the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 05 Nov 2025
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 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 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 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.
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Status: open (until 17 Dec 2025)
- RC1: 'Comment on egusphere-2025-4818', Anonymous Referee #1, 20 Nov 2025 reply
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Measurement report: Altitudinal Shift of Ozone Regimes in a Mountainous Background Region Y. Yang et al. https://doi.org/10.5281/zenodo.17239121
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- 1
Overall Evaluation
This manuscript presents a comprehensive and valuable study on the altitudinal distribution of ozone (O₃) and its precursors on Mount Fanjing, a remote background site on the Yunnan-Guizhou Plateau in Southwest China. The research is timely and addresses a critical knowledge gap, as high-altitude observations, particularly in this understudied region, are sparse. The experimental design is robust, incorporating a multi-platform approach with gradient observations, advanced statistical analysis (Random Forest with SHAP), chemical box modeling (OBM-MCM), and Concentration-Weighted Trajectory (CWT) model. The key findings-a positive O₃ gradient with altitude, a shift from net O₃ destruction at the foot to net production aloft, and an altitude-dependent shift in chemical regimes-are well-supported by the data and clearly presented. The study makes a significant contribution to the field of mountain atmospheric chemistry and provides actionable insights for region-specific air quality management. I recommend publication after minor revisions to address the points outlined below.
Major Strengths
1.This is the first detailed altitudinal gradient study of O₃ and its precursors in the Fanjingshan region. The data provides a crucial benchmark for understanding background pollution in Southwest China.
2.The combination of in-situ measurements, machine learning for driver attribution, and detailed chemical modeling is a powerful and modern approach that strengthens the conclusions significantly.
3.The clear demonstration of shifting O₃ regimes with altitude-from NOₓ-dominated titration at the foot to VOC-sensitive production influenced by temperature and transport aloft-is a key scientific result. The discussion of the decoupling between VOC concentration and OH reactivity (e.g., isoprene) is particularly insightful.
4.The conclusion that O₃ control strategies must be altitude-specific is well-argued and has practical implications for regional air quality planning.
Specific Comments and Suggestions for Revision
1)The manuscript mentions 57 VOCs species were measured. It would be highly beneficial to include a table in the supplement listing these species and their average concentrations at each site. This is critical for reproducibility and for readers to assess the VOCs mix.
2)The 0-D box model (OBM-MCM) is a suitable tool, but its inherent limitation in not accounting for advective transport should be explicitly stated in the methodology or discussion. Acknowledging that the calculated R_trans is a residual helps, but a sentence on the model's limitations would strengthen the manuscript.
1)Global Comparison (Figure 3): The comparison is useful for context. However, to make it more robust, please consider adding the time period (year/season) of the compared data in the figure or its caption, as O₃ levels can have temporal trends.
2)Negative RIR Values: The negative RIR for VOCs at the mountain foot is a critical finding. The explanation is correct (strong NOₓ-limited regime where VOC reduction can increase O₃), but this non-intuitive concept could be elaborated upon slightly for clarity, perhaps with a reference to the classical EKMA diagram concept.
1)While the figures are informative, some captions are very dense (e.g., Figure 2). Consider streamlining the captions and moving detailed descriptions of plot elements (e.g., the "cloud," "raindrop" components in Figure 2) to the main text or supplement.
2)Some sentences, particularly in the abstract and introduction, are very long and complex. A thorough proofread to break down overly long sentences would improve readability.
3)Check for consistency in reference formatting (e.g., journal name abbreviations, use of "et al.").
Typographical and Minor Errors
Conclusion
This is an excellent piece of work that provides a valuable dataset and insightful analysis of ozone photochemistry in a complex, high-altitude terrain. The minor revisions suggested above will further polish the manuscript and solidify its arguments. I look forward to seeing the publication.