Study On The Spatial Distribution Of Dust And Its Heavy Metals In Beijing And Its Rainfall Erosion Characteristics

The heavy metal pollution in runoff caused by street dust washoff has been an increasingly prominent problem in the context of rapid urbanization in China. Urban Dusts(UDs) is an important carrier of the diffused pollution. This study analyzed The UDs from two directions which containing the “level” and the “vertical”.Analyzing the partical size distribution of UDs in this area through studying the Urban Sediments in different functional areas of the “level” direction and different underlying surface of “vertical” direction. Through the evolution and innovation of the Contribution Rate(CR) of Heavy Metals,another new kind of method was discussed along height gradient.The results showed that with the increase of particle size, the concentration of Heavy Metals in the dust particles was decreased. The larger CR was mainly distributed in the partical size of 44-250μm, especially in the 44-105μm, and the CR of to the Heavy Metals was 45%;In dense commercial areas and major transport corridors, the CR of road traffic lane to heavy metals is the largest, but in the leisure area, the roof of the CR of heavy metals is the largest.The distribution trend of the UDs mass of the unit area under different underlying surface is:On the ground motor vehical, commercial area > main road > leisure area;At the bus stop level, main road > leisure area > commercial area;On the footbridge level, commercial area > leisure area > main road; At the top of the building, leisure area > main road > commercial area.The amount of UDs per unit area increased first and then decreased, then increased and then decreased in Shuangfeng, and the peak was between the partical sizeof 105-149μm.The heavy metal concentration decreased from "static" street dust to "dynamic" runoff particulate in the same diameter particles. Heavy metals in street dust were dissolved and extracted during washoff. The average loss proportion of the five metals(Cr, Cu, Ni, Pb, Zn) were 24.3%,56.8%,34.3%,22.8%,27.3%, respectively. The loss proportion increased with the decrease of the particle size of street dust. Proportion of extracted form dust was higher in street than that in washoff samples, which suggested some dissolved loss in water. In washoff samples, dissolved metals of waterphase did not have significant changes; however, heavy metals with particle state in waterphase reduced rapidly during runoff. Meanwhile, heavy metals of solid-phase particle reduced during runoff. Street dust with small particle size had higher loss rate during runoff. The variation rate of street dust loss among different particle sizes varied from 4.6% to 62.1%. Street dust with smaller particle size had higher migration ability in runoff, which was more risky to urban water pollution.Based on the morphology of the RAC risk assessment method for the analysis of heavy metal speciation we found that :street dust and runoff particles of heavy metals in RAC risk evaluation index the order: Zn > Ni > Pb > Cu > Cr, which change by the particle diameter. Zn belongs to high pollution, Ni belongs to moderate pollution, Cr, Cu, Pb belong to low pollution.