Characteristics and influencing factors of ambient ozone pollution in Hangzhou in the relative humidity range with high ozone levels

With the rapid economic growth and urbanization, despite improvements in particulate matter (PM_2.5) pollution control, ozone (O₃) pollution has emerged as a pressing environmental issue in China. This study systematically investigates O₃ pollution dynamics in Hangzhou (2021–2023) in the relative humidity (RH) range with high ozone levels using observational data and generalized additive modeling (GAM). Key findings reveal distinct temporal patterns: diurnal O₃ peaks at 14:00 (lagging solar radiation by about 2 h), seasonal maxima in summer (157.58 μg m⁻³) driven by temperature-photochemistry coupling, and non-monotonic annual trends (116.21–123.77 μg m⁻³) despite NO_x decline, reflecting transitional chemical regimes. Alkenes (36.94 %) and oxygenated VOCs (OVOCs, 36.54 %) dominated O₃ formation potential, with acetaldehyde, ethylene, and 1-butene as top contributors, highlighting the significant contributions of industrial and vehicular emission sources to O₃ formation. GAM analysis identified temperature as the primary driver, exhibiting exponential O₃ enhancement above 30 °C. In particular, NO₂ and peroxyacetyl nitrate showed synergistic effects, suggesting their dual roles as both precursors and indicators of radical cycling efficiency under the RH range. The 40 %–60 % RH range optimizes hydroxyl radical production while minimizes aerosol hydration, establishing it as a critical threshold for photochemical O₃ generation. These findings emphasize the importance of considering specific RH ranges and precursor reactivity in formulating refined O₃ pollution control strategies, providing a new theoretical basis for O₃ pollution control in the Yangtze River Delta region.