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The Aerodyne aerosol mass spectrometer (AMS) was used to characterize physical and chemical properties of secondary organic aerosol (SOA) formed during ozonolysis of cycloalkenes and biogenic hydrocarbons and photooxidation of m-xylene. Comparison of mass and volume distributions from the AMS and differential mobility analyzers yielded estimates of “effective” density of the SOA in the range of 0.64−1.45 g/cm³, depending on the particular system. Increased contribution of the fragment at m/z 44, CO₂⁺ ion fragment of oxygenated organics, and higher “Δ” values, based on ion series analysis of the mass spectra, in nucleation experiments of cycloalkenes suggest greater contribution of more oxygenated molecules to the SOA as compared to those formed under seeded experiments. Dominant negative “Δ” values of SOA formed during ozonolysis of biogenics indicates the presence of terpene derivative …