Exploring the causes for co-pollution of O3 and PM2.5 in summer over North China

Co-pollution of surface O-3 and PM2.5 has become the most predominant type of air pollutions in Beijing-Tianjin-Hebei region in the hot season since 2017, particularly in May-July. Analysis based on observational data showed that co-pollution was always accompanied by high temperature, moderate relative humidity, extremely high SO2, and higher NO2. We also found that the meteorology and precursor dependence of O-3 was similar between co-pollution and O-3- single pollution. While PM2.5 in co-pollution was more related to temperature, relative humidity, and precursors, that in PM2.5-singe pollution were more related to small winds. These results indicate that co-pollution seemed to be more affected by atmospheric chemistry. According to the PM2.5 components, secondary inorganic aerosols (SIA) composed 44.3-48.7% of PM2.5 in co-pollution, while those accounting for 42.1-46.5% and 41.2 +1.3%, respectively, in O-3- and PM2.5-single pollution, which further confirmed the relatively stronger atmospheric chemistry processes in co-pollution. And the high proportion of SIA in co-pollution was mainly attributed to SO42-, which was observed to rapidly boom in non-refractory submicron aerosol (NR-PM1) on the condition of high level of O-3 at daytime. Additionally, we further explored the interactions of O-3 and PM2.5 in co-pollution. It was found that most (similar to 61.9%) co-pollution episodes were initiated by high O-3 at daytime; while for other episodes, high PM2.5 firstly occurred under the more stable meteorological conditions, and then accumulation of precursors further induced high O-3. A higher SIA concentration was observed in O-3-initiated co-pollution, indicating that the atmospheric oxidation in co-pollution caused by chemical processes was stronger than that by physical processes, which was further approved by the higher values of SOR and NOR in O-3-initiated co-pollution. This observational study revealed that controlling O-3 and precursor SO2 is the key to abating co-pollution in the hot season.