Abstrakt
People in big cities spend up to several hours a day on the subway to commute. Studies from all over the world have revealed that levels of non-exhaust metal-rich particulate matter (PM) on underground platforms and in tunnels are at health concerning levels that would often exceed ambient PM limits (Chang et al, 2021). However, only a limited number of subway systems have been comprehensively characterized. Due to significant differences between them, site-specific measurements would be required as a basis for effective measures. In this study, we optimized a methodology based on the use of a custom-built mobile system for identification of hot-spots, sources and monitoring of personal exposure during typical rides. Further estimation of PM sources and health risks has been obtained through analysis of elemental composition. The new system consists of a water-proof ventilated box with omni-directional inlets in the breathing zone, attached either to a stroller or a frame-rucksack. Dynamics of PM (OPS 3300, TSI), particle number concentration (PNC; DISCmini, Testo) and black carbon (eBC; MA200, Aethlabs) were measured in 1-10 s resolution and data were geo-referenced using GPS (64s, Garmin) or notes. PM was sampled by personal impactors (Sioutas, SKC) and filter holder for subsequent elemental analysis (ICP-MS, Agilent), gravimetry (Cubis 2, Sartorius) and scanning electron microscopy (GeminiSEM 360, Carl Zeiss) with an EDX probe for single-particle elemental analysis (Ultim Max 40, Oxford instruments). Our study was conducted in the Munich subway (Germany) and consisted of 1) mobile measurements and cumulative sampling during typical subway rides, 2) repetition of one-hour ride several times a day for spatio-temporal variability 3) platform measurements for elemental analysis and gravimetric OPS correction, and 4) reference measurements and sampling of the urban background for indoor/outdoor comparison. The highest average PM concentrations were found at the Hauptbahnhof transfer station with 220+-32, 72+-7 and 20+-0.3 µg m-3 for PM10, PM2.5 and PM1, respectively. The majority of the aerosol consisted of iron oxides from rails and wheel abrasion (Fe up to 66 µg m-3 in PM2.5). The effective density of PM2.5 was 2.1 g cm-3. Significant differences between platforms were observed, and the spatial variability in PM was generally more important than temporal. During the rides, air exchange between the train and the tunnel was high in both the new air-conditioned and old passively ventilated cars. Peak PM concentrations on platforms were associated with arriving/departing trains. PNC were not significantly elevated, but few cases of traffic-related particles intake from the streets above were observed. Our experimental mobile system is suitable for use in any subway system, providing comparable results. The system is useful for proposing targeted effective measures, such as the design of station ventilation and air conditioning filters in wagons. It can also be used for potential air quality control and is less demanding to operate and organize than conventional stationary measurements on platforms.
This work is funded by dtec.bw - Digitalization and Technology Research Center of the Bundeswehr [project MORE and LUKAS]. dtec.bw is funded by the European Union - NextGenerationEU. This research was also funded by the project ULTRHAS under the EU's Research and Innovation programme Horizon 2020, Grant Agreement No. 955390.
Reference: Chang, L., Chong, W.T., Wang, X.P. et al (2021) Environ. Sci.: Processes Impacts 23, 642-663.