Influence of stratosphere-to-troposphere exchange on the seasonal cycle of surface ozone at Mount Waliguan in western China

Surface ozone over the Qinghai-Tibetan Plateau exhibits a summertime maximum contrasting to a spring peak observed at many remote locations in the Northern Hemisphere. In this study we conducted meteorological simulations for April - August 2003, when intensive measurement of trace gases was carried out at remote Mount Waliguan, to elucidate the influence of atmospheric dynamics on the observed ozone events and seasonal cycle. Examinations of potential vorticity and trace gases suggest that most of the synoptic-scale ozone enhancements were due to stratospheric intrusions, as opposed to transport of anthropogenic pollutions. Case studies show that the existence of jet stream is the main dynamical cause for these intrusions. Further analysis of upper-level mean zonal winds suggests that stronger subtropical jet streams lead to more frequent stratospheric intrusions in summer compared to spring, which may have contributed to the higher summertime surface ozone there and possibly also in other regions of Central Asia.