The air ventilation rate,
Composition of PM size fraction was highly influenced by the air ventilation rate, which is related to the depth of the subway station. Indoor PMx increased with outdoor PM10, and the ratio of PM1 to PM10 exhibited higher levels at concourses than platforms, especially during the winter. Both PM10 and CO2 concentrations were increased in the order of non-operation hours, normal-operating hours, and rush hour. By comparing indoor- and outdoor air quality, outdoor PM10 was determined to be the most dominant factor which controls indoor PMx concentrations in all stations. The number of trains in operation was associated with the PM10 concentration at platforms.