Dominant role of humidity thresholds in driving seasonal ozone production regimes in urban Guiyang, Yunnan-Guizhou Plateau

Ground-level ozone (O 3 ) pollution has emerged as a significant environmental challenge in Guiyang, a plateau city in southwestern China, despite its reputation for high ecological quality. This study investigates the seasonality of O 3 production rates and sensitivities through comprehensive in situ measurements of O 3 and its precursors from August 2022 to August 2024. Our results reveal that O 3 concentrations in Guiyang peak during spring (35.0 ± 17.0 ppb) and autumn (28.1 ± 17.0 ppb). This contrasts sharply with the summer-dominant patterns observed in the North China Plain and Yangtze River Delta regions. This anomaly is attributed to the interplay of meteorological factors, such as high humidity and monsoon rains in summer, which suppress O 3 accumulation. Random forest modeling integrated with SHapley Additive exPlanations (SHAP) identify relative humidity (RH) as the dominant factor influencing O 3 variability, exhibiting threshold-dependent suppression effects (RH > 80% in spring/summer; >75% in autumn/winter). Chemical budget analyses highlight that O 3 production is primarily driven by RO 2 + NO reactions (52.4%–56.8%) in non-winter seasons, shifting to HO 2 + NO (54.6%) in winter, while O 3 destruction is dominated by photolysis (46.8%–53.3%). Regional transport plays a minor role, with photochemical production dominating daytime accumulation versus physical advection prevailing at night. Sensitivity simulations indicate a NO x -limited regime for O 3 control in spring, summer, and autumn, advocating for targeted NO x reductions, whereas a transitional (VOCs-NO x co-limited) regime in winter necessitate coordinated control of both precursors. These findings underscore the need for seasonally tailored emission control strategies to mitigate O 3 pollution in plateau regions, emphasizing the interplay of meteorology, chemistry, and precursor emissions.