@@ 1.Introduction The carbonaceous material constitutes a significant component of total aerosol mass and it is composed by two main fractions, organic carbon (OC) and elemental carbon (EC). OC is a mixture of hydrocarbons and oxygenates, which can be directly emitted from sources (primary OC) or produced from atmospheric reactions involving gaseous organic precursors (secondary OC) (Pandis et al, 1992; Turpin and Huntzicker, 1995). With a chemical structure similar to impure graphite, generally EC is generated by the combustion of carbon-containing fuels. EC may not only intervene in some important chemical reactions in the atmosphere (Gundel et al., 1989) but also play an important role in global climate change by affecting radiative forcing and be the second most important component of global warming, after CO2 (Hansen et al, 2000; Jacobson, 2001). Menon et al. (2002) reported that over the past decades in China, the increased rainfall in the south and drought in the north might be related to the increased EC aerosol.Beijing, with an area of 16,807 km2 and a population over 16.3 million, is the economic, politics and culture center of China. Despite the evident significance of OC, EC and PAHs in atmospheric environmental process and human health, the study on their concentrations, sources and size distribution is quite poor in Beijing.
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