Magnetic, Geochemical, and Microstructural Characteristics of Road Dust on Roadsides with Different Traffic Volumes—Case Study from Finland

The combination of magnetic, geochemical, and microstructural methods was used to characterize road dust properties in roadside soils of two sites in southern Finland: a highway (Tuusula no. 45, high traffic volume) and a local road (Mikkeli no. 13, low traffic volume). Significant differences in horizontal (κ) and vertical (χ) distribution of magnetic susceptibility were observed in the investigated roadside soils. These variations were concluded to be mostly associated to traffic volume, which is considered to be a major factor that determines the degree of particle emissions derived from vehicle traffic. Magnetic parameters of hysteresis, isothermal remanence magnetization (IRM) acquisition curves, and thermomagnetic data indicated coarse-grained (pseudo-single-domain/multi-domain) magnetite as the primary magnetic carrier. Scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDX) analyses identified two groups of magnetic materials: angular/aggregate particles (diameter ~2-100 μm) derived from circulation of motor vehicles (e.g., vehicle exhaust emission, cycling of dust in suspension due to vehicular movement, dispersion of road construction materials, and abrasion of tires, brake linings, and road surface), and magnetic spherules (d ~ 3-15 μm) possibly originating from industrial and domestic heating systems. Concentrations of selected trace elements Cu, Zn, and Pb in highway roadside soils were significantly higher than those in local roadside soils.