Fine particulate matter (PM2.5) and tropospheric ozone (O-3) are currently the two air pollutants of the greatest concern in China, affecting air quality and human health. Since 2013, Beijing has implemented multiple measures such as the "Air Pollution Prevention and Control Action Plan" and the "Three-Year Action Plan for Winning the Blue Sky Defense War", which have significantly reduced primary emissions and improved air quality. This study focuses on the two major pollutants: PM2.5 and O-3, which are tightly related to the secondary formation chemistry in ambient air. The statistical data on the number of days during the period from 2013 to 2020 on which the concentration of PM2.5 and O-3 in Beijing exceeds the standard were obtained from the China National Environmental Monitoring Center, as well as the concentration data of PM2.5 and O-3 from 2015 to 2020. The pollutant concentrations and meteorological parameters of the typical pollution process are the observation results of the typical urban site "PKUERS" located on top of the Science Building No.1 of Peking University campus in Haidian District, Beijing. In addition, the characteristics of the annual evolution of PM2.5 and O-3 pollution, as well as the "generation-development-elimination" law of the typical pollution processes are revealed. Studies have shown that the frequency, duration, and peak concentration of the PM2.5 pollution process are all decreasing year by year. Compared with 155 d of PM2.5 exceeding the standard in 2013, there are only 35 d of PM2.5 exceeding the standard in 2020 (a decrease of 77%), indicating that Beijing's atmospheric PM2.5 control has made remarkable progress. However, in the months of October, November, January and February of 2020, there are still 6-8 d of PM2.5 exceedance every month, or 1.5-2 d of exceedance per week, which implies that PM2.5 pollution always exists in the cold season. In contrast, the trend of interannual change of O-3 pollution process is not evident, about 1/3 of the time from May to July of each year is exposed to O-3 pollution. In 2018, the number of days when O-3 exceeded the standard surpassed PM2.5 for the first time, suggesting that O-3 may gradually replace PM2.5 as the most important pollutant in Beijing. Moreover, this paper summarizes three PM2.5 pollution processes, three O-3 pollution processes, and three PM2.5 and O-3 dual pollution processes. It is shown that high NOx concentrations are often associated with the occurrence of PM2.5 and O-3. PM2.5 episodes mainly occur in winter, and a pollution process lasts for a long time, the change of PM2.5 concentration shows a peak or a step pattern; while O-3 episodes mainly occur in summer, the O-3 concentration shows an obvious daily variation, high concentration during the day and low concentration at night. When O-3 pollution lasts for a long time, it can turn into a double episode of O-3 and PM2.5 pollution. In this case, the concentration of both pollutants may show an alternating peakand-valley phenomenon, and this is consistent with the daily variation in temperature and relative humidity. In terms of dominant PM2.5 components, organic matter (including primary and secondary organic matter) accounted for the highest proportion of the three types of pollution processes, followed by nitrate and sulfate. The proportion of nitrate has increased over the years, while the proportion of sulfate has declined. In summary, this study could provide a theoretical basis for relevant departments to clarify and coordinate the control of PM2.5 and O-3, and further improve the air quality based on the substantial progress made in air pollution prevention and control at the current stage.
