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Understanding characteristics of diurnal particle concentration variation in an underground subway tunnel is important to reduce subway passengers' exposure to high levels of toxic particle pollution. In this study, real-time particle monitoring for eight consecutive days was done at a shelter located in the middle of a one-way underground subway tunnel in Seoul, Republic of Korea, during the summer of 2015. Particle mass concentration was measured using a dust monitor and particle number concentration using an optical particle counter. From the diurnal variations in PM 10 , PM 2.5 , and PM 1 , concentrations of particles larger than 0.54 μm optical particle diameter were affected by train frequency whereas those of particles smaller than 0.54 μm optical particle diameter were not changed by train frequency. Number concentration of particles smaller than 1.15 μm optical particle diameter was dependent on outdoor ambient air particle concentration level, whereas that of particles larger than 1.15 μm optical particle diameter was independent of outdoor ambient air due to low ventilation system transmission efficiency of micrometer-sized particles. In addition, an equation was suggested to predict the diurnal particle concentration in an underground tunnel by considering emission, ventilation, and deposition effects.

Given that around eight million commuters use the Seoul Metropolitan Subway (SMS) each day, the indoor air quality (IAQ) of its stations has attracted much public attention. We have monitored the concentration of particulate matters (PMx) (i.e., PM10, PM2.5, and PM1) in six major transfer stations per minute for three weeks during the summer, autumn, and winter in 2014 and 2015. The data were analyzed to investigate the relationship between PMx concentration and multivariate environmental factors using statistical methods. The average PM concentration observed was approximately two or three times higher than outdoor PM10 concentration, showing similar temporal patterns at concourses and platforms. This implies that outdoor PM10 is the most significant factor in controlling indoor PM concentration. In addition, the station depth and number of trains passing through stations were found to be additional influences on PMx. Principal component analysis (PCA) and self-organizing map (SOM) were employed, through which we found that the number of trains influences PM concentration in the vicinity of platforms only, and PMx hotspots were determined. This study identifies the external and internal factors affecting PMx characteristics in six SMS stations, which can assist in the development of effective IAQ management plans to improve public health., (Copyright © 2015 Elsevier B.V. All rights reserved.)