Exposure assessment and particle number size distribution in train carriages in the London subway
File(s)
Published version
Author(s)
Type
Journal Article
Abstract
Subway systems provide an important mode of sustainable urban public transportation reducing traffic congestion and vehicle emissions. However, there are increasing concerns over the elevated levels of particulate matter (PM) concentrations found in the subway and the potential health effects from train-related emissions. These emission sources are common worldwide with the highest concentrations driven by lack of ventilation, tunnel depth and distance from tunnel openings. The aim of this study was to assess PM2.5 mass concentrations, particle number concentrations and particle number size distributions by taking measurements inside a train carriage in a London Underground deep-tunnelled line. A fully portable, battery powered Mobile Reference Station (MoRS) was developed to obtain simultaneous measurements of oxides of nitrogen, ozone and all size fractions of PM in train carriages across 16 distinct Bakerloo Line subway journeys. In the tunnelled sections of the Bakerloo line mean PM2.5 concentrations were 343 (115) μg m−3, with mean concentrations peaking at 550 (45) μg m−3 at Marylebone Station. A moderate negative correlation between PM2.5 and NO2 was demonstrated likely influenced by a complex relationship linking tunnel depth, ventilation, ambient air, PM2.5 and NO2 concentrations. PM2.5 concentrations and particle size distributions were seen to be driven by the piston effect of moving trains and settling processes. Using particle number size distributions, ultrafine particles were found to constitute 84 % of subway particle number concentrations with 99 % of subway particles having a diameter less than 530 nm. Lung Deposited Surface Area (LDSA) was used to estimate PM exposure during the subway journeys. Importantly, total LDSA for PM2.5 was higher in the subway than at a London roadside site. Overall, this study provides a basis to begin to control specific train related emission sources and to improve air quality management through focused mitigation strategies in the London Underground and worldwide subway systems.
Date Issued
2026-02-15
Date Acceptance
2025-12-10
Citation
Atmospheric Environment, 2026, 367
ISSN
1352-2310
Publisher
Elsevier
Journal / Book Title
Atmospheric Environment
Volume
367
Copyright Statement
© 2025 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
License URL
Subjects
AIRBORNE PARTICLES
AIR-POLLUTION
COMMUTER EXPOSURE
Environmental Sciences
Environmental Sciences & Ecology
Life Sciences & Biomedicine
Meteorology & Atmospheric Sciences
OXIDATIVE STRESS
PARTICULATE MATTER CONCENTRATIONS
Physical Sciences
PM10
PM2.5
PUBLIC TRANSPORT
Science & Technology
SEOUL
ULTRAFINE PARTICLES
Publication Status
Published
Article Number
121745
Date Publish Online
2025-12-11