| Summary: | Hazards related to particulate matter (PM) in subway systems necessitate improvement of the air quality. As a first step toward establishing a management strategy, we assessed the physicochemical characteristics of PM in a subway system in Seoul, South Korea. The mean mass of PM<sub>10</sub> and PM<sub>2.5</sub> concentrations (<i>n</i> = 13) were 213.7 ± 50.4 and 78.4 ± 8.8 µg/m<sup>3</sup>, with 86.0% and 85.9% of mass concentration. Chemical analysis using a thermal⁻optical elemental/organic carbon (EC⁻OC) analyzer, ion chromatography (IC), and inductively coupled plasma (ICP) spectroscopy indicated that the chemical components in the subway tunnel comprised 86.0% and 85.9% mass concentration of PM<sub>10</sub> and PM<sub>2.5</sub>. Fe was the most abundant element in subway tunnels, accounting for higher proportions of PM, and was detected in PM with diameters >94 nm. Fe was present mostly as iron oxides, which were emitted from the wheel⁻rail⁻brake and pantograph⁻catenary wire interfaces. Copper particles were 96⁻150 nm in diameter and were likely emitted via catenary wire arc discharges. Furthermore, X-ray diffraction analysis (XRD) showed that the PM in subway tunnels was composed of calcium carbonate (CaCO<sub>3</sub>), quartz (SiO<sub>2</sub>), and iron oxides (hematite (<i>α</i>-Fe<sub>2</sub>O<sub>3</sub>) and maghemite-C (<i>γ</i>-Fe<sub>2</sub>O<sub>3</sub>)). Transmission electron microscopy images revealed that the PM in subway tunnels existed as agglomerates of iron oxide particle clusters a few nanometers in diameter, which were presumably generated at the aforementioned interfaces and subsequently attached onto other PM, enabling the growth of aggregates. Our results can help inform the management of PM sources from subway operation.
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