Abstract: In order to explore the characteristics of PM_2.5 and ozone (O₃) pollution in Nanjing, and reveal the evolution characteristics of ambient air pollutants and 10nm-10μm aerosol size distribution during the PM_2.5 and O₃ combined air pollution, the hourly concentration data of ambient air pollutants was analyzed in Nanjing from 2015 to 202, and a detailed analysis of typical air combined pollution case from October 12 to 17, 2015. The results showed that the variation characteristics of different ambient air pollutants were different in Nanjing from 2015 to 2021. The concentrations of PM_2.5, PM₁₀ and SO₂ were significantly decreased, with an average annual decrease rate of 8.9%/a, 6.2%/a and 15.4%/a, respectively. NO₂ showed an increasing trend from 2015 to 2019. After reaching its peak in 2016, CO began to enter a decline trend, with an average annual decline rate of 7.6%/a. O₃ concentration variation was small from 2015 to 2021. The air pollution situation had undergone great changes in Nanjing, and the air pollution dominated by PM_2.5 pollution had transformed into the O₃ pollution. The frequency and intensity of PM_2.5 pollution decreased significantly, while the frequency and intensity of O₃ pollution did not change significantly. The number concentration spectra of aerosols in different stages of the air combined pollution were bimodal, but the peak particle size and concentration were different. The peak concentration of size distribution was the lowest in the PM_2.5 pollution stage, the peak of particle size distribution in the first peak shifted to the small particle size segment in NO₂ pollution stage, and the first peak particle size shifted to the large particle size segment in the PM_2.5 and O₃ compound pollution stage. In the O₃ pollution stage, the peak particle size of the number concentration spectra did not change, but the peak concentration increased significantly, especially the first peak concentration, which was 3-5 times than before the pollution. Under different pollution stages, the diurnal variations of the different ambient air pollutants and aerosol number concentration of different mode exhibited great variability.