Characteristics of particulate matter (PM) concentrations inﬂuenced by piston wind and train door opening in the Shanghai subway system /Jiajia Wang a,b , Laijun Zhao a,b, ⇑ , Daoli Zhu a,b , H. Oliver Gao a,b,c , Yujing Xie d , Huiyong Li d , Xiang Xu b , Hongbo Wang - Wang,Jiajia