Resolving the Formation Mechanism of HONO via Ammonia-Promoted Photosensitized Conversion of Monomeric NO<sub>2</sub> on Urban Glass Surfaces
Jiangping Liu, Bai Li, Huifan Deng, Yan Yang, Wei Song, Xinming Wang, Yongming Luo, Joseph S. Francisco, Lei Li, Sasho Gligorovski
Abstract
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.