To meet the objectives proposed in Action B1, air quality monitoring campaigns were performed focused on variations in key factors affecting subway air quality. These sampling campaigns included some specifically aimed at assessing the impact on passenger exposure to air pollutants after making changes to normal conditions on platforms and inside trains. In this context, from January 2015 to September 2016, sampling campaigns carried out in the framework of this action followed four main purposes: to check the impact on passenger exposure to air pollutants attributed to (1) maintenance works in tunnels, (2) use of different brake pads, (3) use of pantographs with different composition (copper versus graphite), and (4) the deterioration of air conditioner filters inside trains. The same sampling protocol was applied on the platform of all the selected metro stations during the project. Sampling devices were located at the end of the platform corresponding to the train entry point, behind a light fence for safety protection.
- (1) Maintenance works in tunnels: the first measurement campaign was carried out in the underground station of Sagrera, where major works were planned aimed to mitigate track vibrations at the station by replacing ballast, sleepers and rails. In addition, to evaluate in more detail the effect on air quality of rail replacement in tunnels and platforms, two more sampling campaigns were performed. The first one took place at the metro station of Santa Coloma (L1) from 1st October to 3rd November 2015, and the second at Joanic (L4) from 3rd November to 22nd December 2015. Both sampling periods included days under normal conditions and days affected by works related to the replacement of rail tracks, consisting of i) material transport using diesel vehicles, ii) rail replacement including abrasive cutting with gasoline-powered machinery, and iii) welding work. PM2.5 ratios between subway and outdoor levels were in the range of 2, indicating the importance of indoor-generated particles. In general values were higher during non-operating hours presumably due to the use of vehicles for renewal and maintenance activities. High short-duration increases of PM2.5, N0.3-10 and CO levels can be observed for some specific days, such as those observed on 3rd, 4th, 5th, 30th and 31st of March 2015. Our data demonstrated that the mix of activities performed at Sagrera station, including rail cutting and welding, as well as ballast removal, replacement and subsequent levelling, had an important influence on pollutant concentrations in ambient air on the platform, especially for PM2.5 and CO at the moment when the activity was carried out (renewal activities were always from 1:00 to 4:00 h CET). These works could increase PM2.5, N0.3-10 and CO levels by up to 90%, although the duration of these extreme work-related pollution spikes was typically short-lived, lasting less than 30 minutes. When looking at the effect of these work activities on the air quality of platforms the next day, higher PM2.5 and N0.3-10 concentrations were commonly registered after the nights involving renewal works, typically when the work activity was extended in time and produced a series of peaks during the night. Thus, for example, on the platforms of Sagrera and Joanic, PM2.5 and N concentrations were on average 18-19% and 8-13% higher respectively the days after renewal works were carried out.
- (2) Use of different brake pads: Four different types of brakes are used in the Barcelona subway system, each with differing chemical compositions. Friction-generated particle emissions generated from these brakes as the slowing train enters the station will contribute to ambient PM present on the platform. The IMPROVE LIFE program is involving the sampling and full chemical analysis of hundreds of platform PM samples, a number unprecedented in the study of subway air, and these may enable us to see whether different brake emissions can be identified. We initially concentrated this investigation on line L5 because all trains use just one of these four types of brake shoes. With this in mind we chose the station of Sant Ildefons (L5) as the site of a specific campaign designed to investigate whether the chemistry of these brakes could be detected in ambient air. This campaign took place from 3rd to 29th February 2016. One of the most obvious chemical characteristics of subway brakes is that they are always enriched in barium (Ba), which is used as a filler and is stable at high temperatures. This element therefore acts as a chemical useful tracer for brake emissions. Furthermore, the relationship between Ba and other trace elements also known to be enriched in brake shoes, such as Sr, Sb and Sn, may enable us to identify specific sources such as a particular brake shoe type. This work has been completed with all chemical analyses available from all stations and subway lines.
- (3) Use of pantographs with different composition (copper versus graphite): the sampling campaign in Joanic (L4) from 3/11-22/12/2015 also permitted us to study the effect of Cu pantographs in this line compared to the graphitic pantographs used in the rest of the Barcelona subway lines. Looking at the scatter plots between Cu, Fe and Cr in ambient PM2.5 sampled from subway platforms different groupings reveal chemical differences in ambient air between different stations. The yellow line (L4) clearly shows the highest Cu concentrations, which is consistent with the fact that Cu pantograph is used in this line for 30% of the trains, while only graphite pantographs are used in the other subway lines
- (4) Deterioration of air conditioner filters inside trains: In order to check the effect of the deterioration of air conditioning filters on air quality inside trains, a DustTrak monitor was installed inside one of the driver’s cabins in one train of L3 (air inside this cab is representative of the passengers’ exposure as it has the same ventilation system as the rest of the carriage). Measurements started on 2nd April 2015, when all air conditioning filters along the train were replaced (TRAIN 1). Filters were changed again after one month, on 2nd May 2015, following the established TMB protocol. These filters were maintained until the end of the campaign on 3rd July, in order to evaluate the effects on air quality of extending the service life of filters from one to two months. To investigate the reproducibility of the results, a second campaign was carried out in a different train of L3. PM measurements started on 10th July and finished on 15th September 2015 (TRAIN 2). In this case, however air conditioner filters in the selected train had been changed one month before the beginning of the sampling campaign, and were not replaced again until the end of the campaign, that is after being 3 months in use.These campaigns produced several interesting results: (a) PM2.5 levels inside the train were higher (c.20%) during weekdays as compared to weekends (45 and 35 μgm-3 respectively), emphasizing how train frequency is a key parameter controlling the PM levels in subway systems; (b) high amplitude transient peaks recorded during the measurements were related to activities during non-operational hours when the train was located in the parking area (e.g. graffiti removal, technical cleaning with compressed air and other forms of cleaning); (c) NO2 concentrations inside the train averaged 47±4 μgm-3, this being 25-30% lower than on platforms; and (d) the air quality benefits of operating air conditioning inside train carriages are maintained for at least three months without changing the filters. If energy efficiency is similarly unaffected over this time span (or more), then there is a strong case for revising existing protocols requiring monthly replacement of such filters.