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

Abstract

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 <2.5 µm) concentrations with respect to those on station platforms, the highest dose was observed inside the trains due to the longer exposure time. Overall, during a subway commuting travel, roughly 80% of the inhaled mass of subway PM2.5 was deposited in the respiratory tract. The dose, and its distribution on the different regions of the respiratory tract, is mainly dependent on the particle size and exposure concentrations.

Chemically, subway PM2.5 on the platforms is comprised of iron, total carbon, crustal matter, secondary inorganic compounds, insoluble sulphate, halite and trace elements. Fe was the most abundant element in PM2.5 found on the platforms, with relative contribution to the bulk PM2.5 ranging from 19 to 46%, which is generated mainly from mechanical wear and friction processes at rail-wheel-brake interfaces. The trace elements with highest enrichment in the subway PM2.5 were Ba, Cu, Mn, Zn, Cr, Sb, Sr, Ni, Sn, As, Co and Zr. Differences in metal concentrations in PM among the stations and subway systems might be associated to the different chemical composition of wheels, rails, brakes, and power supply materials.

A source apportionment analysis allowed the identification of outdoor (sea salt, fuel oil combustion and secondary) and subway sources on platforms, the latter including all emissions generated within the subway system.