Reply on RC2

Abstract

The outbreak of COVID-19 promoted strict restrictions to human activities in China, which led to dramatic decrease in most air pollutant concentrations (e.g., PM2.5, PM10, NOx, SO2, and CO). However, abnormal increase of ozone (O3) concentrations was found during the lockdown period in most urban areas of China. In this study, we conducted a field measurement 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 for the Yangtze River Delta (YRD) city cluster of China. We further applied the integrated methodology including machine learning, observation-based model (OBM), and sensitivity analysis to get insights into the reasons causing the abnormal increase of ozone. Major findings include: (1) By deweathered calculation, we found changes in precursor emissions contributed 5.1 ppbv to the observed O3 during the Full-lockdown period, while meteorological conditions only contributed 0.5 ppbv to the O3 changes. (2) By using an OBM model, we found that although significant reduction of O3 precursors was observed during Full-lockdown period, the photochemical formation of O3 was stronger than that during the Pre-lockdown period. (3) The NOx / VOCs ratio dropped dramatically from 1.84 during Pre-lockdown to 0.79 in Full-lockdown period, which switched O3 formation from VOCs-limited regime to the conjunction of NOx- and VOC-limited regime. Additionally, the decrease in NOx / VOCs ratio during Full-lockdown period was supposed to increase the MeanO3 by 2.4 ppbv. Results of this study investigate insights into the relationship between O3 and its precursors in urban area, demonstrating reasons causing the abnormal increase of O3 in most urban areas of China during the COVID-19 lock-down period. This study also underlines the necessity of controlling anthropogenic OVOCs, alkenes, and aromatics in the sustained campaign of reducing O3 pollution in China.