Spatiotemporal characteristics and trends of combined PM2.5 and O₃ pollution across the central plains: a decadal analysis of Henan, China (2016–2025)

The co-occurrence of fine particulate matter (PM 2.5 ) and surface ozone (O₃) pos es an environmental challenge to high-quality development in China. Henan Province, a major agricultural and industrial center in the Central Plains, faces particular difficulties in coordinating PM 2.5 and O₃ control because of its industrial structure, energy mix and geography. Using hourly ground-station air quality and meteorological datasets spanning 2016–2025, we applied statistical and spatial analyses to characterize the spatiotemporal evolution of PM 2.5 and the 90th percentile of the daily maximum 8-h average O₃ concentration (MDA8-O₃), and their associations with meteorological factors and pollutant precursors. PM 2.5 concentrations decreased by 46.6% to 39 μg/m 3 , while MDA8-O₃ increased from 63 μg/m 3 to 78 μg/m 3 , revealing a pronounced inverse inter-annual covariation (the ‘seesaw effect’) Spatially, PM 2.5 concentrations were higher in northern than southern Henan, whereas MDA8-O₃ concentrations were higher in northern and western Henan and lower in the south. Long-term PM 2.5 –MDA8-O₃ background-coupling categories (HH, HL, LH and LL) evolved through three distinct phases. Associations between pollution levels and meteorological variables or precursor pollutants also varied by season and region. These findings support targeted, coordinated PM 2.5 and MDA8-O₃ mitigation across the Central Plains and North China Plain.