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

Area of Science:

• Atmospheric Chemistry
• Environmental Science
• Climate Change Mitigation

Background:

• Ozone (O3) pollution control is vital for China's dual goals of pollution reduction and carbon mitigation.
• Stratospheric intrusion (SI) is a major natural source of tropospheric O3, complicating control efforts.
• The COVID-19 pandemic lockdowns offered a unique opportunity to study SI's impact under reduced emissions.

Purpose of the Study:

• To investigate the influence of stratospheric intrusion (SI) on ozone (O3) pollution control strategies.
• To compare SI events before and during COVID-19 lockdowns in Yunnan, China.
• To determine optimal O3 mitigation strategies considering varying SI contributions.

Main Methods:

• Utilized multiple diagnostic tools to quantify SI impacts in Yunnan case studies (2019 vs. 2020).
• Employed Integrated Process Rate (IPR) analysis to identify key O3 enhancement contributors.
• Analyzed O3 control strategy shifts under different emission reduction scenarios and SI contributions.

Main Results:

• The SI event during the 2020 lockdown was more severe (deeper altitude, longer duration, greater impact) than in 2019.
• Horizontal advection was identified as the primary driver of tropospheric O3 enhancement.
• A critical threshold (11.2 ppb SI contribution) was identified, necessitating a shift in mitigation focus from VOCs to NOx.

Conclusions:

• Stratospheric intrusion (SI) presents a significant, often underestimated, challenge for ozone (O3) pollution control.
• Effective O3 mitigation requires accounting for the additional O3 burden from SI.
• Optimal control strategies must be dynamic, adapting to the level of SI influence, particularly under emission reduction scenarios.