Size-dependent characteristics of diurnal particle concentration variation in an underground subway tunnel.

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 PM10, PM2.5, and PM1, 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.