Study On The Characteristics Of PM_2.5 And PM₁₀ Pollution Change And Regional Transmission In Lanzhou Under The Influence Of COVID-19

With the rapid economic development,the pollution of air environment becomes more and more serious.As a typical heavy industry city in northwest China,Lanzhou has a long history of air pollution problems.Therefore,it is particularly important to study the change characteristics of PM_2.5 and PM₁₀ pollution in Lanzhou.In this paper,high-resolution remote sensing data and PM_2.5 and PM₁₀ concentration data from Lanzhou air quality monitoring stations were used to make a statistical analysis of the spatial and temporal variation characteristics of PM_2.5 and PM₁₀ concentration in Lanzhou from 2015 to 2022.The research focuses on the annual,seasonal and monthly variation trends as well as the excess rates of PM_2.5 and PM₁₀.At the same time,the change of PM_2.5 and PM₁₀ concentration in Lanzhou during three different seasons of epidemic containment period from 2020 to 2022 was analyzed,revealing the impact of local pollution emission on atmospheric pollutant concentration in Lanzhou.The GM（1,1）model was established by using the grey relational degree method.Considering the sudden onset of the epidemic,the concentration values from 2020 to2022 were fitted and compared with the actual values,and the PM_2.5 and PM₁₀concentration trends were predicted in the next three years from 2023 to 2025 in Lanzhou.Finally,the HYSPLIT model of backward trajectory,the potential source contribution factor method（PSCF）and the weighted concentration trajectory method（CWT）were used to identify the air pollution transmission path,the distribution of external potential sources and the contribution degree of Lanzhou City,considering the impact of"emission reduction"during the epidemic lockdown period,and the following conclusions were drawn:（1）Temporal and spatial variation characteristics of PM_2.5 and PM₁₀concentrations in Lanzhou City and the impact of COVID-19 on themThe average annual concentration of PM_2.5 and PM₁₀ in Lanzhou showed a decreasing trend from 2015 to 2022,with the highest concentration in 2016 and the lowest concentration in 2021.They are mainly distributed in Xigu District,Chengguan District,Anning District and the north of Qilihe District,while the concentrations of PM_2.5 and PM₁₀ are low in the northwest of Yuzhong County and Yongdeng County.The exceedance frequency of PM_2.5 and PM₁₀ decreased in spring,summer and autumn,but the exceedance in winter did not change,with the maximum value appearing 54.44%in 2016 and 31.11%in 2017,respectively.The containment of the epidemic in 2020reduced PM_2.5 and PM₁₀ concentrations by 22.88%and 23.01%,respectively.During the epidemic,both of them were high in the middle and low in the edge.The effect of containment on PM₁₀ concentration was greater than that of PM_2.5.After containment,PM₁₀ rapidly decreased,and then rapidly increased after uncontainment.During the epidemic containment period in 2021,PM_2.5 concentration in the main urban area increased significantly,exceeding 60μg/m3.Except for the main urban area,the PM₁₀concentration increased significantly at the junction of Gaolin County and Yongdeng County,with the concentration exceeding 60μg/m3.In 2022,the concentration of PM_2.5and PM₁₀ was low on the whole,and decreased significantly and rapidly after entering the sealing period,and then increased rapidly after unlocking.Under better natural conditions,the transient human activities have a more significant and direct impact on pollutant concentration.（2）Prediction of the change trend of PM_2.5 and PM₁₀ concentrations in Lanzhou under the influence of COVID-19The correlation degree of natural meteorological factors（including average temperature,average wind speed,relative humidity,average air pressure,precipitation and visibility,etc.）,sulfur dioxide,nitrogen oxide and PM_2.5 and PM₁₀ concentration in Lanzhou from 2015 to 2019 was calculated.The order was average temperature>average wind speed>relative humidity>average air pressure>precipitation>visibility>sulfur dioxide>nitrogen oxide.Anthropogenic emissions of sulfur dioxide and nitrogen oxides have less impact on PM_2.5 and PM₁₀.In addition,the grey prediction model GM（1,1）for PM_2.5 and PM₁₀ concentration has a good effect,with the maximum relative errors being 2.77%and 3.04%,the variance ratio C being 0.312 and 0.216,and the small error probability p being 0.956 and 1.000,respectively.All these models have passed the accuracy test.The results of the grey prediction model show that the actual concentrations of PM_2.5 and PM₁₀ in 2020,2021 and 2022 are all lower than the predicted values.It can be seen that the large-scale,rapid and substantial reduction of human activities will affect the concentration changes of PM_2.5 and PM₁₀ to varying degrees.In addition,in the next three years（2025）,the PM_2.5 and PM₁₀ concentrations in Lanzhou will both decrease and fall below the national secondary standard limits(PM_2.5=35μg/m3 and PM₁₀=70μg/m3).（3）Analysis of regional transmission functionThe atmospheric backward track of 24h at a height of 500 meters in Lanzhou city mainly comes from the southeast of Lanzhou,Dingxi and Longnan regions,accounting for about 40%-50%.The air mass arriving in Lanzhou in spring,autumn and winter moves fast and passes through several dust sources,causing serious PM_2.5 and PM₁₀pollution in Lanzhou.In summer,the backward track comes from north and southeast,and the moving speed is slow.Combined with the near-surface wind field,the mild pollution in Lanzhou is mainly caused by the accumulation of local pollutants in light wind conditions,while the moderate pollution may be caused by the accumulation of local pollutants superimposed by external pollutants.In Lanzhou,air pollution is often associated with fast near-surface wind speed,but when the wind speed is too fast,it is conducive to the diffusion of PM_2.5 and PM₁₀.In spring,the potential sources mainly concentrated in the southern part of the Badain Jaran Desert in Inner Mongolia,and contributed more than 45μg/m3 and 75μg/m3 to PM_2.5 and PM₁₀ concentrations in Lanzhou,respectively.In autumn and winter,the potential sources were mainly concentrated in the north of Huangnan Tibetan Autonomous Prefecture of Qinghai Province and the west of Linxia Hui Autonomous Prefecture of Gansu Province,contributing more than 45μg/m3 and 110μg/m3 to the average annual concentration of PM_2.5 and PM₁₀ in Lanzhou,respectively.In summer,the potential source of PM₁₀ in Lanzhou was mainly located in the southern Tengger Desert,the border area of Gansu and Ningxia,and contributed more than 120μg/m3 to the average annual PM₁₀ concentration in Lanzhou.During the epidemic containment period in 2020,the southeast wind appeared with high concentrations of PM_2.5 and PM₁₀.The contribution of PM_2.5 and PM₁₀ in the northwest of Linxia and the area near Longnan to Lanzhou exceeded 40μg/m3 and 100μg/m3,respectively.Meanwhile,the southern Tengger Desert and the southern Badan Jaran Desert in Inner Mongolia were also major dust sources.In 2021,the containment period was short,and the contribution of foreign potential sources to the daily concentration of PM_2.5 in Lanzhou was no more than 30μg/m3.The southwesterlies with high PM₁₀ concentration confirmed that the junction of Huanghaizhou and Linxia was a potential source of PM₁₀,contributing more than 70μg/m3.Therefore,the air pollution prevention and control work in Lanzhou not only needs to strengthen the sandstorm from Inner Mongolia in spring,but also needs to study related pollution emissions in Linxia,Longnan,Dingxi and other cities and states,so as to better prevent PM_2.5 and PM₁₀ pollution.