Measurement report: Altitudinal shift of ozone regimes in a mountainous background region

Elevated background ozone (O 3 ) poses significant challenges for regional air quality management. Understanding the vertical distribution of O 3 and its precursors is critical, yet remains poorly characterized in Southwest China. This study presents the first comprehensive altitudinal gradient analysis (550, 1774, 2119 m above sea level – a.s.l.) in the Fanjingshan National Nature Reserve, a remote high-altitude site on the Yunnan-Guizhou Plateau. Continuous measurements (March–August 2024) revealed a marked positive gradient in O 3 (14.8 ± 15.2 ppb at mountain foot to 40.2 ± 14.7 ppb at mountaintop), contrasting with declining precursor concentrations. Random Forest–SHAP analysis identified relative humidity and NO x as dominant controls at the mountain foot, whereas temperature and reactive VOCs governed O 3 variability aloft. Chemical box modeling (OBM-MCM v3.3.1) demonstrated net O 3 destruction at mountain foot (−1.93 ppb h −1 ) due to NO titration, shifting to net production at mountainside (0.35 ppb h −1 ) and mountaintop (0.29 ppb h −1 ). While O 3 formation remained NO x -limited across all sites, sensitivity to anthropogenic hydrocarbons increased with altitude (RIR: −0.12 mountain foot to 0.51 mountaintop). Transport analysis indicated O 3 accumulation at mountain foot via regional transport, contrasting with mountainside and mountaintop, which function as net source regions. These findings necessitate altitude-specific O 3 control: prioritizing NO x reduction at lower elevations while coordinating NO x and VOC controls at higher altitudes. Expanding high-altitude monitoring, especially in under-monitored areas like Southwest China, is crucial for characterizing regional background pollution. Future studies require vertical monitoring with improved models to assess transboundary impacts and changing emissions.