Comprehensive Analysis of PM2.5 Concentrations in the Seoul Metro Underground Stations: Relationships with Indoor Sources and Outdoor Air Quality

This study examined the observed PM_2.5 concentration across 247 underground stations  consisting of Line-1 to Line-8 of the Seoul Metro from April 2021 to March 2023 in order to understand general characteristics of underground PM_2.5 air quality. Approximately, in one-thirds of underground stations (85 stations), annual averaged PM_2.5 concentration are over 35 µg m⁻³. Moreover, in 30 underground stations (approximately 12%), it exceeds 50 µg m⁻³, the recommended 24-hour maintenance standard for PM_2.5 concentration in underground stations. We found that PM_2.5 concentration is considerably influenced by both internal and external factors. Among the internal factors (i.e., depth, the number of passengers and operation frequency), the frequency of subway operation significantly affects changes in PM_2.5 concentration however, various internal factors may act in combination. In terms of external factor, there are positive correlation coefficients (r = 0.15–0.95) between daily averaged PM_2.5 concentration in underground station and that of the outdoor observatory closest to each underground station. In particular, in underground stations with high PM_2.5 concentration, the correlation with outdoor PM_2.5 air quality was low, suggesting that for better air quality in underground stations, we need to focus more on reducing the inherent emission from underground stations in highly polluted stations, but for less polluted stations, we need to improve outdoor air quality as well. We believe that this study may provide insights for effective future PM_2.5 air quality management in underground stations.