An Integrated Methodological Framework for the Characterization of Fine Urban Road Dust (<50 μm): Magnetic, Mineralogical, and Organic Components

Fine fractions of urban road dust represent an environmentally relevant and potentially hazardous component of particulate matter due to their high mobility, efficient atmospheric resuspension, and capacity to accumulate anthropogenic pollutants. This study provides an integrated mineralogical, physicochemical, magnetic, and organic components characterization of the finest fraction (<50 μm) of road dust collected from seven urban locations (V1-V7) in Vienna, Austria, with particular emphasis on sources of minerals and organic components of road dust.

Magnetic properties were assessed using mass-specific magnetic susceptibility (χ), frequency-dependent susceptibility (χfd%), anhysteretic remanent magnetization susceptibility (χARM), hysteresis loop parameters, and thermomagnetic κ(T) curves. Complementary mineralogical and physicochemical analysis included Scanning Electron Microscopy with Energy-Dispersive X-ray Spectroscopy (SEM-EDS), quantitative X-ray Diffraction (QXRD), Fourier Transform Infrared Spectroscopy (FTIR), Thermogravimetry coupled with Quadrupole Mass Spectrometry (TG-QMS), and elemental CHNS and TOC analyses, supported by multivariate statistics adapted for compositional data.

All samples showed a complete dominance (≈100%) of strongly magnetic material in the <50 μm fraction. Mass-specific magnetic susceptibility values ranged from 362 × 10⁻⁸ m³ kg⁻¹ (V4) to 911 × 10⁻⁸ m³ kg⁻¹ (V6), indicating substantial enrichment in ferrimagnetic particles. Frequency-dependent susceptibility values ranged from 4.1% to 6.5%, confirming a significant contribution of ultrafine superparamagnetic grains. Hysteresis loop parameters (Mrs/Ms = 0.057-0.089; Hcr/Hc = 4.44-6.18) and King plot relationships indicate a dominance of stable single-domain magnetite with a minor superparamagnetic fraction, characteristic of anthropogenic sources such as brake abrasion, fuel combustion, and industrial emissions. Thermomagnetic κ(T) curves revealed Curie temperatures consistent with magnetite and evidence of magnetic enhancement during heating, suggesting the formation of secondary magnetite from combustion-related precursors.

QXRD analysis showed a mineralogical composition dominated by carbonates and silicates, including quartz (24–46 wt%), dolomite (12-36 wt%), calcite (~10-15 wt%), feldspars (6-12 wt%), and muscovite (5-15 wt%), accounting for ~90 wt% of the samples. Minor phases included chlorite, kaolinite, amphiboles, and biotite (7-10 wt%), while iron oxides occurred below the quantitative detection limit of QXRD but were confirmed by XRD, FTIR, and SEM observations.

FTIR and TG-QMS analyses revealed abundant aliphatic C-H functional groups and hydrocarbon fragments indicative of organic matter derived from tire wear, asphalt binders, lubricants, fuel residues, and polymeric materials. TG-derived organic matter contents ranged from 3.0 to 8.6 wt%, closely matching TOC values (2.9-8.6 wt%). SEM-EDS provided direct evidence of microplastic particles, including carbon- and oxygen-rich fibers, irregular polymeric fragments, and significant amount of slag glasses (balls, rods).

The combined magnetic–chemical approach demonstrates that ultrafine road dust acts as an efficient carrier of strongly magnetic particles, C-H-rich organic pollutants, and microplastics. These findings highlight the environmental and health relevance of fine road dust as a vector for inhalable anthropogenic contaminants and emphasize the value of integrating magnetic indicators with mineralogical and organic analyses in urban pollution assessments.

This research was funded in whole by the National Science Centre, Poland under grant number 2021/43/D/ST10/00996.