兰州市街道尘埃磁学特征及其环境意义

兰州市是全球大气污染最为严重的城市之一，为城市颗粒物污染研究的理想场所.分析了兰州市街道尘埃样品的磁学参数(包括磁化率、无磁滞磁化率、等温剩磁、磁滞回线和热磁曲线)变化特征.结果表明，兰州市街道尘埃中磁性矿物含量较高，主要磁性矿物是铁磁矿、磁赤铁矿和赤铁矿，并伴有少量顺磁性矿物；磁性矿物粒度主要是准单畴（PSD）；市区污染春季较夏季严重，春季高频率的强沙尘暴可能带来了更多的磁性矿物；兰州市污染物的沉积和分布在很大程度上受到气象条件和地形地貌格局的影响，市区每个盆地东端高于其它地区；兰州市污染以人为活动产物为主，人为活动贡献约为81.9%，其中包含交通约22.6%的贡献，自然降尘贡献约为18.1%；特殊的地形和气象条件，使得兰州市区的大气环境容量比平原城市的环境容量小得多，进而导致严重的大气污染.

Magnetic Characteristics of Street Dust in Lanzhou and Its Environmental Significance

Lanzhou has been one of the most seriously polluted cities in the world due to its special geographical location and weather conditions, which make it an ideal place for urban pollution studies. A set of environmental magnetic parameters (chi lf, chi fd%, chi ARM, chi HIRM, SOFT and back field IRM) of the street dust sampled in Lanzhou during spring and summer 2005 were analyzed. These samples were collected along a main street across the urban area and a mountain route from foot to the top. Results demonstrate that the main magnetic minerals in the street dust are magnetite, maghaemite and haematite with some paramagnetic minerals. Plots of Mrs/Ms vs. Bcr/Bc and chi fd% vs. chi ARM/SIRM indicate that the main grain size of magnetic minerals in the street dust are pseudo single domain (PSD). The results reveal that the Lanzhou city is more polluted in spring than in summer. The main pollution sources are anthropogenic activities (81.9%) and natural dust (18.1%), especially the former. The spatial distribution of pollution is controlled by the topography and weather conditions.