Vânia Martins

Exposure to airborne particulate matter in the subway system.

The Barcelona subway system comprises eight subway lines, at different depths, with different tunnel dimensions, station designs and train frequencies. An extensive measurement campaign was performed in this subway system in order to characterise the airborne particulate matter (PM) measuring its concentration and investigating its variability, both inside trains and on platforms, in two different seasonal periods (warmer and colder), to better understand the main factors controlling it, and therefore the way to improve air quality. The majority of PM in the underground stations is generated within the subway system, due to abrasion and wear of rail tracks, wheels and braking pads caused during the motion of the trains. Substantial variation in average PM concentrations between underground stations was observed, which might be associated to different ventilation and air conditioning systems, characteristics/design of each station and variations in the train frequency. Average PM2.5 concentrations on the platforms in the subway operating hours ranged from 20 to 51 and from 41 to 91 μg m(-3) in the warmer and colder period, respectively, mainly related to the seasonal changes in the subway ventilation systems. The new subway lines with platform screen doors showed PM2.5 concentrations lower than those in the conventional system, which is probably attributable not only to the more advanced ventilation setup, but also to the lower train frequency and the design of the stations. PM concentrations inside the trains were generally lower than those on the platforms, which is attributable to the air conditioning systems operating inside the trains, which are equipped with air filters. This study allows the analysis and quantification of the impact of different ventilation settings on air quality, which provides an improvement on the knowledge for the general understanding and good management of air quality in the subway system.

Factors controlling air quality in different European subway systems

Sampling campaigns using the same equipment and methodology were conducted to assess and compare the air quality at three South European subway systems (Barcelona, Athens and Oporto), focusing on concentrations and chemical composition of PM2.5 on subway platforms, as well as PM2.5 concentrations inside trains. Experimental results showed that the mean PM2.5 concentrations widely varied among the European subway systems, and even among different platforms within the same underground system, which might be associated to distinct station and tunnel designs and ventilation systems. In all cases PM2.5 concentrations on the platforms were higher than those in the urban ambient air, evidencing that there is generation of PM2.5 associated with the subway systems operation. Subway PM2.5 consisted of elemental iron, total carbon, crustal matter, secondary inorganic compounds, insoluble sulphate, halite and trace elements. Of all metals, Fe was the most abundant, accounting for 29-43% of the total PM2.5 mass (41-61% if Fe2O3 is considered), indicating the existence of an Fe source in the subway system, which could have its origin in mechanical friction and wear processes between rails, wheels and brakes. The trace elements with the highest enrichment in the subway PM2.5 were Ba, Cu, Mn, Zn, Cr, Sb, Sr, Ni, Sn, Co, Zr and Mo. Similar PM2.5 diurnal trends were observed on platforms from different subway systems, with higher concentrations during subway operating hours than during the transport service interruption, and lower levels on weekends than on weekdays. PM2.5 concentrations depended largely on the operation and frequency of the trains and the ventilation system, and were lower inside the trains, when air conditioning system was operating properly, than on the platforms. However, the PM2.5 concentrations increased considerably when the train windows were open. The PM2.5 levels inside the trains decreased with the trains passage in aboveground sections.

A new look at inhalable metalliferous airborne particles on rail subway platforms

Most particles breathed on rail subway platforms are highly ferruginous (FePM) and extremely small (nanometric to a few microns in size). High magnification observations of particle texture and chemistry on airborne PM₁₀ samples collected from the Barcelona Metro, combined with published experimental work on particle generation by frictional sliding, allow us to propose a general model to explain the origin of most subway FePM. Particle generation occurs by mechanical wear at the brake-wheel and wheel-rail interfaces, where magnetic metallic flakes and splinters are released and undergo progressive atmospheric oxidation from metallic iron to magnetite and maghemite. Flakes of magnetite typically comprise mottled mosaics of octahedral nanocrystals (10-20 nm) that become pseudomorphed by maghemite. Continued oxidation results in extensive alteration of the magnetic nanostructure to more rounded aggregates of non-magnetic hematite nanocrystals, with magnetic precursors (including iron metal) still preserved in some particle cores. Particles derived from steel wheel and rails contain a characteristic trace element chemistry, typically with Mn/Fe=0.01. Flakes released from brakes are chemically very distinctive, depending on the pad composition, being always carbonaceous, commonly barium-rich, and texturally inhomogeneous, with trace elements present in nanominerals incorporated within the crystalline structure. In the studied subway lines of Barcelona at least there appears to be only a minimal aerosol contribution from high temperature processes such as sparking. To date there is no strong evidence that these chemically and texturally complex inhalable metallic materials are any more or less toxic than street-level urban particles, and as with outdoor air, the priority in subway air quality should be to reduce high mass concentrations of aerosol present in some stations.

