Impact of the East Asian Stratospheric Intrusion on Lower Tropospheric Ozone in the Yangtze River Delta

Abstract

Tropospheric ozone (O₃) pollution has aroused increasing attention in past decades, especially in China, with escalating near-surface O₃ levels. Apart from photochemical reactions, stratospheric intrusion (SI) also contributes to tropospheric O₃ pollution. In this study, a strong SI event that greatly influenced the near-surface O₃ pollution was identified in May 2023. To quantitatively analyze the contribution to surface O₃ over the Yangtze River Delta (YRD), meteorological reanalysis data, in situ observations, and a regional meteorology-chemistry coupled model were integrated. Our findings reveal that the severe O₃ pollution observed over the YRD region cannot be solely attributed to photochemical processes. Noteworthily, a distinct signal of stratospheric air masses injecting into the troposphere was observed, indicating a more complex interplay between atmospheric chemical and physical processes. The clustering analysis of the backward trajectories shows that the O₃-rich air masses injected into the lower troposphere are primarily driven by westerly jets and downwelling behind the troughs accompanying the low-pressure weather system at 46°N–60°N. The stratospheric O₃-rich air masses can be transported to the YRD region driven by the strong downwelling occurring with a high-altitude wind field toward the south. Using the regional meteorology-chemistry model Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) with real-time input of the upper chemical boundary conditions, it is estimated that such regionally transported SI O₃ contributed more than 12 ppb to surface O₃ pollution over the YRD region.