Differences between PM_2.5 chemical sources at semi-closed transport environments: comparing bus depots and metro...

Chemical composition and source contributions of PM_2.5 were investigated at a public-restricted bus depot in Valladolid (Spain) and two metro stations (Alto dos Moinhos and Quinta das Conchas) in Lisbon (Portugal) using positive matrix factorization (PMF). Locally derived sources dominated PM_2.5 in both environments, with OC-rich combustion emissions prevailing at the bus depot and Fe-rich wear processes at the metro stations as major components. Trace element signatures (ng m^-3) differed accordingly, with Zn dominating at the bus depot, reflecting tire wear and maintenance, and Mn prevailing at the metro stations, indicative of metro wear (e.g. rail-wheel-brake wear). Seven sources were resolved at the bus depot, led by mineral/construction (27.6%), followed by secondary sulfate (15.8%), tire wear (14.8%), exhaust (11.5%), washing/resuspension (11.4%), maintenance (9.9%), and brake wear (8.9%). However, both metro stations were dominated by metro wear emissions (∼45-46%), with additional contributions from sea spray (∼16-17.5%), secondary sulfate (8.4-16.5%), and mineral/construction (11.5-20.6%). Traffic-related infiltration was identified only at Alto dos Moinhos (8.1%), whereas biomass burning was resolved at Quinta das Conchas (9.7%). The different source profiles indicate the need for specific exposure-control strategies targeting indoor emissions, particularly combustion- and activity-related sources at the bus depot and mechanically generated metal-rich wear particles at the metro stations. Source-specific responses to air purifier operation suggest that filtration can complement source-control measures, although its effectiveness may depend on particle characteristics, spatiotemporal distribution, and continuous source generation. These findings support targeted air quality management in semi-closed transport environments.