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.