[Concentration Characteristics and Sources of Trace Metals in PM_2.5 During Wintertime in Beijing]

To study the characteristics and sources of trace metals in PM_2.5 during wintertime in Beijing, PM_2.5 samples were collected from December 2014 to January 2015 by a middle volume sampler in the urban area of Beijing for 30 consecutive days. The mass concentration of PM_2.5 was measured by filter membrane weighting method, and 16 kinds of trace metals were determined by inductively couple plasma-mass spectrometry (ICP-MS). In addition, the pollution characteristics and sources of trace metals were analyzed by enrichment factor (EF) method and factor analysis, respectively. The results showed that the concentrations of five elements (i. e. K, Ca, Fe, Al and Mg) accounted for 90.7% of the total metal elements. The concentrations of the metal elements changed obviously between day and night. Compared with daytime, crustal elements like Mg and Al decreased by more than 30% while anthropogenic elements like Cu and Pb increased by more than 40% in nighttime. Although the concentrations of metal elements increased by nearly one time in heavy pollution days compared with clean days, the mass percent of which in PM_2.5 decreased from 10.9% in clean days to 4.6% in heavy pollution days. This result suggested the accumulation of metal elements in heavy pollution days had a minor contribution to the increased mass concentration of PM_2.5. As the pollution episode progressed, anthropogenic elements (Cu,Zn,As,Se,Ag and Cd) increased faster than crustal elements (Al,Mg,Ca,Mn and Fe), which showed ratios ranging from 2.9 to 5.3 for anthropogenic elements and ratios ranging from 1.2 to 1.8 for crustal elements, when compared between heavy pollution days and clean days. In addition, the EF value of anthropogenic elements was also increased in the pollution days, indicating the concentrations of these elements was further influenced by the anthropogenic sources. Factor analysis showed that metal elements of PM_2.5 during wintertime of Beijing were mainly from coal combustion and biomass burning, motor vehicle and industry emissions, and re-suspension of road dust, with the contributions of 34.2%,25.5% and 17.1%, respectively.