Abstract
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 PM10 and PM2.5 concentrations (n = 13) were 213.7 ± 50.4 and 78.4 ± 8.8 µg/m3, 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 PM10 and PM2.5. 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 (CaCO3), quartz (SiO2), and iron oxides (hematite (α-Fe2O3) and maghemite-C (γ-Fe2O3)). 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.
Highlights
Subway systems relieve traffic congestion in metropolitan areas as an environmentally friendly means of transportation [1,2,3,4,5]; human exposure to air pollutants in subway systems is a concern [6]
The normal concentration of carbonate estimated by ion balance was converted into mass concentration, and the carbonate carbon (CC) concentrations were subtracted from the organic carbon (OC) concentrations to avoid overlap with the carbonate estimated from the ion balance in Equation (1) [29]
These results indicated that Particulate matter (PM) in the subway tunnels
Summary
Subway systems relieve traffic congestion in metropolitan areas as an environmentally friendly means of transportation [1,2,3,4,5]; human exposure to air pollutants in subway systems is a concern [6]. Reduce the risk of accidents and level increase platform significantly reduced [15]; the accumulation of PM in the tunnel operationisefficiency of heating, ventilation, and air-conditioning systems [14,15]. Systems interface and outflow via electrical the ventilation in PM the of 6.98 and 165.5 nm is emitted from the wheel–rail during brakingsystem [2] This tunnel into tunnels the city.could penetrate the subway cabin via heating, ventilation, and air-conditioning in subway. Information properties and sources of PM the in ventilation subway tunnels needed to and NO transported from the outdoor air through systemis[2,16,28,29,30,31,32]. We characterized the PM in subway tunnels beneath Seoul, Korea
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