Liquid-liquid phase separation in secondary organic aerosol particles produced from α-pinene ozonolysis and α-pinene photo-oxidation with/without ammonia

Abstract. Recently, liquid–liquid phase separation (LLPS) of secondary organic aerosol (SOA) particles free of inorganic salts has been intensively studied because of their importance on cloud condensation nuclei (CCN) properties. Herein, we investigated LLPS in four different types of SOA particles generated from α-pinene ozonolysis and α-pinene photo-oxidation in the absence and presence of NH 3 . LLPS was observed in SOA particles produced from α-pinene ozonolysis at ~ 95.8 % relative humidity (RH) and α-pinene ozonolysis with NH 3 at ~ 95.4 % RH. However, LLPS was not observed in SOA particles produced from α-pinene photo-oxidation and α-pinene photo-oxidation with NH 3 . With datasets of average oxygen to carbon elemental ratio (O : C) for different types of SOA particles of this study and previous studies, LLPS occurred when the O : C ratio was less than ~ 0.44 and LLPS did not occur when the O : C ratio was greater than ~ 0.40. When LLPS was observed, the two liquid phases were present up to ~ 100 % RH. This result can help to predict more accurate results of CCN properties of organic aerosol particles.