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Carbonaceous aerosols and PM₁₀ were monitored from September 8 to November 30, 2002, in a semi-urban site (Tsinghua University) in Beijing. Daily concentrations of OC and EC ranged from 7.1 to 65.9μgCm⁻³ and from 1.3 to 26.1μgCm⁻³, with the overall average concentrations of 21.2μgCm⁻³ and 7.3μgCm⁻³, respectively. The diurnal variation of carbonaceous concentrations on 2h basis presented two-peak trend, which was attributed to the cooperative effect of local meteorological conditions and anthropogenic sources such as traffic exhaust and human outdoor activities. Daily average OC/EC ratio varied between 1.5 and 5.3 with an average of 3.0. Strong correlation between OC and EC (R²=0.8) indicated that their main sources were common. The frequency of OC/EC ratio presented Gaussian normal distribution trend in fall, of which the peak value appeared in the range of 2.8–5. In winter, it presented …