Research On The Impact Of Harvesting Process On The Components And Sources Of PM_2.5 In Surrounding Cities

Open burning of wheat straw can lead to a sharp deterioration of air quality.Its biomass combustion is affected by wind direction and tends to spread to the north and south direction,causing certain pollution to the city and surrounding cities.During the sampling period in Baoding City,the average mass concentration of PM_2.5varied greatly at different stages.During the wheat harvest period（June18th-June 20th）in the surrounding areas,the average PM_2.5mass concentration was137μg·m^-3,which was much higher than the value of 75μg·m^-3 in China National Secondary Standard for Ambient Air Quality（GB3095-2012）.During this period,the PM_2.5 concentration in Baoding showed a higher trend.In the second part of the sampling period（June 21st-July 7th）,the average mass concentration of PM_2.5was 66.3μg·m^-3,which was lower than the secondary national ambient air quality standard in China.During the sampling period,the surrounding counties have gone through the wheat harvest,and a large number of harvester activities have ended,the"migration effect"of pollutants is greatly reduced.During harvest,anions SO₄^2-and NO₃^-accounted for 32.15%and 38.98%of PM_2.5 concentration,respectively.The cationic NH₄⁺accounts for 25.34%of the PM_2.5 concentration.After wheat harvest,the proportions of SO₄^2-and NO₃^-were 39.83%and 26.67%,respectively.The proportion of cation NH₄⁺was 27.64%.NO₃^-accounted for from38.98%to 26.67%.It was related to the tail gas emitted by harvester and the contribution of straw burning during wheat harvest in Baoding.Source apportionment PMF results showed that there were 6 main pollution sources in Baoding during the sampling period,namely biomass combustion,dust,secondary sources,vehicle exhaust,coal combustion and industrial sources.Among them,biomass burning contributed the largest change,increasing from 1.68%to 14.39%,and the generated biological sources reached the urban area through cross-regional transfer.Five trajectories were obtained through backward trajectory clustering analysis.Among them,trajectory 3 and trajectory 5 overlap in the subordinate rural areas of Baoding City and finally reach the urban area of Baoding.Through the analysis of potential source areas and the analysis of biomass combustion sources by PSCF,it was found that the main sources of pollution sources were distributed in the southern part of Baoding City,and the secondary external contribution sources were western and southern Shandong province,and northern Henan Province.The results of enrichment factor analysis and geoaccumulation index analysis showed that the pollution severity of Cu,Zn,As,Mo,Cd,Sn,Sb,W,Tl,Pb and Bi in the first stage was several times higher than that in the second stage.The risk value of Zn intake through respiration in the first stage was higher than that in the second stage,which is related to the regional migration of particulate matter generated by harvester harvesting activities in the surrounding towns.As caused by coal burning during wheat harvest and surrounding industries in surrounding areas brings great risks to children’s health.