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Cooking organic aerosol (COA) constitutes a considerable proportion of fine particles in populated area, while there is a lack of understanding on its impact on climate. Using a data set from field observations in Beijing, we characterize the variations of COA in fine particulate matter (PM) and its effect on aerosols hygroscopicity and cloud condensation nuclei (CCN) activity. The observed mass concentrations of COA from two campaigns of winter 2016 and summer 2017 were 5.5 ± 5.6 µg m⁻³ and 1.9 ± 2.1 µg m⁻³, respectively, corresponding to mass fraction of COA to PM₁ and total OA of 8% ± 9% and 17% ± 13% in winter, 10% ± 9% and 19% ± 13% in summer. The strong COA sources are evidenced by apparent peak values in mass concentrations at lunch and dinner times. With increase of volume fraction of COA from ∼10% during noncooking time to ∼40% during dinner time, hygroscopic parameter of OA (κ …