兰州市大气降尘磁学特征及其环境意义

Environmental magnetic measurements were carried out on the samples of Lanzhou dustfall. These samples were collected monthly during April 2007 to June 2011 at Lanzhou University. The magnetic parameters (χ_1f, SIRM, χ_ARM, κ-T curves and magnetic hysteresis parameters) indicate a high concentration of magnetic minerals in Lanzhou, dominated by pseudo-single domain (PSD) magnetite, but also contain maghemite, lepidocrocite and hematite. They mainly come from pollution of human activities. The parameter χ_1f can effectively denote the concentration variations of magnetic mineralogy of polluted dustfall. Previous studies have shown that χ_1f value in Lanzhou dustfall has significant variations during the year. The value of χ_1f is distinctly high in the heating months, while low in other months. Although March is also in the heating period, its χ_1f value is obviously lower than other that of heating months, which might be due to the unique inversion phenomenon of Lanzhou basin: low intensity of ground inversion being conducive to the diffusion of pollutants. April and May have the lowest values throughout the year, and the reasons could be 1) low intensity of ground inversion, and 2) dilution effect of frequent occurrence of dust storms (with low χ_1f) in these months. The air quality of Lanzhou has been greatly improved in winter for the last four years. In 2010, the χ_1f value is 38% lower than that in 2007, indicating that environmental magnetism can effectively monitor urban pollution.