Influence of stratosphere-to-troposphere transport on summertime surface O3 changes in North China Plain in 2019

Abstract

The North China Plain (NCP) is a major region of summer O3 pollution in China. However, the contribution of stratosphere-to-troposphere transport (STT) to summertime surface O3 in the NCP and the meteorological mechanisms are poorly understood. In this study, meteorological reanalysis data, O3 observations, and Lagrange modeling were used to analyze the contribution of the STT to the change in surface O3 in the NCP, the vertical transport pathways, and the associated meteorological mechanism. In summer 2019, the STT contributed an estimated 5.7%–18.8% to the surface O3 concentrations in the NCP. The influence of the STT in the NCP is closely related to the changes in westerly circulation in the upper troposphere and lower stratosphere (UTLS) over the Eurasian region. The troughs and vortices in the westerlies led to the convergences of O3 in the UTLS and tropopause folding, forming downward intrusions of O3-rich air from the UTLS to the lower troposphere. Controlled by the westerly circulation, the O3 source regions of STT in the stratosphere are found between 40° and 70°N, oscillating zonally with changing transport periods, where a key source region of STT is situated steadily over 60°-100°E northwest of the NCP. In association with the cut-off low of westerly circulation in the UTLS over Siberia evolved from the splitting of the polar vortex, a slanted channel of O3 downward intrusion was set up from the UTLS to the lower troposphere over the NCP in 7 days. As the cut-off low in westerly circulation moves southeastward close to the NCP, the channel of the O3 downward intrusion is set upright with a shorter period of vertical transport to the NCP ground. Our results provide insights into the the O3 source–receptor relationship in the STT with vertical O3 transport structures in changes in the atmospheric environment.