While the elevated ambient levels of particulate matters with aerodynamic diameter of 2.5 µm or less (PM_2.5) are alleviated largely with the implementation of effective emission control measures, an opposite trend with a rapid increase has been seen in surface ozone (O₃) in the North China Plain (NCP) region over the past several years. It is critical to determine the real culprit causing such a large increase in surface O₃. In this study, 7-year surface observations and satellite retrieval data are analyzed to determine the long-term change in surface O₃ as well as driving factors. Results indicate that anthropogenic emission control strategies and changes in aerosol concentrations as well as aerosol optical properties such as single-scattering albedo (SSA) are the most important factors driving such a large increase in surface O₃. Numerical simulations with the National Center for Atmospheric Research (NCAR) Master Mechanism (MM) model suggest that reduction of O₃ precursor emissions and aerosol radiative effect accounted for 45 % and 23 % of the total change in surface O₃ in summertime during 2013–2019, respectively. Planetary boundary layer (PBL) height with an increase of 0.21 km and surface air temperature with an increase of 2.1 ^∘C contributed 18 % and 12 % to the total change in surface O₃, respectively. The combined effect of these factors was responsible for the rest of the change. Decrease in SSA or strengthened absorption property of aerosols may offset the impact of aerosol optical depth (AOD) reduction on surface O₃ substantially. While the MM model enables quantification of an individual factor's percentage contributions, it requires further refinement with aerosol chemistry included in the future investigation. The study indicates an important role of aerosol radiative effect in development of more effective emission control strategies on reduction of ambient levels of O₃ as well as alleviation of national air quality standard exceedance events.

Stratospheric intrusions are an important source of ozone (O₃) in the troposphere. In this study, 17 years of O₃ sounding data from the Hong Kong Observatory with a weekly sampling frequency are analyzed to identify stratospheric intrusions and quantify their impact on O₃ enhancement events in springtime from 2004 to 2020. 24.7% of O₃ enhancement events are related to stratospheric intrusion whereas 31.7% of intrusions lead to the enhancement of O₃ in the lower troposphere. Occurrences of stratospheric intrusion in springtime are closely related to tropopause folding and tropospheric convective activities in subtropical regions. The Weather Research and Forecasting model coupled with Chemistry simulations are conducted with the integrated process analysis to quantify the impact of stratospheric intrusions on the enhancement of O₃ in the lower free troposphere as well as near the surface under different synoptic patterns. Synoptic patterns associated with stratospheric intrusion‐driving O₃ enhancements are classified into two categories: Saddle Point (“S”) and Cold Front (“F”). While downward transport of O₃‐enriched air from the enhancement layer exerts an important impact on surface O₃ for synoptic pattern “S,” the impact on surface O₃ is limited for surface pattern “F.” The impact of stratospheric intrusions together with entrainment on surface O₃ may have a strong indication on the development of effective emission control strategies on surface O₃ reduction in Hong Kong.