Commuters' exposure to PM2.5 and CO2 in metro carriages of Shanghai metro system

Air quality inside transportation carriages has become a public concern. A comprehensive measurement campaign was conducted to examine the commuters' exposure to PM2.5 (dp ⩽ 2.5 μm) and CO2 in Shanghai metro system under different conditions. The PM2.5 and CO2 concentrations inside all the measured metro lines were observed at 84 ± 42 μg/m3 and 1253.1 ± 449.1 ppm, respectively. The factors that determine the in-carriage PM2.5 and CO2 concentrations were quantitatively investigated. The metro in-carriage PM2.5 concentrations were significantly affected by the ventilation systems, out-carriage PM2.5 concentrations and the passenger numbers. The largest in-carriage PM2.5 and CO2 concentrations were observed at 132 μg/m3 and 1855.0 ppm inside the carriages equipped with the oldest ventilation systems. The average PM2.5 and CO2 concentrations increased by 24.14% and 9.93% as the metro was driven from underground to overground. The average in-carriage PM2.5 concentrations increased by 17.19% and CO2 concentration decreased by 16.97% as the metro was driven from urban to the suburban area. It was found that PM2.5 concentration is proportional to the on-board passenger number at a ratio of 0.4 μg/m3·passenger. A mass-balance model was developed to estimate the in-carriage PM2.5 concentration under different driving conditions.