Impact of Stratospheric Intrusions on Ozone Enhancement in the Lower Troposphere and Implication to Air Quality in Hong Kong and Other South China Regions.

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. Key Points: Stratospheric intrusion is an important factor to the enhancement of ozone (O3) in the lower troposphere observed over Hong Kong during springtimeOccurrence of stratospheric intrusion in the subtropical region is attributed to the large‐scale subsidence associated with the upper‐level subtropical jetThe impact of stratospheric intrusion can be extended to the atmospheric boundary layer, which may lead to the increase of surface O3 [ABSTRACT FROM AUTHOR]