Nitrous acid (HONO) is an important photolytic source of hydroxyl radicals, yet its sources in coastal and marine atmospheres remain poorly constrained. We combined three field campaigns conducted across the Yellow Sea–Bohai Sea region during 2025–2026, including urban–coastal observations, a coastal sea-fog campaign, and a cruise measurement campaign, to examine reactive nitrogen cycling from urban to marine environments.
Simultaneous measurements in Beijing and Qingdao showed distinct HONO formation regimes. Although HONO concentrations were lower in Qingdao, its apparent NO₂-to-HONO conversion efficiency was 50–60% higher. Beijing was characterized by humidity-dependent nighttime accumulation and daytime photolytic loss, whereas Qingdao exhibited a weak daytime HONO peak. The absence of a clear algae influence suggests additional photochemical sources, potentially including particulate nitrate photolysis and photosensitized NO₂ reduction at the sea surface.  Cruise measurement further showed abrupt HONO enhancements of up to 1.56 ppbv in ship plumes. During background marine periods, midday maxima in HONO and HONO/NO₂ indicated an additional photochemical source independent of direct emissions. Primary pollutants declined from Beijing toward the open sea, whereas O₃ remained higher over marine areas, likely because of weaker NO titration and sustained photochemistry.
These observations demonstrate that ship emissions, fog processing, and marine photochemistry jointly regulate HONO and reactive nitrogen cycling over coastal seas, with potential implications for marine atmospheric oxidation and O₃ formation.