Measurement report: Altitudinal Shift of Ozone Regimes in a Mountainous Background Region

Abstract. Elevated background ozone (O₃) poses significant challenges for regional air quality management. Understanding the vertical distribution of O₃ and its precursors is critical yet underexplored in Southwest China. This study presents the first comprehensive altitudinal gradient analysis (550 m, 1,774 m, 2,119 m 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 O₃ (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 O₃ variability aloft. Chemical box modeling (OBM-MCM v3.3.1) demonstrated net O₃ destruction at mountain foot (−1.93 ppb hr^-1) due to NO titration, shifting to net production at mountainside (0.35 ppb hr^-1) and mountaintop (0.29 ppb hr^-1). While O₃ 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 O₃ accumulation at mountain foot via regional transport, contrasting with mountainside and mountaintop, which function as net source regions. These findings necessitate altitude-specific O₃ 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.