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Aerosol samples of PM_2.5 and PM₁₀ were collected in both summer and winter seasons from 2002 to 2003 synchronously at a traffic site, an industrial site, and a residential site in Beijing, which could basically be the representatives over Beijing. Twenty-three elements and 15 ions together with organic carbon and elemental carbon were analyzed systematically for characterization of Beijing aerosol. PM_2.5 was the major part of the inhalable particles (PM₁₀), as the ratios of PM_2.5/PM₁₀ were 0.45–0.48 in summer and 0.52–0.73 in winter. SO₄²⁻, NO₃⁻, NH₄⁺, organic matter, crustal matter, and element carbon were the six dominant species, which totally accounted for 85.8–97.7% of PM_2.5. Secondary aerosol (SO₄²⁻, NO₃⁻, and NH₄⁺), road dust or/and long-range transported dust from outside Beijing, industry and motor vehicles emission, coal burning were the major contributors to the air-borne particulate …