Ozone Pollution in a Heavy-Industrial City with Complex Terrain: VOC Reactivity, Source Apportionment, and Meteorological Drivers
Surface ozone (O3) pollution has become an increasingly important constraint on further improvements in urban air quality, particularly in industrial cities where complex terrain, local emissions, and meteorological conditions interact. In this study, hourly air pollutants, meteorological parameters, and high-time-resolution volatile organic compound (VOC) observations collected at a single urban-core site during September from 2021 to 2024 were used to investigate O3 pollution characteristics, VOC reactivity, source contributions, and driving mechanisms in a resource-based heavy-industrial city in northwestern Henan Province, China. Ozone formation potential (OFP), diagnostic ratios, positive matrix factorization (PMF), meteorological normalization, and extreme gradient boosting combined with Shapley additive explanations (XGBoost-SHAP) were integrated to identify key reactive species, major sources, and meteorological–precursor interactions. The mean maximum daily 8 h average O3 concentrations were 113.44, 150.32, 123.59, and 154.18 μg·m−3 from 2021 to 2024, respectively, with the highest level observed in 2024 despite the lowest nitrogen dioxide (NO2) and carbon monoxide (CO) concentrations. O3 was positively correlated with temperature and negatively correlated with relative humidity, indicating the importance of hot and relatively dry conditions. Total VOC OFP first increased and then declined, with alkenes dominating in 2021 and aromatics exceeding alkenes after 2022. Ethene, m/p-xylene, toluene, and vinyl chloride were identified as priority reactive species. PMF results showed that mixed industrial processes and vehicle exhaust were the dominant VOC sources, contributing 32.3% and 23.8%, respectively. Under the original meteorological-normalization specification, represented meteorological features accounted for 64.7% of the modeled O3 increase during the study period. Sensitivity specifications retained meteorological dominance but showed that the exact share was model dependent. SHAP analysis further identified temperature, short-term temperature variation, relative humidity, alkenes, and NO2 as key drivers. These results suggest that O3 pollution in this heavy-industrial city is jointly shaped by favorable meteorological conditions, reactive VOCs, nitrogen oxides (NOx) chemistry, and combined industrial and traffic emissions. Accordingly, industrial processes, vehicle exhaust, and highly reactive VOC species are likely priority targets for mitigation, while the effectiveness of coordinated VOC–NOx control still warrants further regime-specific evaluation.