Resolving the Formation Mechanism of HONO via Ammonia-Promoted Photosensitized Conversion of Monomeric NO₂ on Urban Glass Surfaces

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.

A Novel Mechanism for NO2-to-HONO Conversion on Soot: Synergistic Effect of Elemental Carbon and Organic Carbon

“…The HONO yield (Figure c) increases from 38.6% at RH0% to 69.7% at RH40%, demonstrating that the existence of water vapor strongly promotes the conversion of NO 2 to HONO on EC. It was recently found that RH significantly increases the HONO yield during the photoreaction of NO 2 in the presence of NH 3 on the urban glass surfaces, where H 2 O is considered as the hydrogen source for the HONO formation from NO 2 . In our study, after the removal of OC from soot, no sufficient protons are available for the formation of HONO on EC.…”

SO2 Photoaging Enhances the Surface Conversion of NO2-to-HONO on Elemental Carbon

“…In addition, a most recent work reported that the UV-A fraction of the sunlight excites polycyclic aromatic hydrocarbons (PAHs, common OC in soot) to the triplet state and enhances the electronic interaction between the PAHs and the NO 2 , thus promoting the HONO formation in the presence of NH 3 . It is worth noting that these proposed NO 2 reduction pathways appear to be a redox reaction between NO 2 and soot rather than a catalyzed disproportionation reaction.…”

Enhanced HONO Formation from Aqueous Nitrate Photochemistry in the Presence of Marine Relevant Organics: Impact of Marine-Dissolved Organic Matter (m-DOM) Concentration on HONO Yields and Potential Synergistic Effects of Compounds within m-DOM

“…Most studies over the last decade have focused on photochemical pathways for HONO formation from urban areas, ice sheets, and plant leaves. − However, recently it has been suggested that the marine boundary layer (MBL) has significant daytime HONO, with the photoreduction of nitrate as a likely source. − Additionally, despite the fact that HONO photodissociates within the solar actinic spectral region, its daytime concentration, especially in the MBL, has been found to reach quasi-steady-state levels, suggesting daytime photolytic sources. ,,, Recent field studies have found that nitrate photolysis reactions are likely the main contributor of photochemical HONO formation in the marine boundary layer (MBL). − , The source of nitrate in the MBL comes from the displacement of chloride with nitrate in aged sea spray aerosols due to heterogeneous chemistry with gas-phase nitrogen oxides (NO 2 , HNO 3 , and N 2 O 5 ). , …”