| Street dust,as an important medium of pollutants in the urban area,is an important source of house dust and urban atmospheric particulate matter.Street dust has received much attention due to its potential and direct risks to the ecosystem,such as soil and water system,and human health.Lanzhou is one of the key industrial cities after the founding of the nation.The severe air pollution problems of Lanzhou are not only caused by the large emission from industrial and domestic pollution sources,the special geographical position, topographical condition and meteorological condition,unreasonable industrial layout,and also affected by industry,agriculture,traffic and other multiplex human activities. Lanzhou has become one of the most seriously polluted cities in the world.Selecting street dust in Lanzhou as the research object,a total of 178 dust samples were collected in different functions regions of Lanzhou in different seasons by a large-scale field survey.The high-resolution magnetic and geochemical research were carried out,including magnetic measurements,grain size,element contents,organic matter content etc.,basing on the geosciences statistical software.(ArcGIS),the temporal and spatial distributions of magnetic properties,geochemical elements(heavy metals), grain size and organic matter content were discussed;the pollution sources,main pollutants and polluted characteristics of the urban street dust were analyzed as well.In addition,understand the relationship between the magnetic parameters and the organic matter content,heavy metals content were understood by the Mathematics and Statistics Software(SPSS).The main conclusions are as follows:Through the research of the dust particle size analysis,basic characteristics of the particle size were revealed,we found that the main dust of Lanzhou city streets is sand or below silt sand level(＜63μm) of the particles,the size distribution was mainly double-peak and multi-peak normal distribution.With greater wind power,regional average size was coarser,and the sorting feature was poor;On the other hand,in the region with small wind energy,mainly fine particulate matter.The spatial distribution was characterized as below,dust samples with coarse particle size in the east and west streets, finer particle size in the middle streets.The results of element content analysis showed that street dust of Lanzhou City mainly contained Si,Al,Ca,Fe,Na,Mg,K,S,Ti,Cl,P,Mn and Ba etc.,the average content of the former seven elements accounted for 79.22%of total content,rest of the elements with relatively small percentage.The elements content had a significant difference in the season;the winter heating period with increased coal-fired,the special geographical features of Lanzhou City,as well as strong winter temperature inversion phenomenon were the two main factors which caused seasonal differences characteristics of.In the spring,the element content increment of the soil in the street dust has a close relationship,with the wind and dust storms.The spatial analysis results showed that the spatial distribution difference of the urban street dust elements was significant: Chengguan district and the Xigu district's pollution are serious while Anning district is better.Xigu District is a high-value area of unknown-source elements,such as Bi and Cs, which were initially inferred from industrial pollution.Through analyzing the organic matter content of the street dust in the study area,it is revealed that Lanzhou street dust contained higher average levels of organic matter,with the distinct seasonal-spatial distribution difference,the highest value in winter and higher in spring,the lowest in autumn;the organic matter content was from high to low in Chengguan,Xigu,Qilihe and Anning district.The results showed that the organic matter of Lanzhou street dust may also come from industrial pollution,automobile exhaust emissions and a variety of pollutants from the daily life of human beings besides coming from soil particles.Magnetic measurements indicate a high concentration of magnetic minerals in Lanzhou street dust,the mainly magnetic minerals are magnetite with a small amount of hematite and magnetic minerals.The magnetic properties of samples were mainly controlled by the ferrimagnetic minerals and a small amount of incomplete antiferromagnetic minerals,ferrimagnetic minerals dominated by pseudo-single domain (PSD) magnetite and multi-domain particles(MD).Magnetic parameters(Xlf,SIRM,XARM, HIRM,SOFT and SOFT%),which were related to the concentration of magnetic materials is distinctly high in winter and spring,low in autumn.Similarly,higher concentrations associated with heavy industry,concentrated residential development,and vehicular traffic suggest mixed contributions of magnetic material from both anthropogenic and natural sources.However,the parameters which reflected the characteristics and types of the magnetic mineral grain,such as Xfd%,S-ratio,XARM/SIRM,SIRM/Xlf and XARM/Xlf.,got a small change.On the one hand,due to the special geographical location of Lanzhou,terrain,weather conditions,the content and particle size of the magnetic minerals were different in different season and sampling points;On the other hand,because of unreasonable industrial layout,the various impact of human activity on the urban environment,which led to different forms of migration, transformation,dissolution of magnetic minerals of the street dust,and the magnetic characteristics was also affected.The analysis of a single-element of variance(ANOVA) of magnetic parameters showed that:many magnetic parameters,Xlf and SOFT%are effective magnetic parameters that denote seasonal differences among magnetic properties in street dust,convenient and economical methods for monitoring street dust pollution.Through analyzing the correlationship of the magnetic parameters and the organic matter content,it is showed that six main magnetic parameters which reflected the magnetic mineral concentration,such as Xlf,SIRM,HIRM,SOFT,XARM and S-20,had a very significant positive correlation with the content of organic matter,but no relation with other magnetic parameters.The positive correlation between XARM and oganic matter content indicated the contribution of single domain magnetic particles to organic matter; at the same time,the significant correlation with the SIRM,HIRM and SOFT has shown the contribution of ferimagnetic particles to the organic matters of street dust.Therefore, magnetic parameters can be used as substitutive indexes of organic matter content in street dust.The results of the heavy metal elements analysis,such as arsenic(As),bismuth(Bi), chromium(Cr),copper(Cu),manganese(Mn),nickel(Ni),Lead(Pb),titanium(Ti),zinc (Zn),iron(Fe),showed that Bi and Cr had a consistent change trend,the overall content level of the western region is relatively higher than the east,which is the further evidence of its relation with industrial activities;As,Cu,Pb Zn also showed the same trends and changed in a larger range,it is inferred that the change of the road communication condition induced this phenomenon,other heavy metal elements(Mn,Ni,Ti,Fe) had shown different trends separately.The correlation analysis of magnetic parameters and heavy metals of street dust showed that heavy metal elements(As,Cu,Mn,Ni,Pb,Zn,Fe) content had a significant positive correlation with the magnetic parameters(Xlf,SIRM,HIRM,SOFT,XARM)which reflected the magnetic particles content,The negative correlation between ratio parameters (XARM/Xlf,XARM/SIRM) and heavy metals content reflected the influence of the size composition of sediment on the heavy metal content.Xfd%had a negative correlation with Cu,Ni,Zn and Pb,but unconspicuous correlation with other heavy metals.S-ratio had a positive correlation with Cu,Pb,Zn and Fe,but inconspicuous correlation with other heavy metals.It indicates that the magnetic components of street dust are interdependent with some heavy metal elements,so magnetic parameters can be used as an index of these heavy metals to survey the pollution status,and monitor the industrial and traffic pollution. And it could be used to track the variation of the heavy metal and the distribution of pollutants.The research showed that the comprehensive analysis of the magnetic parameters can be used to reveal the distribution situation of the street dust pollution,determine the source of pollution,analyze the response extent of human activities and extract pollution information effectively basing on acquiring a large number of experimental and using multi-element statistical methods,geographical statistical techniques with the appropriate geochemical analysis.In addition,comparing with other study objects,street dust has the features of the large spatial scale and the smaller time scale in the environmental record. As a result,collecting and analyzing the environmental magnetic information of the street dust periodically can reflect sensitively changes features of the urban environment pollution.In short,the study showed that magnetic and geochemical analysis of the street dust, is a new,economic,rapid and effective method of carrying out large-scale,high-resolution and magnetic research under the urban environments,this new method can quickly,easily and economically provide the pollution information. |
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