| Modern atmospheric dustfall is the production of dust’s derivation, transportation and sedimentation. And this process plays a remarkable effect on the atmosphere and the earth’s surface environment. Therefore, more and more attention had been paid on the study of this issue. The study of modern dustfall not only could helpful to find the source region of the dust sedimentation, but also can find out the origin of atmospheric pollution. Determine the factors of atmospheric dustfall, is conductive to the management of atmospheric environment. At present, a large number of studies on dustfall are concentrated in the north area of China, and are dominated on short-time surveillance. As known to all, area of the lower reaches of Yangtze River is belong to the humid monsoon climate, developed economy and intensive human activities, and there exists a big difference to the north in physical geography. Where is its source? What are the main control factors? In order to answer these questions, we need to investigate its change law, including the variation of temporal and spatial.For this purpose, we chose Nanjing, one of the most important industrial cities of this area, as the object of study. We took measures of multipoint, regular intervals and irregular intervals to collect samples and analyzed the change rules of its dustfall for the past 3 years. There are 3 locations belong to the city ground (CG), the roof of city high building (RHB) and suburb summit (SS), respectively. When we collected dustfall monthly (this is one type of the dustfall), we also took other 3 types, they are long-term dry dustfall, dry and wet dustfall in dust weather. From the analysis of the 4 types of dustfall in the aspects of flux, grain size, element and magnetic susceptibility, we revealed its temporal and spatial characterization and analyzed the probable factors. Moreover, on the foundation of those, I combined general atmospheric circulation and applied mathematical statistics method, such as enrichment factors (EF), factorial analysis (FA) and principal component analysis (PCA), to determine its source qualitatively or quantitatively. We got some conclusions and consequences as follows:First, on the aspect of flux of 4 types of atmospheric dustfall in this city, except the dry dustfall from normal weather (DDN), the largest is the dry dustfall from sand and dust weather (DDSD), the second is the wet dustfall from sand and dust weather (WDSD), the smallest is the dry and wet complex dustfall from normal weather (DWCDN), and their fluxes are 2.46 g/m2·d,0.49 g/m2·d, and 0.13 g/m2·d, respectively. It means that DDSD and WDSD are the accidential dust from external area. Furthermore, on the behalf of fluxes from different locations, it reveals the characteristic that CG is larger than RHB, and LA is larger than SS, and their fluxes are 56.35 g/m2·yr,50.86 g/m2·yr and 41.61 g/m2·yr, respectively. Obviously, it is the production of human activities, which lead to the release of dust. In terms of season flux, according to the number from large to small is spring, winter, summer and autumn. High level of flux in spring and winter may well reflect the influence of accidental dust, probably, it relates to the sand and dust storm. While, the monthly changes of flux are dominated evidently by the meteorological conditions, such as monthly total precipitation and wind speed.Second, all types of dustfall are relatively fine, composed evidently by 10-50μm and below 10μm these two component, which contribute 90%. Their grain size shapes are dominated by doublet, positive skewness, and very broad peaks. However, there are also many differences among them, especially the median size, mean size and skewness. On the mean size, DDN is the most coarse, DWCDN is second, then is DDSD, WDSD is the finest and from fine to coarse is SS, RHB,CG; spring, summer or winter, autumn. From the analysis of source characteristics and wind power these two main factors which play big roles on grain size, it indicates that the sedimentation mechanism is very complicated. Grain size feature is impacted by human activities, on the other hand, is controlled by sorting process and sedimentation patterns. It is worthily to notice that sorting process was influenced by the dust sources and the patterns was directly related to its settlement dynamics.Third, Major elements content are almost equal except SiO2, CaO, LOI in each types and locations. On the behalf of trace elements, the heavy metal elements’ content in wet dustfall is much higher than dry dustfall; it is related to the sedimentation mechanisms and properties. It’s not very hard to understand the pulling effect of rainfall and most heavy metals are inclined to fine particles. The concentration of Cd, Pb, Zn, Sn from high to low is DWCDN, WDSD, DDN, DDSD; RHB,SS,CG. High value appeared between June and September, especially July and September, while March or April is the minimum, especially April. From the analysis of REE distribution patterns, it’s obvious that although discrepancies in settlement model exist between DDSD and WDSD, they are come from similar area, probably come directly from desert-loess area in north of China. There are no differences between DDN and Xiashu Loess this area on the side of REE distribution patterns, it indicates that the restraining significance of Xiashu Loess as parent material for the dustfall in this area.Fourth, magnetic susceptibility value from maximum to minimum is DWCDN, WDSD, DDN or DDSD, and this feature may relate to settle models. High magnetic susceptibility means a higher magnetic mineral content, which contributed by the urban pollution. Magnetic susceptibility value from high to low is RHB, CG and SS, which may emphases the role of human activities. In one year, magnetic susceptibility reveals apparently doublet or double dip. High level values are summer and winter, July in summer, December, January or February in winter. Low level values are spring and autumn, April or October. Human activities and atmospheric or meteorological conditions are the main influencing factors.Fifth,53 species elements were analyzed in each type of dustfall by the way of enrichment factors (EF), I found that Cu, Zn, Mo, Cd, Sn, W, Pb and Bi 8 elements are severe to extremely severe enriched. In total, DDN and DWCDN are apparently higher than DDSD and WDSD, which indicates that DDN and DWCDN are contaminated seriously by man-made pollution, such as combustion of coal and petrol, non-ferrous metal metallurgy and petrochemical industry. According to the analysis above, atmospheric dustfall in Nanjing city could be treated as the mixture of 2 components in terms of different proportion, that is local source and nonlocal source, the former is characterize by high level content of contaminated particles and the latter is opposite. The mixed proportion of the two sources is dominated by climate and meteorology. Along with the addition of nonlocal dust, proportion of polluted particles is reduced in sand and dust storm weather. However, fine contaminated particles are almost completely eluted when it rains, which leads to a higher proportion of polluted particles. Moreover, a higher proportion of polluted particles also produced by a worse air condition.Sixth, DWCDNs’ grain size data was analyzed by the way of factorial analysis (FA), which based on the different dynamic sources of each place. CG has two factors, Fl and F2.F1 was particles below 10μm, its contribution proportion is 52.12%.F2 was particles 5-20μm, especially 10-20μm, its contribution proportion is 29.40%. RHB and SS have 3 factors, respectively. F1 and F2 are below 2μm and 2-10μm, and their contribution proportion is 28.80-34.10% and 34.19%-34.75%, respectively. F3 in RHB contributes 5-20μm, especially 10-20μm, and its contribution proportion is 27.00%. Because of contamination when sampling, F3 in SS is out of consideration. It is obviously that main contribution factors are extremely fine and fine particles, which indicates that urban dustfall is impacted evidently by the human production activities. Meanwhile, there are also some differences from one to another, and it’s the consequence of different settle intension to particles, which impact by precipitation and wind speed of different places.Seventh, elements were further analyzed by means of principal component analysis (PCA) and got 4 principal components. It means that DWCDN are composed of 4 types of source dust materials. First of all are dust material discharged from combustion of coal and petroleum, petrochemical industry, non-ferrous metallurgy and electronic industry. Second are the materials from UCC. Third, are the dust discharged from iron and steel industry. Least, are the Construction dust and long distance dust from the north of China. Their contribution proportions are 34.93%, 25.41%,11.11% and 6.46%. Therefore, I can draw the conclusion that the contribution proportion from nature source and artificial source are 25.41%-31.87% and 46.04%-52.50% in DWCDN. Furthermore, the source of DDN is similar to DWCDN, while it has a higher rate of construction dust. |
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