Magnetic properties of urban dustfall in Lanzhou, China, and its environmental implications

Lanzhou has been one of the most seriously polluted cities in the world due to its special geographical location, weather conditions and large amount of emissions from coal-fired factories. These factors make it an ideal place for urban particulate pollution study. In this study, a set of environmental magnetic parameters (χlf, χfd%, χARM, saturation isothermal remanent magnetization (SIRM), HIRM, SOFT and backfield IRM) were measured on dustfall samples collected monthly during April 1997–May 2000 and November 2004–October 2005 at the Lanzhou University campus (KLD). Results show that the magnetic assemblage in the dustfall is dominated by pseudo-single domain (PSD) magnetite associated with maghaemite and haematite. Anthropogenic activities (mainly from coal burning) are the main sources for urban particulate pollutants in Lanzhou. The great efforts of the government and local authority in reducing local industrial and domestic emissions during the past several decades has led to improved air quality in winter in Lanzhou; however, the air quality in summer has not been significantly improved.