[Characteristics and Influencing Factors of PM2.5-O3 Composite Pollution in Henan Province]

The PM2.5-O3 composite pollution in Henan Province has notable impacts on the economy, society, and daily life of the region, exhibiting a more complex nature than single pollution. Therefore, this study utilized air quality data, ground-based meteorological data, boundary layer height, CO concentration, emission inventory, and other information from 17 cities in Henan Province to investigate the spatial-temporal distribution, formation causes, and influencing factors of PM2.5-O3 composite pollution from May 2014 to December 2023. The PSCF and CWT methods were used to identify possible source areas of composite pollution. The results showed that： ① Due to the government's control of PM2.5 pollution and the impact of the epidemic, the PM2.5-O3 composite pollution events in Henan Province from 2014 to 2023 showed an overall downward trend, however, rebounded during the resumption of work and production after the epidemic in 2022. Before 2018, O3 concentration was the major factor determining the occurrence of composite pollution due to severe PM2.5 pollution. After 2018, the decrease in PM2.5 concentration increased its importance, especially the decrease in PM2.5 concentration from May to September in Henan Province, which was the primary reason for the reduction in composite pollution frequency in recent years. ② The monthly distribution of composite pollution showed a bimodal pattern, with peaks in April and October, corresponding to two periods when O3 increased while PM2.5 decreased and O3 decreased while PM2.5 increased, respectively, whereas the rainy season in July and August had a mitigating effect on composite pollution. ③ The severity of composite pollution in Henan Province was in the order of north > central > south and the topography also had a certain impact on the severity of composite pollution. ④ The meteorological conditions during the occurrence of composite pollution were moderate temperature, specific humidity, and CO concentration between PM2.5 pollution and O3 pollution, with low wind speed and low boundary layer height. ⑤ During the period of compound pollution in Henan Province, in addition to the transportation of pollutants from cities inside the province, especially the eastern and northern parts of the province, the eastern area （Anhui Province） and northeastern area （Shandong Province） to Henan Province were the major source areas of pollutants. Emissions from the source area, combined with emissions from cities inside Henan Province, were important causes for the PM2.5-O3 pollution during this period.