Stratospheric influence on surface ozone pollution in China.

Events of stratospheric intrusions to the surface (SITS) can lead to severe ozone (O 3 ) pollution. Still, to what extent SITS events impact surface O 3 on a national scale over years remains a long-lasting question, mainly due to difficulty of resolving three key SITS metrics: frequency, duration and intensity. Here, we identify 27,616 SITS events over China during 2015-2022 based on spatiotemporally dense surface measurements of O 3 and carbon monoxide, two effective indicators of SITS. An overview of the three metrics is presented, illustrating large influences of SITS on surface O 3 in China. We find that SITS events occur preferentially in high-elevation regions, while those in plain regions are more intense. SITS enhances surface O 3 by 20 ppbv on average, contributing to 30-45% of O 3 during SITS periods. Nationally, SITS-induced O 3 peaks in spring and autumn, while over 70% of SITS events during the warm months exacerbate O 3 pollution. Over 2015-2022, SITS-induced O 3 shows a declining trend. Our observation-based results can have implications for O 3 mitigation policies in short and long terms.