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

Surface ozone (O 3 ) 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 O 3 increases, but the chemical mechanisms involved are unclear. Here we report field observations in the Shanghai lockdown that reveals increasing photochemical formation of O 3 from VOC oxidation with decreasing NO x . Analyses of the VOC profiles and NO/NO 2 indicate that the O 3 increases by the NO x reduction counteracted the O 3 decreases through the VOC emission reduction in the VOC-limited region, and this may have been the main mechanism for this net O 3 increase. The mechanism may have involved accelerated OH-HO 2 -RO 2 radical cycling. The NO x reductions for increasing O 3 production could explain why O 3 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 O 3 increase under current NO x abatement conditions.