Evaluation of particulate matter concentration in Shanghai’s metro system and strategy for improvement Laijun Zhao a,b , Jiajia Wang a,b,⇑ , H. Oliver Gao a,c , Yujing Xie d , Ran Jiang b , Qingmi Hu b , Yan Sun a a Sino-US Global Logistics Institute, Shanghai Jiao Tong University, Shanghai 200030, PR China b Antai College of Economics and Management, Shanghai Jiao Tong University, Shanghai 200030, PR China c School of Civil and Environmental Engineering, Cornell University, NY 14853, USA d School of Management, Shanghai University, Shanghai 200444, PR China article info Article history: Keywords: Shanghai metro system Particulate matter Contamination evaluation Causation analysis Improvement strategy abstract The air quality in Shanghai’s subway system has become a big concern. At present, the sys- tem is the longest in the world, and its daily passenger volume exceeds 9 Â 10 6 travelers every work day. In this study, we comprehensively assessed the fine particulate matter (PM 2.5 ) concentrations in the 14 lines of Shanghai’s metro system through field measure- ments in the metro (subway) system and real-time data acquisition at the nearest state-run air sampling sites. We ranked and clustered the 14 lines according to the PM 2.5 concentrations and the relative concentrations in the halls and on the platform of the metro station and inside the train for each line. We identified the factors that influence the PM 2.5 concentration, and found that the external environment appears to have the strongest influence on air quality. In addition, the age of the line, type of platform (screen door versus half-height security door), air-conditioning filtration system, and other factors influenced the PM 2.5 concentration for each line. Based on our evaluation of the contami- nation and its causes, we propose potential solutions, such as reducing particulate matter invasion from pollution sources, updating the environmental protection hardware (i.e., filtration systems), developing a more scientific cleaning program, and optimizing the travel behavior of passengers and working conditions of merchants to improve the air quality and reduce traveler exposure to pollution in the metro system. Ó 2017 Elsevier Ltd. All rights reserved. 1. Introduction Metro (subway) systems have quickly become the main mode of transportation in the world’s cities due to their conve- nience, safety, high speed, and environmental benefits. Currently, metro passenger volumes in London, Paris, New York, and Tokyo account for 35%, 37%, 54%, and 86%, respectively, of their total transportation demand. In China, the average daily pas- senger volume of Beijing’s metro in 2015 was 7.728 Â 10 6 , versus 8.395 Â 10 6 in Shanghai. With the continuous expansion of the metro network, travelers are staying longer in this part of the transportation system. The typical one-way commute time is 48, 52, and 51 min in New York, Beijing, and Shanghai, respectively. The metro employees and merchants stay even longer in the metro system. Therefore, the air quality of metro systems has attracted increasing attention, particularly since some http://dx.doi.org/10.1016/j.trd.2017.04.010 1361-9209/Ó 2017 Elsevier Ltd. All rights reserved. ⇑ Corresponding author at: Sino-US Global Logistics Institute, Shanghai Jiao Tong University, Shanghai 200030, PR China. E-mail address: wangjjsjz@126.com (J. Wang). Transportation Research Part D 53 (2017) 115–127 Contents lists available at ScienceDirect Transportation Research Part D journal homepage: www.elsevier.com/locate/trd