[Evolutionary Characteristics of Nocturnal PM2.5 and O3 Mass Concentrations in China and Their Synergistic Effects on Nighttime Atmospheric Compound Pollution].

This study systematically analyzed the evolutionary characteristics of nighttime PM_2.5 and O₃ mass concentrations in China and their synergistic effects on nighttime atmospheric compound pollution using hourly PM_2.5 and O₃ monitoring data from the China Environmental Monitoring General Station from 2015 to 2023. The study obtained the following results： ① The annual mean PM_2.5 mass concentration at night decreased by 2.7 μg·（m³·a）^-1 during the study period， while the annual mean O₃ mass concentration increased by 0.87 μg·（m³·a）^-1. ② The mass concentrations of PM_2.5 and O₃ between nighttime and daytime exhibited a synergistic pattern of change. The correlation coefficient between nighttime and daytime PM_2.5 mass concentrations was 0.92； the correlation coefficient between nighttime and daytime O₃ mass concentrations was 0.68. ③ Spatial statistical analyses revealed significant decreases in nighttime PM_2.5 mass concentration in 98.0% of the cities， especially in the Beijing-Tianjin-Hebei urban agglomeration and its neighboring cities， while 65.6% of the cities， mainly in the central-eastern and southern regions， showed a significant increasing trend in nighttime O₃ mass concentration. ④ The number of cities with nighttime PM_2.5-O₃ compound pollution showed a pattern of first increasing （2015-2017）， then decreasing （2017-2020）， and finally increasing continuously （2020-2023）. Additionally， the spatial distribution of cities exhibited a shift from dispersion （2015-2017） to agglomeration （2017-2023）， with further expansion observed in central and southern China. ⑤ The spatial synergistic variations of PM_2.5 and O₃ demonstrated that the two exhibit significant positive and negative correlations in diverse regions of the country at night， indicating the complexity of the compound pollution. The findings of this study provide a scientific foundation and crucial point of reference for the management of nighttime air pollution in China， particularly with regard to the control of nighttime O₃ pollution.