Characterization Of Atmospheric Black Carbon And Secondary Aerosols Using Electron Probe X-Ray Microanalysis

Black carbon aerosols and secondary aerosols,as important components of atmospheric particulates,have attracted extensive attention at home and abroad for their unique role in haze formation.For further research morphology characteristics and transformation of black carbon and secondary aerosol on atmospheric environment,the paper used the low atomic number particle electron probe microanalysis technology(low-Z particle EPMA),Taiyuan and Wuxiang county,Shanxi Province as sampling sites,classified and analyzed different atmospheric aerosol particle size during the haze and the non haze.Based on the analysis of the characteristics of various types of particles at the same time,and this paper put great emphasis on measure the shape,size,mixing state and chemical composition of black carbon and secondary aerosols and compared their differences in different years,seasons and periods,to lay a theoretical foundation for exploring their sources and environmental hazards.By using a seven-stage MAY cascade impactor,atmospheric particles in the size ranges of 0.25-0.5?m,0.5-1.0?m and 1.0-2.0?m in aerodynamic diameter,respectively,were collected on the roof of the college of environment and resource science in Shanxi University,Taiyuan city at every 4h from 8:00am 12/30/2011 to 4:00am 01/01/2012 and from 8:00am 11/25/2018 to 4:00am 11/27/2018 when haze episodes occurred.In addition,the center of the county(S1)and the edge of the city(S2)were selected,Dekati three-stage impingement particle sampler was used to collect single particle samples with aerodynamic diameters ranging from 10-2.5?m and 2.5-1.0?m from 2014/10/24 to 2014/10/29 and 2015/1/16 to 2015/1/22.After that,the secondary electron images and X-ray spectra of particles were determined using a scanning electron microscopy equipped with ultrathin-window energy dispersive X-rayspectrometer and elemental atomic concentrations in each particle were calculated based on a Monte Carlo simulation program using the quantitative electron probe X-ray microanalysis.The particles were identified and classified according to their morphology and element composition.The results showed :(1)Black carbon aerosol(soot)is composed of beaded the carbon spheres,atoms of carbon and oxygen concentration is greater than the sum of 90%,and the C content is three times more than O.According to different degrees of aging,there were four types,i.e.floc or spreading structure,branched chain,agglomeration,and impaction(aggregation inlaid with or adhered to other substances),each of the elongation(E),the aspect ratio(R),the concentration of C and O,C and S atoms than all have significant difference(P < 0.05).(2)The relative abundance of black carbon aerosols ranged from 10% ~40%,and those with particle size of less than 1?m accounted for more than 75%.The relative abundance of black carbon aerosols was greater in the country center than in the country edge and in the winter than the autumn.(3)During the haze period in 2018,floc or spreading structure and impaction black carbon particles were more than the agglomerated and branched carbon particles,and the embedded black carbon particles mostly appeared in the daytime.During periods of severe haze in 2011,the agglomerated and branched black carbon particles gradually increased and mostly appeared at night,and the chemical composition became complex.(4)The X-ray energy spectrum of secondary aerosols is mainly composed of the spectral peaks of four elements C,N,O and S(sometimes the peak of N cannot be observed).After X-ray measurement,the brighter round or oval particles will disappear or change in shape;Morphology and C,N,O,S elements atomic concentration of secondary aerosols with the haze condition and type.During the severe haze,the content of C and S increases,and the secondary electron image appear as blackened droplet,and most of them appear at noon and midnight.(6)the relative abundance ofsecondary aerosols increases with the decrease of particle size.The amount of particles with Particle size under 0.5 ?m>with particle size0.5 ~ 1?m > with particle size 1 ~ 2?m.In addition,the relative abundance of secondary particles in the central area of the city is greater than that in the edge of the city,and the haze period is greater than that in the non-haze period.The above results show that the low atomic number particle electron probe microanalysis technology can well analyze the change process of black carbon and secondary aerosols in the atmosphere,and the pollution characteristics could be inferred from their physical and chemical characteristics and content,revealing the generation and interaction rule of particles and their transformation mechanism,and providing scientific basis for the treatment of air pollution.