Detection of stratospheric intrusion events and their role in ozone enhancement at a mountain background site in sub-tropical East Asia

In this study, we investigated the influences of stratospheric intrusion (SI) on the tropospheric ozone (O₃) at Lulin Atmospheric Background Station (LABS, 2862 m MSL), a representative high-altitude site in East Asia. Thresholds for surface O₃, carbon monoxide (CO), and relative humidity (RH) were set to identify SI events at LABS. Accordingly, 3.0% of the observation period was impacted by SI leading to a net O₃ enhancement of 14.6 ± 9.6 ppb at LABS. Factors influencing the two most common SI event types affecting LABS, i.e. tropopause folds (major) and tropical cyclones (minor), were discussed. We utilized ERA5 reanalysis products (meteorology and O₃) to trace and analyze the pathway and mechanism of these SI types over sub-tropical East Asia. With a conceptual model, we illustrate two driving circulations for SI air at ∼200 hPa that affect subtropical East Asia. SI air generated due to Rossby wave breaks are transported equatorward and downward into the troposphere. The southern westerlies passing south of the Tibetan Plateau can further push the SI air eastward. A SI event in 2007 with a significant O₃ enhancement of 43.4 ppb caused by typhoon Nari was detected and discussed to depict the second SI type.