A mobile measurement system for complex characterization of particulate matter (PM) was developed together with the proposed methodology and applied in the subway system of Munich, Germany. The main objectives were to observe the spatio-temporal variability of PM, personal exposure, identify hot-spots and pollution sources. Particle mass (PMx) and number (PNC) concentrations, and equivalent black carbon (eBC) were measured at 0.1–1 Hz. On the U5 subway line, PM10, PM2.5 and PM1 concentrations at platforms ranged from 59 to 220, 27–80, and 9–21 μg m−3, respectively. During rides towards downtown, average PM10, PM2.5 and PM1 levels gradually increased from 8 to 220, 2 to 71 and 2–20 μg m−3, respectively, with a similar dynamic of decrease on the return journey. Spatial variability of PM was generally more important than temporal, and significant differences were observed between platforms. During the rides, air exchange between train and tunnel was high in both air-conditioned and old passively ventilated trains. Peak PM concentrations on platforms were associated with arriving/departing trains. Subway PNC were not significantly elevated, but a few cases of intake of traffic-related particles from outside were observed, otherwise air exchange was considered low. Generally, most of the aerosol mass was composed of iron corrosion products from rails and wheels (Fe up to 66 μg m−3 in PM2.5). The effective density of PM2.5 was 2.1 g cm−3. Particles were classified as 75.4% iron oxides, 5.35% metallic Fe, 1.23% aluminosilicates and 17% carbon and oxygen rich particles. The iron oxide particles consisted predominantly of Fe (63.4 ± 8.7 wt%) and O (36.2 ± 8.2 wt%). To effectively monitor subway PM and reduce overall PM exposure, we propose to identify hot-spots using our methodology and focus on improving their ventilation, as well as installing filters in air-conditioned wagons.


Graphical Abstract

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