How stratospheric intrusion modulates the optimal mitigation pathway for ozone under sharp emission reductions.

The prevention and control of ozone (O3) pollution is a critical foundation for achieving the Chinese government's strategy of coordinated improvement in pollution reduction and carbon mitigation. As the largest natural source of tropospheric O3, stratospheric intrusion (SI) poses unforeseen additional challenges for O3 pollution control. The abrupt lockdowns during the COVID-19 pandemic provided a striking real-world scenario for studying the impact of SI on O3 control strategies amid dramatic emission reductions. Multiple diagnostic tools quantified the impact of SI in two representative Yunnan case studies from February 8-12, 2019 before the lockdown and the same period in 2020 during the lockdown. Compared to the pre-lockdown case, the SI event during the lockdown featured a deeper intrusion altitude, longer duration, and greater impact. Integrated process rate (IPR) analysis identified horizontal advection as the main contributor to tropospheric O3 enhancement. Critically, under substantial emission reduction scenarios, the optimal control strategy shifts with increasing SI contribution. Specifically, when the SI contribution exceeds 11.2 ppb during the lockdown period, the focus of mitigation should be dynamically recalibrated from primarily targeting VOCs to prioritizing NOx reductions. Our findings underscore the need to account for the additional O3 burden from SI and to dynamically adjust optimal mitigation pathways to counteract its negative impact.