Vertical distribution of tropospheric ozone and its sources of precursors over Beijing: Results from ∼ 20 years of ozonesonde measurements based on clustering analysis

This study comprehensively analyzed nearly 20 years of ozonesonde data from Beijing on the North China Plain collected between 2001 and 2019 to determine the vertical structure of tropospheric ozone as well as its sources of precursors through a combination of radiosonde data, satellite products, and ERA5 reanalysis data. Quantitative analysis involved the self-organizing map (SOM) algorithm, calculating backward trajectories, and applying the Potential Source Contribution Function (PSCF) algorithm. The SOM clustering results (3 × 3 classes) showed that the ozone profiles over Beijing were primarily affected by seasonal variability, and the vertical ozone structure and its precursors exhibited obvious seasonal patterns (high VOCs in warm seasons and high NOx in cold seasons). The tropospheric ozone concentrations in Beijing, which were influenced by both ozone precursors and seasonal meteorological factors, were high in warm seasons and low in cold seasons. Further detailed investigations of SOM clustering results found that biomass burning, pollutant transport and stratospheric intrusion contributed significantly to tropospheric ozone over Beijing. Ozone in spring and summer was influenced by biomass burning activities south of Beijing, resulting in an increase in tropospheric ozone concentrations (at 0–10 km) of 60.6 ppbv. Biomass burning activities on the North China Plain have been adequately controlled since the implementation of the Clean Air Action Plan in 2013, which greatly reduced the frequency of ultrahigh tropospheric ozone concentrations in Beijing during the spring and summer. Stratospheric intrusions, which are closely associated with ozone transport from high latitudes (including the influence of Brewer-Dobson circulation subsidence), occur more frequently during spring and winter and increase ozone concentrations in the middle and upper troposphere in Beijing.