中文摘要：

兴建轨道交通被认为是解决当前城市交通拥堵与空气污染的一项有效措施，但传统研究往往受困于内生性所导致的估计偏误。本文利用DID、RD以及RDID等准实验方法估计了中国14个城市新开通的45条线路对空气污染的影响．发现轨道交通的开通具有显著且稳健的污染治理效应。此外．本文分别考察了轨道交通减排效应的城市异质性与污染物异质性。前者发现污染治理效应随入口规模、人口密度及污染程度的提升而增强；后者发现CO、SO扒NO，及悬浮颗粒（PM）的减排效果比较显著，且在交通高峰与凌晨非高峰期存在显著差异。而且还发现轨道交通可以缓解较大规模城市的交通拥堵，而对二、三线城市的作用不显著：对不同交通出行工具的检验表明轨道交通的减排效应是通过对出租车出行的替代实现的。最后．本文探讨了轨道交通的规模效应．发现累积开通里程越长．新开通线路的减排效果越强．而这一规律在人口密度较高的城市表现得较为明显，表明在网络正外部性作用下．当前的城市轨道交通仍未达到最优规模。

英文摘要：

Urban rail transit system is regarded as an effective measure to solve the problem of traffic congestion and air pollution. This paper employs quasi-experimental methods, namely DID, RD and RDID to estimate the impact of 45 newly opened rail lines of 14 cities on air pollution. These estimation, which control endogeneity and show robustness, prove that rail transit can significantly reduce air pollution. Moreover, we discuss the heterogeneous effects of rail transit and find that the pollution abatement effect enhances with increase of population scale and density, as well as air pollution severity. We also discover that abatement effect is particularly notable in terms of the vehicle related emission of S02, NO2, and PMs. In the mechanism analysis, the subway is found to be effective on the alleviation of morning peak-hour traffic congestion, which suggests that traffic substitution is the main channel of pollution alleviation. Due to positive network externality, finally, we find that it appears scale effect in pollution alleviation of rail transit. More mileage of built rail transit lines means more pollution abatement, especially in those cities with high population density.