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Aerosol samples of PM_2.5 and PM₁₀ in a period of intensive haze−fog (HF) events were collected to investigate the chemical characteristics of air pollution in Beijing. The air quality in HF episodes was much worse than that in nonhaze−fog (NHF) days. The concentrations of elements and water-soluble (WS) ions (K⁺, SO₄^2-, and NO₃^-) in HF episodes were more than 10 times higher than those in NHF days. Most of the chemical species in PM_2.5 and the secondary species (NH₄⁺, SO₄^2-, and NO₃^-) in PM₁₀ showed significant difference between HF from westerly direction (HFW) and southerly direction (HFS). The concentrations of secondary species in HFS were much higher than those in HFW, and other chemical species in HFS were lower than those in HFW. The sources of PM_2.5 were more from areas on the regional scale due to its tendency for long-range transport, while PM₁₀ was more limited to the local …