Research On The Impact Of Harvesting Process On PM_2.5 In Rural Areas

Wheat harvesting and subsequent open-air straw burning can seriously affect air quality in rural areas and cause potential health hazards to humans.In this study,the characteristics of PM_2.5,water-soluble ions（WSIs）and inorganic elements（IEs）were analyzed during and after wheat harvest episodes based on a systemetic PM_2.5 sampling campaign conducted in rural areas of Wangdu County,Baoding City from June 18 to July 7,2019.The average concentration of PM_2.5 was 87.7±43.8μg·m^-3 in the entire sampling period.A noticeable increase was found from 75.6±31.9μg·m^-3 after wheat harvest to 156±42.5μg·m^-3 during wheat harvest,which demonstrated the positive effect of harvesters on the PM_2.5 increment,indicating harvesting behavior might be an important source of local PM_2.5.SNA(SO₄^2-,NO₃^-and NH₄⁺)dominated in WSIs,contributing 90.4%of the∑₉WSIs and accounting for 37.6%of PM_2.5.The mean ratio of NO₃^-/SO₄^2-in the entire sampling period was 0.95,indicating the influence by fixed sources.The mean ratio of NO₃^-/SO₄^2-fluctuated from 0.93 after wheat harvest to 0.99during wheat harvest,which implied the conversion of NO_x emitted from the operation of mobile sources such as harvesters to NO₃^-during wheat harvest.The average value of AE/CE in the entire sampling period was 0.79,indicating that the particulate matter was alkaline.Mean AE/CE ratio elevated from 0.76 after wheat harvest to 0.91 during wheat harvest,illustrating more effects of acid gases from fossil fuels burning associated with the wheat harvesting machinery.Si,Ca,Al,K,Fe,Na,and Mg accounted for the largest proportion of inorganic elements.The concentrations of V,Cr,Ni,Cu,As,Sn,Sb,La,Ce and Pb during wheat harvest increased by 269%,227%,146%,126%,869%,128%,231%,234%,131%and 195%,respectively,compared with those after wheat harvest.The enrichment factor method and index of geoaccumulation analysis showed that Cd,Cu,W,Zn,Bi,Sb,Pb and Mo might be affected by anthropogenic activities including industrial emissions,harvesters and burning of fossil fuels.Six sources were identified by PMF model:biomass burning source,fugitive dust source,secondary emission source,vehicle emission source,coal combustion source and industrial emission source,and their contributions were22.50%,22.81%,18.13%,17.47%,9.66%and 9.44%,respectively.With the transition from during wheat harvest period to after wheat harvest period,the contributions of vehicle emission source and fugitive dust source decreased from 19.88%and 26.81%to 17.04%and 22.1%,proving a large amount of exhaust emission and dust caused by intensive agricultural operations.Conversely,the contribution of biomass burning source elevating from 5.27%to 25.54%evidenced open biomass burning.In whole sampling period,the contribution of coal combustion source demonstrated that coal was still an important fuel in rural areas,and the contribution of industrial emission source was the lowest,indicating the industry’s strict emission control measures.The result of backward trajectory analysis showed the sampling site was susceptible to airflow in the south,southeast and northeast directions.The potential source aeras of biomass burning were mainly distributed in Hebei Province on the basis of potential source simulation.Results of human health risk assessment showed that the carcinogenic risk of each element during wheat harvest period was greater than that after wheat harvest and risks of As and Ni through ingestion,dermal contacting and inhaling during wheat harvest were 4.64,4.64 and 9.69 times and 1.20,1.20 and 2.46times of these after wheat harvest.The total non-carcinogenic risk caused by ingestion after wheat harvest was greater than that during wheat harvest period,and risks from dermal contacting and inhaling during wheat harvest were higher than these after wheat harvest.The HI values during wheat harvest period were 0.99,1.43 and 2.76 times of these after wheat harvest,respectively.