[Comparative Analysis of Ambient Ozone and Its Background Concentration in Beijing in Recent Years]

To understand the trends of O₃ pollution changes and the contribution of regional background in Beijing in recent years, this study investigated the temporal variations of O₃ mixing ratios and the exceedance of O₃ pollution at different types of sites, such as urban, suburban, clean, regional, and transportation sites, based on the online monitoring data from 34 stations in Beijing from 2018 to 2022. The regional background estimation method proposed by the Texas Commission on Environmental Quality （TCEQ） was applied to calculate the contributions of regional background and local generation to O₃. The results showed that the annual average value of O₃ mixing ratios in Beijing was the highest in 2019, the lowest in 2021, and rebounded in 2022, from 2018 to 2022. Except for transportation sites, O₃ mixing ratios in Beijing and all other types of sites exhibited decreasing trends. The highest monthly average value of O₃ occurred in June and the monthly average values of clean sites were often higher than those of other sites. The proportions of O₃ exceedance days and heavily polluted days varied over the last five years and days with high O₃ pollution more often occurred at the clean site. The O₃ exceedance days were mostly dominated by light pollution and the average O₃ mixing ratios on heavily polluted days in 2020-2022 were greater than those in 2019. To exclude the influence of NO on the "titration" effect of O₃, the TCEQ method was estimated by using the total oxidant O （O=O₃+NO₂） instead of O₃. The results showed that both the regional background and the local generation in Beijing showed decreasing trends and the contribution of the regional background was significantly higher than that of the local generation. However, the contribution of the regional background decreased in the summer due to the enhancement of the photochemical activities. In general, although the O₃ mixing ratios in Beijing had been decreasing from 2018 to 2022, no such trend was observed in the O₃ mixing ratios on heavily polluted days. To achieve the goal of basically eliminating heavily polluted days in the 14th Five-Year Plan and to effectively control O₃ pollution, it is recommended that Beijing should further optimize the joint prevention and control of atmosphere pollution in Beijing-Tianjin-Hebei and neighboring regions to reduce the contribution of regional background to O₃ pollution while strengthening the control of local O₃ precursor emissions.