Nanoscale physical and chemical properties of individual airborne magnetic particles from vehicle emissions

Abstract

Diesel and petrol powered vehicles are contributors to the magnetic fraction of atmospheric particles in the urban atmosphere. For an assessment of the health effects of vehicle-emitted magnetic particles, the mineralogical characters of particles present in engine exhaust vs. those produced by braking need to be understood. We collected magnetically separated individual particles directly from both brake wear and engine exhaust, from the air inside a car cabin while driving on a motorway, and from the ambient urban atmosphere. The morphologies, sizes, chemical compositions and spatial associations of magnetic particles with other constituents of vehicle emissions were characterized using scanning and transmission electron microscopy techniques. Magnetic separates typically contained particles that were aggregates of a large number of nanocrystals. Particles from engine exhaust, car cabin and urban air were similar, with diameters ∼1–4 μm, consisting predominantly of ∼10 nm-sized, randomly oriented crystals of magnetite and minor hematite. In contrast, brake debris particles were larger (∼1–10 μm in diameter) and contained mostly ∼7 nm-sized hematite, in addition to metallic iron and minor goethite. Particles in all samples comprised carbonaceous constituents besides iron oxides. In the exhaust and ambient particles carbon was present in both amorphous and semi-ordered forms, encompassing the nanocrystals, suggesting a high-temperature formation. In contrast, only amorphous carbon was found in brake wear. Besides the main components (Fe, O, C) several additional elements, including Mg, Al, Si, P, S, Cl, Sn, Ca, Mn, Zn, Cu, Pb and Zr occurred in the particles, with some minor differences between the distinct types of samples. Whereas the overall characteristics of magnetic particles in exhaust emissions were similar to those of ambient particles, the brake emissions produced slightly different particle types, particularly in terms of size and mineralogy. Thus, based on the qualitative assessment of particle properties in direct emissions and ambient air, a significant portion of magnetic particles in urban air appears to originate from vehicle emissions. Although the aggregate particles have sizes in the μm range, they still may represent a potential health risk on inhalation. Synopsis: This article discusses the nanoscale properties of magnetic particles from vehicle emissions, for a better understanding of their sources and health effects. • The magnetic fraction of vehicle-derived airborne particulate matter can be collected using a simple device. • Exhaust emissions may constitute a substantial source of magnetic particles in urban environments. • Magnetic fraction of brake wear predominantly originates from the corrosion of the brake disc. • The primary magnetic agglomerates are small enough to reach the lungs upon inhalation.