Stratospheric intrusion may aggravate widespread ozone pollution through both vertical and horizontal advections in eastern China during summer

Stratospheric intrusion (SI) is an important source of tropospheric ozone (O-3). Here, we used the online coupled Weather Research and Forecasting-Chemistry (WRF-Chem) model to simulate a typical SI event that occurred over eastern China on 15-19 July 2016 and investigate the impacts of SI on near-surface O-3 pollution. The results show that the large-scale circulation of SI was characterized by a deep trough over central China and South Asia high and Western Pacific Subtropical high located to the east and west of the deep trough, respectively. With the evolution of the deep trough, the strong downdrafts behind the trough lead to O-3-rich air injected into the lower troposphere across eastern China. By using a tracer tagging method in WRF-Chem, we quantified the SI contributed up to 6.5 ppb to the surface O-3 concentration. According to the integrated process rate analyses, which were employed to quantify the contribution of different physical/chemical processes to O-3, the advection process dominated variations in troposphere O-3 with positive contribution ranging from 0.1 to 13.8 ppb h(-1). As altitude decreases, the contribution of advection diminishes as the intensity of the SI gradually weakens. Although the vertical advection contributed limited O-3 that directly reached the ground, we revealed that the SI has significant impact on near-surface O-3 over a large territory of eastern China through regional transport by horizontal advection process. Below 691 m, the positive contribution of horizontal advection to O-3 ranges from 1.6 to 13.4 ppb h(-1). This study highlights the natural effect of SI on summertime O-3 pollution in eastern China and will help the development of a future O-3 pollution alert system.