A significant mechanism of stratospheric O₃ intrusion to atmospheric environment: a case study of North China Plain

Abstract. Stratosphere-to-troposphere transport results in the stratospheric intrusion (SI) of O₃ into the free troposphere through the tropopause folding. However, the mechanism of SI influencing the atmospheric environment with the cross-layer transport of O₃ from the stratosphere, free troposphere to the atmospheric boundary layer has not been elucidated thoroughly. In this study, a SI event over the North China Plain (NCP) was taken to investigate the mechanism of the cross-layer transport of stratospheric O₃ with the impact on the near-surface O₃ based on the multi-source reanalysis and observation data and air quality modeling. The results revealed a significant mechanism of stratospheric O₃ intrusion to the atmospheric environment induced by an extratropical cyclone system. The SI with downward transport of stratospheric O₃ to near-surface layer was driven by the extratropical cyclone system with vertical coupling of "upper westerly trough-middle the Northeast Cold Vortex (NECV)-lower extratropical cyclone" in the troposphere. The deep trough in the westerly jet aroused the tropopause folding, and the lower stratospheric O₃ penetrated the folded tropopause into the upper and middle troposphere; the westerly trough was cut off to form a typical cold vortex in the upper and middle troposphere. The compensating downdrafts of the NECV pushed the further downward transport of stratospheric O₃ in the free troposphere; The NECV activated an extratropical cyclone in the lower troposphere, and the vertical cyclonic circulation governed the stratospheric O₃ from the free troposphere across the boundary layer top invading the near-surface atmosphere. In this SI event, the averaged contribution of stratospheric O₃ to near-surface O₃ was accounted for 26.77 %. The proposed meteorological mechanism of vertical transport of stratospheric O₃ into the near-surface atmosphere driven by an extratropical cyclone system could improve the understanding of the influence of stratospheric O₃ on atmospheric environment with implications for the coordinated control of atmospheric pollution.