Exploring the impact of urban emissions on elevated HONO in suburban mountain areas.

Nitrous acid (HONO) is a key precursor of OH radical, initiating daytime photochemistry. But its sources remain highly uncertain, particularly in suburban mountain area affected by urban transport. In summer 2023, intensive observations were conducted at the Mangshan (MS, 659 m) site, located along the prevailing southerly pathway from urban Beijing, with the Institute of Chemistry Chinese Academy of Sciences (ICCAS, 59 m) in the city center as an urban reference. Elevated HONO concentrations were observed at MS (average: 0.42 ± 0.31 ppb; maximum: 1.62 ppb). Multiple lines of evidence demonstrated that high pollutants levels at MS were strongly influenced by emissions from urban area and mountain foot through the "urban-mountain" transport chain and upslope valley winds. Daytime source analysis revealed that NO + OH homogeneous reaction only explained ∼ 7 % of observed HONO, indicating strong additional sources (Pother, average: 0.94 ± 0.42 ppb/h; maximum: 2.35 ppb/h). Photo-enhanced NO2 heterogeneous reactions and nitrate photolysis represented additional sources. The majority of unexplained HONO (∼ 48.7 %) was attributed to transport and secondary formation on transport. HONO photolysis contributed 79.3 % to OH production at MS, higher than 73.6 % at ICCAS and exceeding values at other mountain sites. Overall, Beijing urban emissions significantly enhanced HONO and oxidation capacity in the suburban mountain area through the "urban-mountain" transport chain. This study provides important evidence for understanding pollution transport and oxidation processes under complex terrain, with direct implications for improving regional air quality modeling and formulating targeted pollution control strategies.