Origin of inorganic and organic components of PM2.5 in subway stations of Barcelona, Spain

The present work assesses indoor air quality in stations of the Barcelona subway system. PM2.5 concentrations on the platforms of 4 subway stations were measured during two different seasons and the chemical composition was determined. A Positive Matrix Factorization analysis was performed to identify and quantify the contributions of major PM2.5 sources in the subway stations. Mean PM2.5 concentrations varied according to the stations design and seasonal periods. PM2.5 was composed of haematite, carbonaceous aerosol, crustal matter, secondary inorganic compounds, trace elements, insoluble sulphate and halite. Organic compounds such as PAHs, nicotine, levoglucosan and aromatic musk compounds were also identified. Subway PM2.5 source comprised emissions from rails, wheels, catenaries, brake pads and pantographs. The subway source showed different chemical profiles for each station, but was always dominated by Fe. Control actions on the source are important for the achievement of better air quality in the subway environment.

Bioaerosols in the Barcelona subway system

Subway systems worldwide transport more than 100 million people daily; therefore, air quality on station platforms and inside trains is an important urban air pollution issue. We examined the microbiological composition and abundance in space and time of bioaerosols collected in the Barcelona subway system during a cold period. Quantitative PCR was used to quantify total bacteria, Aspergillus fumigatus, influenza A and B, and rhinoviruses. Multitag 454 pyrosequencing of the 16S rRNA gene was used to assess bacterial community composition and biodiversity. The results showed low bioaerosol concentrations regarding the targeted microorganisms, although the bacterial bioburden was rather high (104 bacteria/m3 ). Airborne bacterial communities presented a high degree of overlap among the different subway environments sampled (inside trains, platforms, and lobbies) and were dominated by a few widespread taxa, with Methylobacterium being the most abundant genus. Human-related microbiota in sequence dataset and ascribed to potentially pathogenic bacteria were found in low proportion (maximum values below 2% of sequence readings) and evenly detected. Hence, no important biological exposure marker was detected in any of the sampled environments. Overall, we found that commuters are not the main source of bioaerosols in the Barcelona subway system.

Characterisation of Airborne Particulate Matter in Different European Subway Systems

Air quality sampling campaigns in three European subway systems (Barcelona, Athens and Oporto) were conducted in order to characterise particulate matter (PM) to better understand the main factors controlling it. PM mass concentrations varied among the European subway platforms, and also within the same underground system, this being mainly associated to differences in the design of the stations and tunnels, system age, train frequency, ventilation and air‐conditioning systems, commuters density, rails geometry and outdoor air quality. PM concentrations displayed clear diurnal patterns, depending largely on the operation and frequency of the trains and the ventilation system. Chemically, subway PM2.5 on the platforms consisted of iron, carbonaceous material, crustal matter, secondary inorganic compounds, insoluble sulphate, halite and trace elements. Fe was the most abundant element, accounting for 19–46% of the bulk PM2.5, which is generated mainly from mechanical wear at rail‐wheel‐brake interfaces. A source apportionment analysis allowed the identification of outdoor (sea salt, fuel‐oil combustion and secondary aerosol) and subway sources on platforms. The use of air‐ conditioning inside the trains was an effective approach to reduce exposure concentra‐ tions, being more efficient removing coarser particles. PM concentrations inside the trains were greatly affected by the surrounding (i.e. platforms and tunnels) air quality conditions.