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An Aerodyne Aerosol Chemical Speciation Monitor (ACSM) was first deployed in Beijing, China for characterization of summer organic and inorganic aerosols. The non-refractory submicron aerosol (NR-PM₁) species, i.e., organics, sulfate, nitrate, ammonium, and chloride were measured in situ at a time resolution of ∼15 min from 26 June to 28 August, 2011. The total NR-PM₁ measured by the ACSM agrees well with the PM_2.5 measured by a Tapered Element Oscillating Microbalance (TEOM). The average total NR-PM₁ mass for the entire study is 50 ± 30 μg m⁻³ with the organics being the major fraction, accounting for 40% on average. High concentration and mass fraction of nitrate were frequently observed in summer in Beijing, likely due to the high humidity and excess gaseous ammonia that facilitate the transformation of HNO₃ to ammonium nitrate particles. Nitrate appears to play an important role in …