River-induced circulations alter ozone distribution, source contributions, and chemical sensitivity in a river–valley city along the Yangtze River

Abstract. Ground-level ozone (O₃) pollution in river–valley cities is influenced by interactions among photochemistry, boundary-layer processes, and terrain-induced circulations, yet their combined effects on O₃ distributions remain insufficiently understood. Using WRF–CAMx coupled with source apportionment and process analysis, we investigated the three-dimensional distribution, source contributions, and formation characteristics of O₃ over the Nanjing section of the Yangtze River. The altitude of the O₃ maximum decreased from ~5.0 km in winter to ~2.0 km in summer, indicating stronger near-surface photochemical production and boundary-layer processes during warm seasons. Distinct spatial heterogeneity was observed between the river corridor and adjacent urban areas. Daytime near-surface O₃ development was weaker over the river in summer, whereas the high-O₃ layer extended downward to greater depths in spring and autumn. Regional background transport accounted for most total O₃, while suburban contributions increased during high-O₃ episodes, reaching 45.5 % under O₃ concentrations of at least 160 µg/m³ in July. Transport and diffusion associated with river-breeze circulations played important roles in regulating O₃ variability within the river corridor. O₃ formation remained volatile organic compound (VOC)-limited in spring and autumn, whereas river-breeze-induced nitrogen oxide (NOx) dilution shifted the river corridor to NOx-limited conditions approximately two hours earlier than in the surrounding urban area in summer. These findings demonstrate that river-induced circulations can substantially modify O₃ distributions, source contributions, and chemical sensitivity within river–valley cities, and that city-averaged and monthly mean characterizations may not fully capture local O₃ variability in complex terrain.