In order to study the temporal and spatial variations of PM2.5 and its chemical compositions in the region of Beijing,Tianjin,and Hebei (BTH),PM2.5 samples were collected at four urban sites in Beijing (BJ),Tianjin (TJ),Shijiazhuang (SJZ),and Chengde (CD),and also one site at Shangdianzi (SDZ) regional background station over four seasons from 2009 to 2010.The samples were weighted for mass concentrations and analyzed in the laboratory for chemical profiles of 19 elements (Al,As,Ba,Ca,Cd,Co,Cr,Cu,Fe,K,Mg,Mn,Ni,P,Pb,Sr,Ti,V,and Zn),eight water-soluble inorganic ions (Na+,NH4 +,K+,Mg2+,Ca2+,Cl-,NO3 -,and SO4 2-),and carbon fractions (OC and EC).The concentrations of PM2.5 and its major chemical species were season dependent and showed spatially similar characteristics in the plain area of BTH.The average annual concentrations of PM2.5 were 71.8-191.2 μgm-3 at the five sites,with more than 90% of sampling days exceeding 50 μg m-3 at BJ,TJ,and SJZ.PM2.5 pollution was most serious at SJZ,and the annual concentrations of PM2.5,secondary inorganic ions,OC,EC,and most crustal elements were all highest.Due to stronger photochemical oxidation,the sum of concentrations of secondary inorganic ions (NH4 +,NO3 -,and SO4 2-) was highest in the summer at SDZ,BJ,TJ,and CD.Analysis of electric charges of water-soluble inorganic ions indicated the existence of nitric acid or hydrochloric acid in PM2.5.For all five sites,the concentrations of OC,EC and also secondary organic carbon (SOC) in the spring and summer were lower than those in the autumn and winter.SOC had more percentages of increase than primary organic carbon (POC) during the winter.The sums of crustal elements (Al,Ca,Fe,Mg,Ti,Ba,and Sr) were higher in the spring and autumn owing to more days with blowing or floating dust.The concentrations of heavy metals were at higher levels in the BTH area by comparison with other studies.In Shijiazhuang and Chengde,the PM2.5 pollution was dominated by coal combustion.Motor vehicle exhausts and coal combustion emissions both played important roles in Tianjin PM2.5 pollution.However,motor vehicle exhausts had played a more important role in Beijing owing to the reduction of coal consumption and sharp increase of cars in recent years.At SDZ,regional transportation of air pollutants from southern urban areas was significant.