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

Surface ozone (O) 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) and volatile organic compounds (VOCs) caused winter ground-level O increases, but the chemical mechanisms involved are unclear. Here we report field observations in the Shanghai lockdown that reveals increasing photochemical formation of O from VOC oxidation with decreasing NO. Analyses of the VOC profiles and NO/NO indicate that the O increases by the NO reduction counteracted the O decreases through the VOC emission reduction in the VOC-limited region, and this may have been the main mechanism for this net O increase. The mechanism may have involved accelerated OH-HO-RO radical cycling. The NO reductions for increasing O production could explain why O increased from 2014 to 2020 in response to NO 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 increase under current NO abatement conditions.