Regional collaboration to simultaneously mitigate PM_2.5 and O₃ pollution in Beijing-Tianjin-Hebei and the surrounding...

Beijing-Tianjin-Hebei and the surrounding area (BTHSA) shows the poorest air quality in China, reflected in sub-standard PM_2.5 and increasingly pronounced O₃ pollution, stressing the urgency for regional cooperation and collaborative control of PM_2.5 and O₃. With the aim to explore the cooperative regions and response mechanisms of PM_2.5 and O₃ in BTHSA, this study applied multiple mathematical models and analytical indicators to multiple data sources, including applying self-organizing map (SOM), response surface model (RSM), random forest (RF), distributed lag nonlinear models (DLNMs), and meta-analysis, on ground observations of air quality and meteorology, ozone monitoring instrument (OMI) observations, and air pollutant emission inventory. The results revealed that BTHSA exhibited clear regional characteristics of air pollution and can be divided into four clusters for enhanced intercity cooperation. Over 2015-2020, anthropogenic factors played more important roles than meteorological ones on the alleviation of PM_2.5 and the deterioration of O₃. RSM based on observations and RF based on emissions both suggested that, in the near future, strengthened abatement of SO₂, PM_2.5 and VOC can be beneficial for controlling PM_2.5 and O₃ pollution, while intensive NOx reduction in PM_2.5-dominant months and mitigatory NOx reduction in O₃-dominant months should be formulated before certifying an obvious transition of O₃-NOx-VOC sensitivity. This study, with multi-model and multi-data fusion, can be expected to provide synthesized fact- and science-based guidance for the next-stage collaborative control of PM_2.5 and O₃ in BTHSA.