Characteristics of Fine Particulate Variations in the Underground Subway Platforms: A Case of Seoul

This study aims to investigate the potential relations affecting indoor air quality (IAQ) of underground subway platforms based on the hourly PM2.5 monitoring data from subway platforms and air quality monitoring station (AQMS) as well as passenger statistics in 2022. Among more than 250 subway stations in lines 1 to 8 of Seoul subway, (1)-Jonggak; (2)-Euljiro1(il)-ga; (3)-Irwon; (4)-Mia; (5)-Gwangnaru; (6)-Beotigogae; (7)-Suraksan; (8)-Songpa were targeted recording the highest annual PM2.5 level in each line. PM2.5 in the underground subway platforms generally showed a significant correlation with the number of passengers by frequent train services during the station operation time, and thus higher-level of PM2.5 was observed during rush hours. Otherwise, the influence of ambient air to subway platform IAQ was more predominant in the station closing time. Eventually, the variation of PM2.5 level in the underground subway platform was mainly determined following the degree of correlation with internal (train frequency and passenger) and external (outdoor air quality) influence factors as temporal variations. In case of specific period such as the highest indoor PM2.5 day and the largest passenger day, the correlations between platform PM2.5 and passenger were increased about 46 similar to 3% and 13 similar to 78%, respectively than the normal period. For high-level PM2.5 (PM2.5 warning) period, indoor PM2.5 level was raised up to 85% comparing normal period. This aspect was also similar on seasonal variations case, because seasonal mean PM2.5 levels in the subway platform showed parallel trend with seasonal mean outdoor air qualities. Therefore, this study would be useful to confirm characteristics of PM2.5 variations in the underground subway platform for further subway IAQ management.