[Evolution Characteristics of PM_2.5 and O₃ and Their Synergistic Effects on Atmospheric Compound Pollution in Tangshan].

The concentrations of atmospheric pollutants PM_2.5, O₃, SO₂, NO₂, and CO together with the meteorological factors of temperature （<i>T</i>）, relative humidity （RH）, wind speed, and other relevant data in Tangshan from 2015 to 2021 were collected to study the variation characteristics of PM_2.5 and O₃ at different periods in Tangshan City in the past seven years and their influencing factors, to discuss the contributions of air mass transport to PM_2.5 and O₃ pollution, and to reveal the synergistic influence mechanism of PM_2.5 and O₃ on atmospheric compound pollution by using correlation analysis and backward trajectory cluster analysis techniques. The results showed that PM_2.5 concentrations in Tangshan decreased year by year from 2015 to 2021, whereas O₃ concentration showed a unimodal trend, with the peak appearing in 2017. Both PM_2.5 and O₃ concentrations showed obvious seasonal variation trends； PM_2.5 was characterized by the highest concentration in winter and the lowest concentration in summer, whereas O₃ was characterized by the highest concentration in summer and the lowest concentration in winter. In addition, the diurnal variation in PM_2.5 showed a bimodal distribution, with the peak occurring during the morning and evening on weekdays, and O₃ showed a unimodal distribution, with the peak value appearing during the period with strong ultraviolet radiation in the afternoon. PM_2.5 had a significant positive correlation with SO₂, NO₂, and CO, whereas O₃ had a significant positive correlation with radiation and temperature. Under the different pollution conditions, PM_2.5 and O₃ were affected by air mass transports from different directions. Being impacted by various factors, the synergistic effect of PM_2.5 and O₃ on atmospheric compound pollution showed an obvious negative effect in winter, whereas there was an obvious positive effect in spring, summer, and autumn. Under the backgrounds of different pollutions, when the concentration of PM_2.5 exceeded 150 μg·m^-3, the synergistic effect of PM_2.5 and O₃ showed an obvious negative effect.