Ozone and its precursors at an urban site in the Yangtze River Delta since clean air action plan phase II in China

In response to regional ozone (O₃) pollution, Chinese government has implemented air pollution control measures in recent years. Here, a case study was performed at an O₃-polluted city, Wuhu, in Yangtze River Delta region of China to investigate O₃ variation trend and the relationship to its precursors after implementation of Clean Air Action Plan Phase II, which aims to reduce O₃ pollution. The results showed that peak O₃ concentration was effectively reduced since Clean Air Action Plan Phase II. Due to significant NOx reduction, O₃ formation tended to shift from volatile organic compound (VOC)-limited regimes to NOx-limited regimes during 2018-2022. VOC/NOx ratios measured in 2022 revealed that peak O₃ concentration tended to respond positively to NOx. Apart from high-O₃ period, Wuhu was still in a VOC-limited regime. The relationship of maximum daily 8-h ozone average and NO₂ followed a lognormal distribution with an inflection point at 20 μg m^-3 of NO₂, suggesting that Wuhu should conduct joint control of VOC and NOx with a focus on VOC reduction before the inflection point. Alkenes and aromatics were suggested to be preferentially controlled due to their higher ozone formation potentials. Using random forest meteorological normalization method, meteorology had a positive effect on O₃ concentration in 2018, 2019 and 2022, but a negative effect in 2020 and 2021. The meteorology could explain 44.0 ± 19.1% of the O₃ variation during 2018-2022. High temperature favors O₃ production and O₃ pollution occurred more easily when temperature was over 25 °C, while high relative humidity inhibits O₃ generation and no O₃ pollution was found at relative humidity above 70%. This study unveils some new insights into the trend of urban O₃ pollution in Yangtze River Delta region since Clean Air Action Plan Phase II and the findings provide important references for formulating control strategies against O₃ pollution.