Characterization and source apportionment of particulate matter and CO2 in Chinese subway stations with varying platform screen door forms.

With the vigorous development of subway systems, particulate matter (PM) and CO2 in subway stations is raising increasing concerns. Here, the characterization of PM and CO2 at seven subway stations with different forms of platform doors in China are studied. The results show that PM concentrations at platforms are higher than those at halls, followed by the outside. CO2 concentrations at platforms are remarkably correlated linearly with passenger number. Subsequently, impacts of piston effect on PM and CO2 concentrations at platforms are assessed. The maximum PM2.5 and PM10 concentrations with half-height platform bailout doors (HPBD) are increased by 20 % and 36 % respectively when the train enters the station, which are higher than those with platform screen doors (PSD). CO2 concentrations are decreased by 1.8 % and 4.6 % under the influence of piston effect at platforms with PSD and HPBD, respectively. Compared with coarse particles (larger than 2.5 µm and <10 µm), fine particles (PM2.5) account for larger mass fraction in total particles (38.22 %-62.57 % at all stations), especially at platforms with PSD. The composition and source apportionment of PM2.5 reveal that Fe is the most abundant element and six pollution sources are identified at all stations. Among them, the indoor source contributes the most to PM2.5 at the station with HPBD. Regardless of the form of platform doors, wheel-rail wear is the largest source of PM2.5 inside each station. The above results are expected to provide reference for the PM control to achieve a healthier underground environment.