Spatial and seasonal variations of PM_2.5 mass and species during 2010 in Xi'an, China

PM_2.5 mass and selected chemical species are measured in 24-h integrated PM_2.5 samples collected simultaneously at the urban and rural regions of Xi'an (six sites in total), China in the four seasons of 2010. The analytes include organic carbon and elemental carbon (OC+EC=total carbon, TC), seven water-soluble inorganic ions (NH₄⁺, K⁺, Mg²⁺, Ca²⁺, Cl^-, SO₄^2-, NO₃^-) and six trace elements (Ti, Mn, Fe, Zn, As, Pb). The average PM_2.5 mass for the entire measurement period is 142.6±102.7μgm^-3, which is more than four times that of the Chinese national ambient air quality standard. Spatial variations in PM_2.5 mass are not pronounced. The PM_2.5 mass and those species measured show a similar seasonal pattern in all six measurement sites, i.e., in the order of winter>autumn>spring>summer. The dominant PM_2.5 composition is OC in winter, soil dust in spring, and sulfate, nitrate, and ammonium in summer and autumn. Seasonal variations of TC/PM_2.5 and OC/EC ratios follow the PM_2.5 changes. Seasonal distributions of (SO₄^2-+NO₃^-+NH₄⁺)/PM_2.5 showed increase in autumn and decrease in winter, while NO₃^-/SO₄^2- ratios increased in autumn and decreased in summer. Eight main PM_2.5 sources are identified based on the positive matrix factorization (PMF) analysis and emissions from fossil fuel combustion (traffic and coal burning) are founded to be the main source responsible for the fine particle pollution in Xi'an. In addition, a decreasing trend in OC/PM_2.5 is observed in comparison with previous studies in Xi'an.