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., PM2.5, PM10, NOx, SO2 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 NOx- 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.