兰州市大气降尘磁学特征及其环境意义

Magnetic Properties of Dustfall in Lanzhou City and Its Environmental Significance

对兰州市大气降尘进行系统的环境磁学测量及分析。结果表明,兰州市大气降尘中磁性矿物总体含量较高,以假单畴磁铁矿为主,并伴有少量的磁赤铁矿、纤铁矿及赤铁矿,而且它们主要是来自人类活动产生的污染。过去研究表明,磁性矿物含量年内变化特征显著,冬季污染值高是由于供暖所致,但3月份虽然处于供暖期,它的污染值却明显低于其它供暖期,我们认为主要是兰州市特有的逆温现象:3月份贴地逆温强度低,利于污染物扩散所导致。4、5月份污染值全年最低,除了逆温层的作用以外,还与该月份频发的沙尘暴带来的低clf值的稀释作用有关。近年来,随着污染防治工作的开展,兰州市大气污染状况有了较大改善,2010年clf值比2007年降低了38%,证明环境磁学方法可以有效地监测城市污染。

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 （clf, SIRM, cARM, k-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 clf can effectively denote the concentration variations of magnetic mineralogy of polluted dustfall. Previous studies have shown that clf value in Lanzhou dustfall has significant variations during the year. The value of clf is distinctly high in the heating months, while low in other months. Although March is also in the heating period, its clf 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 clf） in these months. The air quality of Lanzhou has been greatly improved in winter for the last four years. In 2010, the clf value is 38% lower than that in 2007, indicating that environmental magnetism can effectively monitor urban pollution.