Air quality in subway systems: particulate matter concentrations, chemical composition, sources and personal exposure

Air quality sampling campaigns both on platforms and inside trains of three European subway systems (Barcelona, Athens and Oporto) were conducted in order to characterise PM, investigating its variability, to better understand the main factors controlling it, and therefore the way to improve air quality. PM concentrations varied among the European subway platforms, and also within the same underground system, mainly associated to differences in the design of the stations and tunnels; system age; train frequency; ventilation and air-conditioning systems; passenger densities; power system (catenary vs. third rail); composition of wheels, rail tracks, brake pads and power supply materials; rail tracks geometry (curved vs. straight and sloped vs. levelled); and outdoor air quality. PM concentrations displayed clear diurnal patterns among the three European subway platforms, depending largely on the operation and frequency of the trains and the ventilation system. In the Barcelona subway system, the new stations showed on average lower PM concentrations than those in the old conventional stations, mainly related to the stations design (with platform screen door systems), but also due to the lower train frequency and more advanced ventilation setup. Furthermore, PM concentrations on the platforms in the colder period were higher than in the warmer period, mainly due to weaker ventilation during the colder period. In Athens, the mean PM concentrations in a new station located in the periphery of the line were lower than in a central station, attributed not only to the age and location of the station, but also to the lower train frequency. Measurements carried out in the three subway systems performed on stations with similar platform design were compared. The highest PM concentrations were observed in the Oporto subway station because the line is composed by curved and/or sloping rail tracks (resulting in a higher emission of rail wear particles) and it has a higher train frequency. Furthermore, mechanical forced ventilation is inexistent in this subway system. The use of air-conditioning inside the trains was an effective approach to reduce exposure levels, being more efficient removing coarser particles. Having the carriage windows open promotes the entrance of polluted air from tunnels and platforms into the trains. Nevertheless, even when the carriage windows are closed and the air conditioning system is switched on, the PM concentrations inside the trains continue to be greatly affected by the surrounding air quality conditions. Despite the lower PM2.5 (PM with aerodynamic diameter