Large Proportions of Surface Ozone and Premature Mortality Due to a Stratospheric Intrusion Event in North China

Ozone pollution and corresponding health impacts have garnered significant attention. However, O 3 sources are highly complex, particularly the contribution of stratospheric O 3 to surface concentrations, which remains unclear and severely constrains quantitative understanding of O 3 pollution and associated health risks. This study focuses on a severe tropopause folding event that occurred over North China (NC) from 30 to 31 July 2021, which provided a critical opportunity to investigate the stratospheric contribution to surface ozone levels. Stratospheric intrusion increased surface O 3 by 23 ppbv over NC, accounting for 40% of the total observed concentration. The enhancement of surface O 3 was jointly driven by upper‐level subsidence, boundary layer compression and lower‐tropospheric disturbances, accompanied by the downward extension of a tongue‐shaped ozone‐rich structure that became trapped near the surface. It is estimated that during this 36‐h period of high O 3 pollution in NC, 101 premature deaths occurred, of which 40 were attributed to stratospheric O 3 intrusion. Quantifying the contribution of stratospheric intrusion to surface O 3 helps to better understand the O 3 enhancement caused by human emissions, and also highlights the health risks linked to the stratosphere.