The Pivotal Role of Carbon-Centered Environmentally Persistent Free Radicals in HONO Formation via Heterogeneous Reaction of NO₂ on Diesel Soot

Nitrous acid (HONO) constitutes an essential gaseous pollutant and a significant reservoir of hydroxyl radicals (^•OH), which are crucial for atmospheric oxidation capacity. Environmentally persistent free radicals (EPFRs), long-lived in particulate matter, may promote HONO formation via reactions with NO₂, although the mechanisms remain incompletely elucidated. In this study, the heterogeneous formation of HONO was explored through reactions between NO₂ and diesel soot collected during a field campaign, focusing on the role of EPFRs. EPFRs in diesel soot, primarily composed of carbon-centered radicals (CCRs) within the elemental carbon (EC), were present at 5.40 × 10¹⁸ ± 9.60 × 10¹⁸ spins·m^-3. The combination of CCRs in EC with NO₂ to form surface nitro compounds (RONO) was initially observed and served as the rate-limiting step. Furthermore, the photolysis of carboxyl compounds (ROOH) in organic carbon (OC) generated ^•OH, which reacted with RONO to enhance HONO formation. Gradient RH experiments revealed that H₂O facilitates HONO formation by enhancing NO₂ adsorption or acting as an ^•OH donor. The results indicate that, in typical urban environments, besides HCHO photolysis, EPFRs in diesel soot significantly contribute to HONO formation, playing a major role in ^•OH production with a rate of ∼10⁶ molecules·cm^-3·s^-1 at RH 40-60% under vehicle-related EPFR concentrations (∼10¹³ spins·m^-3, observed in Xi'an, China). This study highlights a novel HONO formation mechanism based on the synergistic effects of OC and EC in soot and underscores the critical role of EPFR-mediated heterogeneous HONO formation in the atmospheric oxidative capacity.