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Distribution features of particle size and heavy metal elements in foliage-captured dust

WANG Hui-xia1,SHI Hui1,LI Yang-yang2,ZHANG Ya-jing1,DU Hong-xia1,DU Heng1(1 School of Environmental and Municipal Engineering,Xi'an University of Architecture and Technology,Xi'an 710055,China;2 Institute of Soil and Water Conservation,Northwest A & F University,Yangling 712100,Shaanxi,China)  
This paper takes as its target to reveal the distribution features of particle size and heavy metal elements(Cu,Zn,Cr,Cd,Pb,Ni) in foliage-captured dust,by using the foliar dust samples of Ligustrum lucidum and Ligustrum quihoui collected from different areas of Xi'an,including its industrial area (IA),commercial and service area(CSA),heavy-traffic area(HTA),residential and educational area(REA) and comparatively pollution-free area(CPFA).Size distribution of the collected samples was determined with a laser particle size analyzer,while the concentrations of heavy metals in the samples were measured with an atomic absorption spectrophotometer.We have also discussed the likely sources of the heavy metals in the dust.The particulate size of the foliar dust proves to be less than 50 μm,and PM10(particulate matter with aerodynamic diameter less than 10 μm) makes up between 13.8% and 63.1% of the particles.There exist two peak values of the foliar dust for Ligustrum lucidum,with its mean and peak values of the dust size being highest in CPFA,and followed by IA,REA,HTA and CSA.For Ligustrum quihoui,however,the size distribution of the foliar dust proves to be normal,with its mean and peak value in the increasing order of CPFA,CSA,HTA,REA,IA.Heavy metals are significantly enriched in foliar dust,with its mean concentrations being(325.5±72.6) mg/kg for Cu,(3 965.6±1 112.9) mg/kg for Zn,(349.2±149.3) mg/kg for Cr,(35.3±6.8) mg/kg for Cd,(1 182.0±355.1) mg/kg for Pb,(324.1±129.5) mg/kg for Ni.Significant differences are found in heavy metals among different urban areas.By contrast,Cu,Zn,Cr,Cd,Pb and Ni concentrations in foliar dust prove no significant difference between Ligustrum lucidum and Ligustrum quihoui.The results of our investigation suggest that the contents of Zn,Pb,Ni and Cr in the foliar dust are the highest in IA with the greatest values observed in HTA for Cu and Cd.However,the contamination of heavy metals proves relatively minor in REA and CPFA.More fine dust particles are found with heavy metals accumulated in the leaves of the two plants.Heavy metals in foliar dust exceed those in background soil in Shaanxi to a factor of 7.9-20.8,29.7-77.9,2.6-11.1,26.4-71.8,6.9-18.9 and 262.8-489.4 for Cu,Zn,Cr, Pb,Ni and Cd,respectively,indicating that the foliar heavy metal dusts are likely to be caused by the industrial pollution,vehicle emission,and/or other anthropogenic sources.The proportion of heavy metal particles in the foliar dust proves to be mutually correlated,revealing the fact that the foliar dust indeed originated from the extraneous source.
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