Exploring the link between ozone pollution and stratospheric intrusion under the influence of tropical cyclone Ampil.

Tropical cyclones (TCs) are synoptic-scale systems with a diameter of up to 2000 km, and may persist for several days to weeks. They can significantly affect the atmospheric conditions and ozone (O) concentrations in coastal areas. They also induce stratospheric intrusion (SI, the injection of stratospheric O into the troposphere) by changing the height of the tropopause. Focusing on the Yangtze River Delta (YRD) region, a highly urbanized coastal area with severe O pollution, we systematically analyze the characteristics of O pollution and SIs as well as their connections under the influence of tropical cyclone Ampil. Based on surface observations, 120-h backward trajectories and ERA5 reanalysis meteorological data, the urban O pollution affected by tropical cyclone Ampil mainly resulted from active photochemical reactions inside the boundary layer and poor diffusion conditions characterized by weak winds. SIs induced by tropical cyclone Ampil were important for the upper and middle troposphere, where O concentration could increase up to 180 ppbv. But they hardly reached the ground over the YRD. Therefore, there was no direct connection between O pollution and SIs. However, the location of SIs can predict urban O pollution. SIs moved with tropical cyclone Ampil but appeared on the northwest side of Ampil, usually 500-1000 km away from tropical cyclone Ampil center. At 200 hPa, they corresponded to a high-PV (>2 PVU) air mass rich in O (>200 ppbv). Below this stratospheric PV signature, urban O concentration was usually high.