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Daily PM_2.5 aerosol samples were collected at five sites in Beijing for a 3-year period from 2001 to 2003. Concentrations of the water-soluble ions (SO₄²⁻, NO₃⁻, Cl⁻, F⁻, PO₄³⁻, NO₂⁻, CH₃COO⁻, HCOO⁻, MSA, C₂O₄²⁻, NH₄⁺, Ca²⁺, K⁺, Mg²⁺, Na⁺) and 23 elements were measured for a total of 334 samples. A relatively even spatial distribution throughout Beijing and a significant seasonal variation were observed. SO₄²⁻, NO₃⁻, Cl⁻, NH₄⁺, Ca²⁺, and K⁺ were the major ions and existed mainly in the form of (NH₄)₂SO₄, NH₄NO₃, NaCl, KCl, and CaCl₂ in aerosol particles. Most ions showed high concentrations in winter and low in summer. Secondary ions, mainly SO₄²⁻, NO₃⁻, exhibited high concentrations in both summer and winter due to the secondary transformation accelerated under high humidity and strong solar radiation in summer and the higher concentration of SO₂ from coal burning and the lower …