (PAHs, common compounds in urban grime). This new mechanism differs from the traditional mechanism, as it does not require the formation of the NO 2 dimer. Instead, the enhanced electronic interaction between the UVA-light excited triplet state of PAHs and NO 2 -H 2 O/NO 2 -NH 3 -H 2 O significantly reduces the energy barrier and facilitates the exothermic formation of HONO from monomeric NO 2 . Furthermore, the performed experiments confirmed our theoretical findings and revealed that the synergistic effect from light-excited PAHs and NH 3 boosts the HONO formation with determined HONO fluxes of 3.6 × 10 10 molecules cm -2 s -1 at 60% relative humidity (RH) higher than any previously reported HONO fluxes. Intriguingly, light-induced NO 2 to HONO conversion yield on authentic urban grime in presence of NH 3 is unprecedented 130% at 60% RH due to the role of NH 3 acting as a hydrogen carrier, facilitating the transfer of hydrogen from H 2 O to NO 2 . These results show that NH 3 -assisted UVA-light-induced NO 2 to HONO conversion on urban surfaces can be a dominant source of HONO in the metropolitan area.