Magnetic properties and Mossbauer spectra of urban atmospheric particulate matter: a case study from Munich, Germany

Due to the high concentrations of iron in anthropogenically-derived atmospheric particulate matter (PM), magnetic techniques are ideal for the study of the behaviour of PM. However, to undertake these studies it is necessary to have a thorough understanding of the magnetic mineralogy of PM. This study reports a detailed examination of the magnetic mineralogy of PM samples collected in Munich. Several different approaches have been utilized in the characterization; from standard mineral magnetic techniques such as magnetic hysteresis and microscopy to non-standard techniques such as Mossbauer spectroscopy. In particular, the study has focused on the smallest magnetic grains (<30 nm), which are mostly likely to be inhaled into the human lung. These have been characterized using low-temperature magnetic and Mossbauer measurements, in conjuncture with the standard frequency dependency of magnetic susceptibility measurements. The primary magnetic minerals were found to be maghemite and metallic iron. The mean magnetic grain size range for these two minerals was 0.1-0.7 mum. The magnetic signature was estimated to come from 60-70 per cent maghemite and the rest from metallic iron. A significant percentage of maghemite in the grain-size range 10-16 nm was identified, and there was evidence for metallic iron in the grain-size range 5-8 nm. This latter fraction is of particular importance as transition metal particles <10 nm in diameter are known to catalyse the production of free radicals which are known to cause many lung complaints such as asthma. It was difficult to give accurate estimates of the total magnetic content by mass due to the mixed mineralogy, however, the absolute limits for the total magnetic content was estimated to be between 0.02-1.2 per cent. It is suggested that the maghemite comes from automobiles and the metallic iron from the street-trams which run near the sampling site.