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 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 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 NOx 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 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 O3 formation remained NOx-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 NOx reduction at lower elevations while coordinating NOx 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.