Magnetic enhancement of road dusts in Shanghai and its implications for the urban environment

Purpose Road dust samples in Baoshan District, Shanghai, were collected to explore magnetic and chemical properties of atmospheric dustfall in urban areas, intensively impacted by anthropogenic activities. Magnetic particles in road dusts were separated and analyzed to track their sources and then to discuss the influences of industrial and traffic emissions on the urban environment. Materials and methods One hundred twenty-two road dust samples in the industrial, traffic, residential, and agricultural areas of Baoshan District, Shanghai, were collected. Magnetic susceptibility (χ lf ) and heavy metal content of the samples were determined. Micromorphological and microchemical features of magnetic particles separated from the road dusts were analyzed by a scanning electron microscope (SEM) equipped with energy spectrum. Results and discussion The road dusts are usually alkaline and strong in magnetic signal, of which, magnetic susceptibility (χ lf ), 838.7 × 10 −8 m 3 kg −1 on average, is much higher than the nearby topsoils. Moreover, χ lf of the industrial and traffic road dusts, 1363.0 × 10 −8 m 3 kg −1 and 775.9 × 10 −8 m 3 kg −1 on average, respectively, is significantly higher than that of the others. Magnetic spherules, mainly composed of Fe oxides, were commonly observed in the road dusts, which are mostly formed during industrial high-temperature processes. A high number of flake-like, rod-like, and other irregular-shaped magnetic particles were also found in the road dusts, which may come from metal processing or vehicular wearing. The road dusts in the study areas are heavily polluted by Cu, Zn, Pb, Cd, and Cr. The principal component analyses (PCA) indicate that χ lf and Zn, Mn, and Fe contents in the road dusts belong to the same principal component. Conclusions Magnetic dustfall commonly occurs in urban areas due to industrial or vehicular emissions, which leads to the enhancement of magnetic signal and heavy metal content in urban road dusts simultaneously. χ lf can indicate the accumulation of toxic heavy metals in the road dusts effectively. This also highlights a fact that the urban environment is continuously and significantly affected by the deposition of artificial atmospheric magnetic particles.