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To evaluate the wintertime regional brown haze in northern China, trace gases and aerosols were measured at an urban site between 9 and 20 November 2009. Ion chromatography and transmission electron microscopy (TEM) were used to investigate soluble ions in PM_2.5 and the mixing state of individual particles. The contrasts between clear and hazy days were examined in detail. Concentrations of the primary gases including NO (55.62 ppbv), NO₂ (54.86 ppbv), SO₂ (83.03 ppbv), and CO (2.07 ppmv) on hazy days were 2 to 6 times higher than those on clear days. In contrast, concentrations of O₃ remained low (5.71 ppbv) on hazy days. Mass concentrations of PM_2.5 (135.90 μg m⁻³) and BC (7.85 μg m⁻³) were 3 times higher on hazy days than on clear days. Based on the estimations from TEM analysis, fractions of both ammoniated sulfate (AS)–soot (20%) and AS‐soot/organic matter/fly ash (20%) were …