Identifying stratospheric ozone intrusion events in Nanjing, China

Abstract Stratospheric ozone (O3) intrusion events (SIE) have been recognized as significant contributors to ground-level O3 pollution. Nanjing, characterized by complex terrain and strong synoptic variability, underwent significant atmospheric changes during 2013-2020 following the national Air Pollution Prevention and Control Action Plan (2013-2017), making it an ideal case to investigate background ozone processes. This study presents a comprehensive approach to identify and analyze SIE events in Nanjing during this critical transitional period, integrating meteorological parameters, PV metrics, and historical O3 data, combined with ERA5 reanalysis and ground-based observations. Our analysis identified several significant SIE events, particularly the notable case on May 2-3, 2020, during which PV anomalies exceeded 4.2 PV units and O3 mixing ratios peaked at 65 ppb, resulting in maximum daily 8-hour average (MDA8) O3 concentrations reaching 160 µg/m3. This event was also characterized by strong inverse correlations between O3 and carbon monoxide (CO) concentrations and relative humidity (RH), further confirming the influence of stratospheric intrusion. A retrospective analysis of 2013-2019 data revealed 45 suspected SIE events in Nanjing, with 67% occurring in spring (March-May) and autumn (September-November), particularly in the years of 2017 and 2019, when enhanced stratosphere-troposphere exchange was observed. In 2017, 12 events were recorded, with PV anomalies exceeding 6 PV units and MDA8 O3 concentrations reaching 223 µg/m3. Seasonal variation was evident, with more frequent events in spring and early summer due to dynamic processes like tropopause folding, and fewer events in autumn under more stable atmospheric conditions. This method offers a practical framework for real-time monitoring and early warning of ozone intrusions and can be extended to other regions with suitable observational infrastructure.