Variations of air pollutants during COVID-19 lockdown in Xi’an, China

【作者】 冯蓉； Hongmei Xu； Zexuan Wang； Pingping Liu； Zhenxing Shen；

【Author】 Rong Feng;Hongmei Xu;Zexuan Wang;Pingping Liu;Zhenxing Shen;Department of Environmental Science and Engineering, Xi’an Jiaotong University;

【机构】 Department of Environmental Science and Engineering, Xi’an Jiaotong University；

【摘要】 In the context of the outbreak of the Coronavirus Disease 2019 （COVID-19） in China,a strict lockdown policy during the epidemic was implemented to control the non-essential human activities （such as industry,agriculture,and transportation） in Xi’an,which greatly affected the emissions from anthropogenic sources.Comparing the air quality data of the three periods （pre-lockdown,dur-lockdown,and post-lockdown of COVID-19）,it was found that AQI,PM_2.5,PM₁₀,SO₂ and CO showed gradual downward trends rather than the lowest level during the lockdown period as expected.This may be due to the unfavorable meteorological factors during the lockdown period.NO₂ exhibited the lowest level during lockdown,which reduced by 52%compared to pre-lockdown,which may be directly related to decreased daily travelling and motor vehicle emissions.The concentration of O₃ was the highest during the lockdown,reaching 88μg m^-3,which was 2.6 times the level before the lockdown of COVID-19.PM_2.5 mass and its carbonaceous fraction as well as water-soluble inorganic ions on the filter samples in the three periods were also analyzed in this study.PM_2.5 concentration in pre-lockdown was1.2 and 1.6 times those in dur-lockdown and post-lockdown,respectively.The proportion of EC in PM_2.5 remained almost unchanged （2.2%-2.7%） in different research intervals,while the proportion of OC in PM_2.5 reached the maximum （11.9%） during the lockdown period,1.1 and 1.3 times those before and after the lockdown of COVID-19.The OC/EC values were all higher than 2.0 in this study,suggesting the presence of secondary organic carbon （SOC）.K⁺concentration during the lockdown was 1.8 and 2.9 times pre-and post-lockdown which is due to the increased consumption of household biomass fuels for heating and cooking.Ca²⁺displayed the lowest value during the lockdown,0.8 and 0.4times pre-and post-lockdown,proving that the reduction of travelling and construction activities during the epidemic lockdown had a greater impact on the concentration of Ca²⁺.The NO₃^-/SO₄^2-ratios during the lockdown （average:1.2±0.4,range:0.6–2.0） was much lower and less scattered than those in the periods of pre-lockdown （average:1.8±0.6,range:0.9–3.1） and post-lockdown （average:2.1±1.2,range:0.2–4.8）.This study indicated that residential combustion sources （for heating in winter） and secondary formation of PM_2.5 have increased significantly in the context of reduced traffic （mobile source） and industrial combustion sources （coal-fired power plants） during the COVID-19 lockdown in Xi’an,China.In this study,we found that control the non-essential human activities did not lead to the reduction of all atmospheric pollutants and the components in PM_2.5,only caused the reduction of the specific components related to the particular anthropogenic sources.Atmospheric pollution was also closely related to meteorological factors,atmospheric oxidation,etc.更多还原

【Abstract】 In the context of the outbreak of the Coronavirus Disease 2019 （COVID-19） in China,a strict lockdown policy during the epidemic was implemented to control the non-essential human activities （such as industry,agriculture,and transportation） in Xi’an,which greatly affected the emissions from anthropogenic sources.Comparing the air quality data of the three periods （pre-lockdown,dur-lockdown,and post-lockdown of COVID-19）,it was found that AQI,PM_2.5,PM₁₀,SO₂ and CO showed gradual downward trends rather than the lowest level during the lockdown period as expected.This may be due to the unfavorable meteorological factors during the lockdown period.NO₂ exhibited the lowest level during lockdown,which reduced by 52%compared to pre-lockdown,which may be directly related to decreased daily travelling and motor vehicle emissions.The concentration of O₃ was the highest during the lockdown,reaching 88μg m^-3,which was 2.6 times the level before the lockdown of COVID-19.PM_2.5 mass and its carbonaceous fraction as well as water-soluble inorganic ions on the filter samples in the three periods were also analyzed in this study.PM_2.5 concentration in pre-lockdown was1.2 and 1.6 times those in dur-lockdown and post-lockdown,respectively.The proportion of EC in PM_2.5 remained almost unchanged （2.2%-2.7%） in different research intervals,while the proportion of OC in PM_2.5 reached the maximum （11.9%） during the lockdown period,1.1 and 1.3 times those before and after the lockdown of COVID-19.The OC/EC values were all higher than 2.0 in this study,suggesting the presence of secondary organic carbon （SOC）.K⁺concentration during the lockdown was 1.8 and 2.9 times pre-and post-lockdown which is due to the increased consumption of household biomass fuels for heating and cooking.Ca²⁺displayed the lowest value during the lockdown,0.8 and 0.4times pre-and post-lockdown,proving that the reduction of travelling and construction activities during the epidemic lockdown had a greater impact on the concentration of Ca²⁺.The NO₃^-/SO₄^2-ratios during the lockdown （average:1.2±0.4,range:0.6–2.0） was much lower and less scattered than those in the periods of pre-lockdown （average:1.8±0.6,range:0.9–3.1） and post-lockdown （average:2.1±1.2,range:0.2–4.8）.This study indicated that residential combustion sources （for heating in winter） and secondary formation of PM_2.5 have increased significantly in the context of reduced traffic （mobile source） and industrial combustion sources （coal-fired power plants） during the COVID-19 lockdown in Xi’an,China.In this study,we found that control the non-essential human activities did not lead to the reduction of all atmospheric pollutants and the components in PM_2.5,only caused the reduction of the specific components related to the particular anthropogenic sources.Atmospheric pollution was also closely related to meteorological factors,atmospheric oxidation,etc.更多还原