“…As these species (i.e. SO 2− 4 , Cl − , OC, EC, Si, Al, Ca 2+ , and Mg 2+ ) were closely associated with coal combustion Liu et al, 2015;Liu et al, 2016;Liu et al, 2017a, b, c), coal combustion for heating in winter probably had a great impact on increasing these chemical species in PM 2.5 . Furthermore, compared to the NCANHP, the concentrations of Cr, Cu, Fe, Mn, Ti, Zn, and Pb increased by 0.02, 0.02, 0.34, 0.02, 0.02, 0.28, and 0.07 µg m −3 , respectively, and the P i−heating values of these species were 72.7, 33.1, 34.4, 21.0, 45.8, 48.3, and 36.2 %, respectively, during the NCAHP.…”