Concentrations of organic carbon (OC) and element carbon (EC) in PM2.5 in the urban and rural areas of Beijing were determined and their sources were qualitatively discussed.PM2.5 samples were collected at five sites of Beijing during summers and winters of 2001 and 2002 and spring of 2003. Daily average 10.7 +/- 3.6 and 5.7 +/- 2.9 ug m-3 in summer and 36.7 +/- 19.4 and 15.2 +/- 11.1 ug m-3 in winter of 2002 for OC and EC, respectively, were observed in Beijing urban area.The concentrations of carbonaceous species contributed 18.4 +/- 3.5% and 37.2 +/- 12.7% of PM2.5 in summer and winter, respectively, of which OC accounted for 70% or more of the total carbon (TC).OC and EC concentrations and their mass percentages in PM2.5 were higher in winter than in summer, which could be attributed to the coal consumption of heating period.Organic aerosols would constitute up to 42% of PM2.5 in winter based on the evaluation of 1.6 for the ratio of OC to organic particulates. Secondary organic carbon (SOC) were estimated to be more than 50% and up to 95% of total OC in summer based on the minimum OC/EC ratio of 0.9. Comparisons of OC and EC with trace elements, As, Zn, K and Pb, at various sites demonstrated that coal combustion was a dominant source of carbonaceous species of PM2.5 in the urban area of Beijing during winter, and biomass burning, traffic and/or industry emission were the major sources of OC and EC in the summer.

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