Evaluation of particulate matter concentration in Shanghai's metro system and strategy for improvement

- We assess the air quality throughout the 14 lines of Shanghai's metro system.
- Ambient air is the strongest influence factor on PM concentration of Shanghai metro.
- PM2.5 inside Shanghai underground trains is lower than aboveground ones in winter.
- Measures are proposed to improve the air quality in Shanghai's metro system.

The air quality in Shanghai's subway system has become a big concern. At present, the system is the longest in the world, and its daily passenger volume exceeds 9 × 106 travelers every work day. In this study, we comprehensively assessed the fine particulate matter (PM2.5) concentrations in the 14 lines of Shanghai's metro system through field measurements 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 PM2.5 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 PM2.5 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 PM2.5 concentration for each line. Based on our evaluation of the contamination 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.