Attributing Increases in Ozone to Accelerated Oxidation of Volatile Organic Compounds at Reduced Nitrogen Oxides...

Surface ozone (O₃) is an important secondary pollutant affecting climate change and air quality in the atmosphere. Observations during the COVID-19 lockdown in urban China show that the co-abatement of nitrogen oxides (NO_x) and volatile organic compounds (VOCs) caused winter ground-level O₃ increases, but the chemical mechanisms involved are unclear. Here we report field observations in the Shanghai lockdown that reveals increasing photochemical formation of O₃ from VOC oxidation with decreasing NO_x. Analyses of the VOC profiles and NO/NO₂ indicate that the O₃ increases by the NO_x reduction counteracted the O₃ decreases through the VOC emission reduction in the VOC-limited region, and this may have been the main mechanism for this net O₃ increase. The mechanism may have involved accelerated OH-HO₂-RO₂ radical cycling. The NO_x reductions for increasing O₃ production could explain why O₃ increased from 2014 to 2020 in response to NO_x emission reduction even as VOC emissions have essentially remained unchanged. Model simulations suggest that aggressive VOC abatement, particularly for alkenes and aromatics, should help reverse the long-term O₃ increase under current NO_x abatement conditions.