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 (O-3) in the troposphere. In this study, 17 years of O-3 sounding data from the Hong Kong Observatory with a weekly sampling frequency are analyzed to identify stratospheric intrusions and quantify their impact on O-3 enhancement events in springtime from 2004 to 2020. 24.7% of O-3 enhancement events are related to stratospheric intrusion whereas 31.7% of intrusions lead to the enhancement of O-3 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 O-3 in the lower free troposphere as well as near the surface under different synoptic patterns. Synoptic patterns associated with stratospheric intrusion-driving O-3 enhancements are classified into two categories: Saddle Point ("S") and Cold Front ("F"). While downward transport of O-3-enriched air from the enhancement layer exerts an important impact on surface O-3 for synoptic pattern "S," the impact on surface O-3 is limited for surface pattern "F." The impact of stratospheric intrusions together with entrainment on surface O-3 may have a strong indication on the development of effective emission control strategies on surface O-3 reduction in Hong Kong.