Magnetic Properties Of Dust And Topsoil In Beijing, And Their Environmental Implications

The aim of this study is to investigate the influence of fallen dust on the surface environment of Beijing. Dust and topsoil samples were collected from 63 locations at 2-2.5km intervals from west to east, north to south, northwest to southeast, and northeast to southwest along the main roads of urban and suburb inside the Six Ring Road of Beijing. Dust samples were swept from the surface of road side with nylon brush at about 5m~2, and topsoil samples were dug respectively at 1cm and 5cm depth, which were within the dust collecting area. Samples were dried at room temperature in the laboratory and through a 0.84mm nylon sieve. Dust storm samples were collected near ground on 17 and 18 April 2006 around the campus of China University of Geosciences (Beijing) . Environmental Magnetic measurements were carried out on all urban dust, topsoil and dust storm samples.The magnetic parameters of samples indicate that there are several differences between the dust samples and topsoil samples at each site, but no significant differences between the topsoil samples of different depth. The mass susceptibility and saturation isothermal remanent magnetism (SIRM) of dust samples are higher than that of topsoil on each site. The temperature dependence of magnetic susceptibility (x-T curves) and S-ratio suggest that all samples are dominated by ferrimagnetic minerals, likely magnetite and some antiferromagnetic minerals (such as iron oxyhydroxides), in both dust and topsoil samples. The results of anhysteretic remanent magnetism (ARM) and hysteresis parameters, which could be used as an indicator of the magnetic grain size, indicate that the size of magnetic particles of dust samples may be bigger than that of topsoil. Dust samples are dominated by multidomain grains, and topsoil are mainly in pseudo-single domain grains. Furthermore, there is metallic iron in some dust samples, but none in topsoil samples. There is a positive correlation of mass susceptibility and saturation isothermal remanent magnetism of topsoil and dust at each site, which suggest that the particles in dust and topsoil have the same source or have some spatial correlations.The composition, concentration and granulometry of magnetic minerals in dust storm samples are similar to topsoil's, but different to dust's. Comparing the hysteresis parameters of dust, topsoil and dust storm samples to natural materials such as Chinese paleosol and loess, we figure out that the dust samples from urban were quite different from the natural material samples. The size of magnetic particles in dust storm is bigger than topsoil's but smaller than urban dust's, which indicate the dust storm sample were mixed well with pollution substances during the transport and falling. The magnetic parameters of the topsoil being similar to that of natural materials may indicate that the material in topsoil has decontaminated the anthropogenic particles in urban dust. The magnetic differences between the urban dust and topsoil may be caused by solutions and/or micro organic activities in the topsoil.