Drivers of ozone episodes during clean and polluted days in eastern China: Insights into precursor characterization,...

Surface ozone (O₃) concentrations in China have either remained high or increased, with pronounced rebound effects and significant regional transport. To investigate O₃ formation mechanisms and their drivers during pollution episodes in eastern China, multi-source observational data were integrated in this study to systematically analyze the pollution characteristics, chemical reactivity, source contributions, regional transport, and associated health risks of O₃ and its precursors from May to September 2022. O₃ exceedance days in Dongying, Zibo, and Heze accounted for 32.7%, 37.3%, and 44.4% of the total observation days, respectively, and were mainly associated with high temperature, low humidity, and elevated planetary boundary layer height (PBLH). Compared to clean days, volatile organic compounds (VOCs) concentrations that serve as O₃ precursors were considerably higher on polluted days, showing clear differences in species composition between cities. Enhanced atmospheric photochemical processes on polluted days were driven by reactive VOCs across cities, as consistently indicated by OFP, OH reactivity, and RIR analyses, with highly reactive alkenes and oxygenated VOCs (OVOCs) playing key roles in O₃ formation and atmospheric oxidation. Positive matrix factorization (PMF) analysis showed the dominance of highly reactive emission sources (e.g., industrial source, oil/gas evaporation, and vehicular exhaust) during polluted days, while source contributions exhibited pronounced city-specific characteristics. Health risk assessments showed that non-carcinogenic risks were significantly elevated on polluted days, primarily driven by acrolein, whereas carcinogenic risks were mainly influenced by highly carcinogenic VOC components and industrial-related sources. Both risk types displayed age-dependent patterns but differed in their responses to pollution conditions. Furthermore, near-surface O₃ diurnal variations across three representative cities reflected the combined effects of local photochemical production and regional transport. Integrating PSCF and CWT revealed that potential O₃ source regions were dispersed with weak contributions under clean conditions, but became concentrated with enhanced contributions near receptor cities and along major transport pathways during pollution episodes. Collectively, these findings provide a robust scientific basis for developing precise and region-specific O₃ mitigation strategies from multiple perspectives.