| Dustfall, as a good carrier and transporter of air pollutants, especially urban particulate matters, it covers a large amount of environmental information. Dustfall in urban area is a potential threat to the urban ecological environment and residents’ daily lives, or even directly endanger to human health, and it affects the sustainable development of the national economy. Lanzhou city, an old industrial city, is one of the most seriously polluted cities in China, its environmental problems are more serious, particularly air pollution problems. These are due to the special geographical position, topographical situation and meteorological situation, unreasonable industrial distribution. To investigate the impact of dustfall to urban environment in Lanzhou, we set up the fixed sampling points from April in2007. These samples were collected monthly during April2007to June2011at Lanzhou University. In addition, we have collected nine topsoil samples and two dust storm samples, a total of45samples.Systematic measurements and analysis were carried out on the samples we collected. The study of the environmental magnetic method is subject, combined with the analysis of the related data, such as the geochemical elements, organic matter content, intensity of inversion and the environmental protection bureau’s data. The magnetic parameters (χ1f, SIRM,χARM, 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 a small amount of iron hydroxides, hematite and paramagnetic minerals. The χ1f value of dustfall is totally higher in Lanzhou, even if its minimum χ1f value is also obviously higher than the topsoil and dust storm samples’ in Lanzhou, indicating that the dustfall mainly affected by anthropogenic sources in Lanzhou; and the χ1f value in winter is clearly higher than that in other seasons, suggesting that the dustfall in winter is more seriously polluted by artificial pollution. Previous studies have shown that χ1f value in Lanzhou dustfall has significant variations during the year. The value of χ1f is distinctly high in the heating months, while low in other months. Although March is also in the heating period, its χ1f value is obviously lower than other that of heating months. That might be due to the unique inversion phenomenon of Lanzhou basin: low intensity of ground inversion being conducive to the diffusion of pollutants. Therefore, the pollution intensity of Lanzhou in winter is not controlled by one single factor, but by dual factors of heating and intensity of inversion. April and May have the lowest values throughout the year, the reasons could be1) low intensity of ground inversion, and2) dilution effect of frequently happening dust storms (with lowχ1f) in these months. The magnetic parameters (eg.χ1f,χARM, SIRM etc.) are consistent with the coal-fired identify elements on the year or inter-annual variations, and poorly consistent with the elements from traffic and industrial pollution, showing that Lanzhou is mainly polluted by coal burning, followed by traffic and industrial contamination. The results of geochemical elements indicate that pollutant is easily adsorbed on the fine particles of dustfall, the consistent change law of elements, organic content and magnetic parameters may be due to they are easily adsorbed on fine particulate matter, or their mutual adsorption results. The air quality of Lanzhou has been greatly improved in winter for last four years, especially in2010, the χ1f value is38%lower than that in2007, this is consistent with the monitoring data of the Environmental Protection Bureau, indicating that environmental magnetism can effectively monitor urban pollution. |
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