A mechanism of stratospheric O.sub.3 intrusion into the atmospheric environment: a case study of the North China Plain

Stratosphere-to-troposphere transport results in the stratospheric intrusion (SI) of O.sub.3 into the free troposphere through the folding of the tropopause. However, the mechanism of SI that influences the atmospheric environment through the cross-layer transport of O.sub.3 from the stratosphere and free troposphere to the atmospheric boundary layer has not been elucidated thoroughly. In this study, an SI event over the North China Plain (NCP; 33-40° N, 114-121° E) during 19-20 May 2019 was chosen to investigate the mechanism of the cross-layer transport of stratospheric O.sub.3 and its impact on the near-surface O.sub.3 based on multi-source reanalysis, observation data, and air quality modeling. The results revealed a mechanism of stratospheric O.sub.3 intrusion into the atmospheric environment induced by an extratropical cyclone system. The SI with downward transport of stratospheric O.sub.3 to the near-surface layer was driven by the extratropical cyclone system, with vertical coupling of the upper westerly trough, the middle of the northeast cold vortex (NECV), and the lower extratropical cyclone, in the troposphere. The deep trough in the westerly jet aroused the tropopause folding, and the lower-stratospheric O.sub.3 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 further pushed the downward transport of stratospheric O.sub.3 in the free troposphere; the NECV activated an extratropical cyclone in the lower troposphere; and the vertical cyclonic circulation governed the stratospheric O.sub.3 from the free troposphere across the boundary layer top, invading the near-surface atmosphere. In this SI event, the average contribution of stratospheric O.sub.3 to near-surface O.sub.3 was accounted for at 26.77 %. The proposed meteorological mechanism of the vertical transport of stratospheric O.sub.3 into the near-surface atmosphere, driven by an extratropical cyclone system, could improve the understanding of the influence of stratospheric O.sub.3 on the atmospheric environment, with implications for the coordinated control of atmospheric pollution.