Atmospheric particulate matter (PM) affects important environmental issues including air quality, regional and global climates, and human health. A one-year sampling campaign for PM2.5 was conducted at six locations in Guanzhong Plain, including the cities of Xi'an, Weinan and Baoji, from March 2012 to March 2013. The 24-h average PM2.5 mass concentration was 134.7 mg m 3, that substantially exceeds the National Ambient Air Quality Standard level of 35 mg m 3. The highest loadings of both organic and elemental carbon (OC and EC) occurred in winter: EC co-varied with OC but showed less variability, presumably due to more stable emissions. The greatest contributions of secondary inorganic ions (SO4 2 , NO3 and NH4 þ) to the total quantified ions also were seen in winter, presumably due to gaseous precursors from coal combustion and biomass burning. Two high PM episodes occurred, one in the autumn and the other in winter. During the autumn episode, regional pollution from biomass burning raised the concentrations of secondary ions while coal combustion was a strong influence during the winter episode. Modeling simulations suggest that the control measures on both primary emissions and secondary aerosol precursors including SO2, NOx, and NH3 are needed to reduce the PM levels of the region.