Spatiotemporal Variations and Sources of Airborne Particulate Matter in Urban Metro Platforms and Carriages in China

Since 2013, China has significantly improved its control over atmospheric air pollution. However, air pollution in indoor environments, particularly in urban metro systems, has been largely neglected. In this study, we comprehensively investigated the spatiotemporal variations in particulate matter (PM) pollution inside metro stations and carriages across seven Chinese cities. Our results revealed consistently high levels of PM2.5 (16–621 μg/m3) within the urban metro platforms. Elevated concentrations of metro PM2.5 can occur even under low outdoor PM2.5 conditions. Furthermore, we found that metro PM2.5 levels exhibited significant spatiotemporal variations across metro environments; specifically, a higher fraction of PM2.5 was present in metro carriages than on platforms. Notably, black carbon (BC), a critical toxic constituent of PM2.5, exhibited significantly higher concentrations in carriages than those on platforms. Real-time monitoring and single-particle analysis revealed distinct sources of metro PM compared to outdoor air PM; particularly, Fe/Ni/Cr/Mn particles mainly originated from the abrasion of rails and brakes, while Cu-bearing particles and BC likely result from the degradation of conductive carbon brushes and wires. Our analysis demonstrated that irregular iron-containing particles derived from rail abrasion constituted approximately 90% of platform PM. These findings highlight the pressing need for air quality control in metro systems and provide valuable insights for public health assessments related to exposure to metro air.