What are the differences between underground and aboveground metro stations in iron-bearing particles?

Iron (Fe)-bearing particle species (e.g., metallic Fe, magnetite, and hematite) play a critical role in determining toxicity, yet few studies compare aboveground and underground metro stations. This study employed multiple approaches, including magnetic measurements, scanning electron microscopy (SEM), and geochemical methods, to analyze the abundance and species of Fe-bearing particles in these environments. Results reveal that total Fe in underground platforms (23.2 ± 4.5 wt%) is approximately three times higher than in aboveground platforms (7.7 ± 0.9 wt%). The proportion of magnetite-derived Fe (Fe_mag) is significantly greater in underground stations (47 ± 4 %) compared to aboveground stations (22 ± 2 %), while silicate mineral-derived Fe (Fe_r) is more abundant in aboveground platforms (8.8 ± 0.9 %) than underground (4.2 ± 1.6 %). Multiple lines of evidence, including magnetic properties, SEM imaging, and Fe species analysis, confirm two primary sources of Fe-bearing particles in platform dust: urban topsoil/street dust and wheel/rail abrasion. Urban topsoil/street dust likely influenced aboveground stations, whereas underground stations were mainly impacted by wheel/rail abrasion. These findings offer key insights for toxicological research, emphasizing particle composition variability in metro environments.