Study On The Distribution Characteristics Of Atmospheric PM_2.5 Organic Pollutants During Wheat Harvest

Wheat harvest and subsequent straw burning for maize planting can cause severe PM_2.5 and PAH pollutions and deteriorate the air quality of nearby cities in consequence.PM_2.5 samples were collected in B urban area（BUA）and adjacent rural site from June18 to July 9 of 2019,during and after wheat harvest（DWH and AWH,respectively）.The“Migration Effect”(i.e.,PM_2.5 and PAHs transferred from rural to urban during DWH and AWH,respectively)was proved by both the later time for appearance of peak values of PM_2.5 and PAHs and the air mass origins in BUA.The daily average PM_2.5(reported inμg m^–3)137 of DWH was 2.58 times 53.1 of AWH for BUA,regardless of its lower levels than the corresponding 156 and 75.6 for an adjacent rural site（ARS）.The reverse trend was found for PAH mass contents(inμg g^–1),AWH possessed much higher value of 139 than 27.8 of DWH,while no significant fluctuations occurred for PAH concentrations due to the varied PM_2.5 levels.Four PAH sources including biomass burning（BB）,coal combustion（CC）,vehicle exhaust（VE）and industrial emissions（IN）were identified using positive matrix factorization（PMF）.In ARS,CC accounted for the highest proportion DWH,indicating that residential coal combustion for cooking,due to its low economic cost,has remained a common practice despite the implementation of the Coal Removal Campaign in 2013.The 11.6%contribution of BB DWH also confirmed the use of biomass fuel for indoor cooking in rural areas of China.VE was the largest contributor of PAHs during the whole sampling period,reflecting the operation of wheat harvesters and rotary cultivators.In BUA,BB was the biggest contributor during whole sampling period（WSP）,followed by CC,VE,and IN.BB increased from 17.3%to 37.1%along with the transition from DWH to AWH,indicated the impact of straw burning for atmosphere.High share of CC suggested that coal was still an important civil fuel.The strict emission reduction measures made the industry smallest contributor in BUA.Lower VE share in BUA than ARS demonstrated that influence of high-intensity operation of wheat harvesters and rotary cultivators.Through cluster analysis of backward trajectories,most of the wind directions in BUA and County ARS originate from the three directions of southeast and west,and these three directions are the main grain-producing areas in North China Plain,Therefore,it is greatly affected by the pollutants in the atmosphere.More than 40%of the atmosphere reaches the urban area of BUA through ARS,indicating that the PAHs produced by the biomass burning phenomenon after the wheat harvest in the rural areas around BUA will affect urban areas.In terms of health risks,the health risks after the wheat harvest were significantly higher than during the wheat harvest,and there was a phenomenon of migration from rural areas to urban areas.