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

PM2.5mass and selected chemical species are measured in 24-h integrated PM2.5samples 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-, SO42-, NO3-) and six trace elements (Ti, Mn, Fe, Zn, As, Pb). The average PM2.5mass 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 PM2.5mass are not pronounced. The PM2.5mass 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 PM2.5composition is OC in winter, soil dust in spring, and sulfate, nitrate, and ammonium in summer and autumn. Seasonal variations of TC/PM2.5and OC/EC ratios follow the PM2.5changes. Seasonal distributions of (SO42-+NO3-+NH4+)/PM2.5showed increase in autumn and decrease in winter, while NO3-/SO42-ratios increased in autumn and decreased in summer. Eight main PM2.5sources 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/PM2.5is observed in comparison with previous studies in Xi'an.