Transport Pathways of Ozone in Beijing–Tianjin–Hebei Based on Causal Networks

Abstract

In recent years, the Beijing–Tianjin–Hebei (BTH) region has experienced severe ozone (O₃) pollution, which constrains further air quality improvement. Identifying O₃ transport pathways is essential for regional joint prevention and control. This study constructed O₃ transport networks for multi-year, summer, and winter periods using complex network methods, enabling assessment of both direction and intensity at the station level. Reliability was confirmed through backward trajectory cluster analysis, potential source contribution analysis, and a nested air quality prediction model. Results show that O₃ transport in BTH is mainly driven by distinct regional patterns. The network exhibited a graph density of 0.62, an average path length of 1.384, and an average clustering coefficient of 0.627, indicating highly interconnected transport, especially in central and southern areas. Southern stations predominantly showed O₃ output, while northern ones were dominated by input: Beijing acted as an input city, whereas Handan and Xingtai were output (exporting) cities. High-value input–output stations were also identified in Shijiazhuang and Tianjin, where both in- and out-weighted degrees exceeded 25. Seasonally, regional transport capacity was stronger in winter than in summer. Dense linkages were observed among southern cities. In summer, O₃ in Shijiazhuang and Baoding was mainly influenced by Handan and Xingtai, while in winter, Hengshui and Cangzhou were primarily connected to surrounding cities. Verification analysis showed transport contributions of 58.64% in summer and 53.85% in winter, confirming interregional transport as a major source of O₃ pollution. Shijiazhuang was most affected by Handan and Xingtai in summer (43.72%) and by Hengshui and Cangzhou in winter (26.96%), along with long-distance inputs from the northwest. These findings align with the identified transport pathways. Additionally, potential O₃ sources were more widely distributed in winter than in summer.