Analysis Of The Mechanism On Changes Of Urban Air Pollutant Mass Concentration Under The Influence Of COVID-19

COVID-19 began spreading globally in 2020,posing a significant threat to human health.Most of the countries affected by the outbreak took control measures to contain the spread of the disease,and there were significant changes in air quality during this period.In the following two years,most countries continued to experience multiple rounds of the outbreak,and the relevant national authorities continued to take control measures.To understand the air quality changes during the new crown epidemic and the impact of different control measures on air quality,and to investigate the mechanism of urban air quality changes,this study takes Taiyuan City,Shanxi Province,as an example,and investigates the air pollutant mass concentration change pattern and its influencing factors before and after the control measures in Taiyuan City from 2019 to 2022 by using time series statistical analysis,positive correlation and partial correlation analysis,and by means of air mass Backward trajectory clustering,potential source contribution factor（PSCF）,source analysis of PM_2.5 water-soluble inorganic ions and carbonaceous fraction were used to study the sources of air pollutants and the role of control measures on them during the three epidemic control periods.The results show that（1）the annual average value of AQI in Taiyuan City from 2019to 2022 shows a decreasing trend,and the same trend as the annual average concentrations of PM_2.5,PM₁₀,CO,SO₂ and NO₂,but the overall O₃ concentration shows an increasing trend;from the monthly variation,the monthly average concentrations of PM_2.5,PM₁₀,CO,SO₂ and NO₂ are In terms of monthly variation,the average monthly concentrations of PM_2.5,PM₁₀,CO,SO₂ and NO₂ are"high in winter and low in summer",but O₃,in contrast to the other five pollutants,shows a"single-peak"structure;in terms of daily variation,the peak of PM_2.5,PM₁₀,CO and SO₂ occurs at 11:00 a.m.,while the peak of O₃occurs at 15:00 p.m.00 pm.（2）The correlation between temperature,relative humidity and the concentration of six air pollutants was the greatest,with temperature being significantly positively correlated with O₃ in spring and autumn,and relative humidity being significantly negatively correlated with pollutants in summer and autumn;air pressure was significantly negatively correlated with SO₂ and NO₂ in summer,and wind speed was positively correlated with pollutants in summer.（3）Compared with the pre-control period,the concentrations of air pollutants,water-soluble inorganic ions and OC and EC decreased significantly during the first two epidemic control periods（January25-February 16,2020 and April 3-May 7,2022）,while O₃ concentrations increased during the first two epidemic control periods and decreased during the third control period（October 20-December 10,2022）;compare Comparing the pollutants and PM_2.5components in the three epidemic control periods,we found that the concentrations of most pollutants,water-soluble inorganic ions and OC and EC were highest in the first control period and lowest in the second control period,which were not only related to anthropogenic emissions,but also closely related to meteorological conditions.（4）According to the trajectory clustering analysis,the first epidemic was mainly influenced by the short-range transport from the local nearby air masses before the control,and the control period was mainly influenced by the shorter-range transport from the local nearby and northwest;the second and third epidemic were mainly influenced by the shorter-range transport from the local nearby and the long-range transport from the northwest before and after the control.（5）The PSCF results show that,except for the local pollution contribution from neighboring cities before and after the third epidemic control,the southern part of Inner Mongolia Autonomous Region,the central-eastern and northern part of Shaanxi Province,and the central-western and southern part of Hebei Province before and after the first two control periods are the main potential source areas affectingρ(PM_2.5)andρ（O₃）in Taiyuan City,so the pollution transmission from these areas to Taiyuan City should be emphasized.The above results show that the meteorological conditions were more unfavourable during the first and third epidemic control periods,but the stricter control measures during the first epidemic control period led to a reduction in pollutant concentrations during the first epidemic control period,while most pollutant concentrations increased during the third epidemic control period,while the stricter control of the second epidemic,combined with favourable meteorological conditions in spring,led to the lowest pollutant concentrations during the second epidemic control period.The lowest pollutant concentrations were recorded during the second outbreak control period.This study can provide basis and ideas for further improvement of air quality and adjustment of control strategies.