Insights into the significant increase in ozone during COVID-19 in a typical urban city of China.

The outbreak of COVID-19 promoted strict restrictions to human activities in China, which led to a dramatic decrease in most air pollutant concentrations (e.g., PM 2.5 , PM 10 , NO x , SO 2 and CO). However, an obvious increase in ozone (O 3) concentrations was found during the lockdown period in most urban areas of China. In this study, we conducted field measurements targeting ozone and its key precursors by utilizing a novel proton transfer reaction time-of-flight mass spectrometer (PTR-TOF-MS) in Changzhou, which is representative of the Yangtze River Delta (YRD) city cluster of China. We further applied the integrated methodology including machine learning, an observation-based model (OBM) and sensitivity analysis to obtain insights into the reasons causing the obvious increase in ozone. Major findings include the following: (1) by deweathered calculation, we found changes in precursor emissions contributed 1.46 ppbv to the increase in the observed O 3 during the full-lockdown period in 2020, while meteorology constrained 3.0 ppbv of O 3 in the full-lockdown period of 2019. (2) By using an OBM, we found that although a significant reduction in O 3 precursors was observed during the full-lockdown period, the photochemical formation of O 3 was stronger than that during the pre-lockdown period. (3) The NOx/VOC ratio dropped dramatically from 1.84 during the pre-lockdown to 0.79 in the full-lockdown period, which switched O 3 formation from a VOC-limited regime to the boundary of a NO x - and VOC-limited regime. Additionally, box model results suggested that the decrease in the NOx/VOC ratio during the full-lockdown period could increase the mean O 3 by 2.4 ppbv. Results of this study give insights into the relationship between O 3 and its precursors in urban area and demonstrate reasons for the obvious increase in O 3 in most urban areas of China during the COVID-19 lockdown period. This study also underlines the necessity of controlling anthropogenic oxygenated volatile organic compounds (OVOCs), alkenes and aromatics in the sustained campaign of reducing O 3 pollution in China. [ABSTRACT FROM AUTHOR]