Magnetic response to air pollution recorded by soil and dust-loaded leaves in a changing industrial environment

Linfen city is one of the World's most polluted cities due to uncontrolled industrial activities of coal combustion releasing huge amounts of heavy metals (HMs) and polycyclic aromatic hydrocarbons (PAHs) into the atmosphere. We used soil and leaves as receptors for atmospheric particulate matter to test the efficiency of magnetic approach for assessing and discriminating past and present pollution. The results indicate that strong magnetic particles in topsoil and leaf samples are mainly low-coercivity magnetite, occurring in a larger grain-size range than in background soil, which is helpful to separate anthropogenic and natural sources. Topsoil magnetic signals reflect pollutants, which accumulated over the last decades. Differences in the vertical distribution of magnetic properties between undisturbed and disturbed (cultivated) soil profiles show that the plowing depth is the most important factor for migration of pollutants in cultivated soils. Magnetic susceptibility (MS) values of leaf samples reflect the present state of pollution and can even trace seasonal changes. Spatial maps of MS identify differences of the past and present environmental conditions caused by the shutdown of industrial sites within the last decade. Correlation coefficients between analyzed HM contents (Fe, Cr, Ni, Cu, Pb) and MS values are significantly positive in leaf samples, and still moderate in topsoil samples. Our results demonstrate the practical and economical value of magnetic techniques for pollution assessment, also for the studied case with a complex pollution history, a relatively high magnetic background and disturbing land use.