The Impact Of Covid-19 Lockdown On The Ozone And Its Precursors In Urban Area Of Wuhan

With the rapid development of national economy,ozone（O₃）pollution has become a serious problem in many cities and regions in China.In recent years,research on the characteristics of O₃ pollution and its formation mechanism has become a hot spot for scholars,and it has practical significance for the formulation of regional O₃ pollution control measures.Since the outbreak of Covid-19 in December 2019,the horrible pandemic swept the world and brought great challenges to human life and health.Wuhan,as the first site of the epidemic,implemented a 76-day"city lockdown"from10:00 on January 23 to 10:00 on April 8.almost all activities including industry,services and transportation were strictly restricted,thus decreasing air pollution in an unprecedented way.Based on the monitoring data from the automatic monitoring station of air pollution of Hubei Environmental Monitoring Center,this study studied the characteristics of ozone and its precursors in different periods of Wuhan-lockdown.The results showed that the concentrations of volatile organic compounds（VOCs）,nitrogen oxides（NOx）and carbon monoxide（CO）decreased to a large extent during lockdown,while the concentration of ozone（O₃）increased rapidly during lockdown.A typical period“from January 19 to January 26”was detailed studied.Ozone photochemistry before and during lockdown were simulated by a photochemical box model incorporating the Master Chemical Mechanism（PBM-MCM）.It was found that the average net ozone production rate during lockdown（1.28ppb/h）was higher than that before lockdown（0.38ppb/h）.In addition,compared with January 19-22,the temperature on January 23-26 were lower,also with the higher relative humidity,higher wind speed,and weaker solar radiation.The meteorological conditions were more conducive to the diffusion of ozone.Therefore,emission change was supposed to be the dominant factor for the ozone increase during lockdown.The changes of ozone sensitivity before and during lockdown were further analyzed.The results showed that ozone formation was VOC-limited in both periods.NO has the highest RIR absolute value,indicating that ozone production was more sensitive to NO.During the city lockdown,the vehicle flow rapidly decreased and the concentration of NO rapidly reduced to a very low level,which weakened the titration consumption of O₃ and lead to the increase of ozone concentration during lockdown.However,the decrease in VOCs was not significant compared to the change in NO.Strengthening the restriction of VOCs emission will be beneficial to ozone control.ethylene,trans-2-butene,propylene and 1-butene were primary species contributing to O₃ production.Cutting the concentrations of olefin species could effectively reduce the ozone concentration in Wuhan urban area.The results of PMF source analysis revealed that exhaust gas of gasoline vehicles and usage of liquefied petroleum gas contributed the most to the concentration of VOCs in Wuhan before lockdown,accounting for 27.1%and 23.7%respectively.During the lockdown,usage of liquefied petroleum gas and organic solvent became the most important source of VOCs in the atmosphere,with contribution of 29.1%and 22.7%,respectively.Combined with the PBM-MCM model,the contribution of various VOCs sources to O₃ production in the two periods was analyzed.Industrial emission and solvent usage contributed the most to ozone before and during the closure of the city.The contribution of gasoline vehicle exhaust dropped from 19.9%before lockdown to6%during the lockdown.Therefore,strengthening the control of VOCs emitted from industrial and solvent sources needs to be considered when formulating ozone control measures in Wuhan.