A100 Key factors influencing air quality in rail subway systems

Abstract Background Air quality plans incentivise the use of public transport to abate atmospheric emissions from private road vehicles in cities. In this context rail subway systems are especially desirable as they are based on electric trains, are energetically/environmentally efficient, and help relieve surface traffic congestion. However, a number of studies have revealed poor air quality underground, especially concerning levels of Particulate Matter. Interestingly, some subway systems appear to be worse than others in terms of particle loading, and it is clearly necessary to identify the key factors involved. Methods Ambient subway aerosol particles present in trains and on platforms in the Barcelona metro system have been (and are being) intensively monitored, sampled and physico-chemically characterised, and different types of stations compared. Air monitoring equipment includes high volume samplers for chemical analysis of particles below 2.5 μm, a light-scattering laser photometer for particle concentrations in 3 sizes, and an indoor air quality meter for carbon oxides. Results Outdoor concentrations of traffic particles and gases in Barcelona city do not significantly influence subway air quality, with most particles being ferruginous and sourced underground. Air quality is better inside trains than on platforms, although passenger density clearly affects carbon dioxide levels in both. Stations with cleanest particle concentrations are those fitted with new platform screen door systems, whereas the poorest air quality is associated with older-style narrow single track-tunnel-platform designs. Particle concentration varies along the platform, being influenced by the number and position of passenger exit tunnels, the train frequency and travel direction. Particle composition and morphologies are similar in all stations, indicating a common mechanism of formation, and they are physically and chemically distinctive from particles breathed in outdoor urban air. The iron-rich particles are mostly nanometric in size and derive from mechanical processes of sliding and wear at the brake-wheel and wheel-rail interface, with minor contributions from high temperature processes such as sparking. Conclusions Air quality in the subway environment is an issue of concern due to the high particle concentrations that can be registered, especially on platforms. The variables influencing these concentrations include type and intensity of ventilation, station design, number and location of accesses to platforms and number of trains and passengers. The detailed study of these parameters in a large number of stations within the ongoing research projects METRO and IMPROVE LIFE is enabling us to develop protocols aimed at producing discernible improvements to rail subway air quality.