The chemical characteristics of PM2.5 from Xian urban area in January and February 2006 and the same period of 2010 were determined in this study.Chemical mass closure analyses were carried out in PM2.5 samples,in order to illustrate the changes in PM2.5 chemical compositions in Xian winter time.Enrichment factors relative to earth crust abundances were evaluated and it was noted that most anthropogenic elements including Ni,Cd,As,and Pb had significant reductions.Correlation and multivariate analysis technique,such as Positive Matrix Factorization (PMF) were used for source apportionment to identify the possible sources of PM2.5 and to determine their contribution changes.Similar sources were identified between these two years.Secondary transformation,fugitive dust,coal combustion and vehicular emissions almost accounted for 80% of the mass concentrations of PM2.5.Moreover,the abundances of these major sources were changed with different years: coal combustion contributed the most (28.9%) of PM2.5 mass concentrations in 2006,followed by vehicular emissions (28.0%) and secondary transformation (15.3%); but vehicular emissions was the most contributor (25.7%) in 2010 and the contribution of fugitive dust increased to 22.5% as well.Besides,industrial emissions and fireworks emissions + biomass burning were the minor sources for PM2.5 in Xian.