Key Laboratory for Semi-Arid Climate Change of the Ministry of Education;
College of Atmospheric Sciences;
Lanzhou University;
Lanzhou 730000;
Meteorological Observatory Unit 95021;
Yichang 441000;
PM2.5; pollution; episodes; synoptic; patterns; Beijing-Tianjin-Hebei; (BTH); Yangtze; River; Delta; (YRD);