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

Recently, liquid-liquid phase separation (LLPS) of secondary organic aerosol (SOA) particles free of inorganic salts has been intensively studied due to the importance of cloud condensation nuclei (CCN) properties. In this study, we investigated LLPS in four different types of SOA particles generated from alpha-pinene ozonolysis and alpha-pinene photooxidation in the absence and presence of ammonia (NH3). LLPS was observed in SOA particles produced from alpha-pinene ozonolysis at similar to 95.8% relative humidity (RH) and alpha-pinene ozonolysis with NH3 at similar to 95.4% RH. However, LLPS was not observed in SOA particles produced from alpha-pinene photooxidation and alpha-pinene photooxidation with NH3. Based on datasets of the average oxygen to carbon elemental ratio (O : C) for different types of SOA particles from this study and from previous studies, there appears to be a relationship between the occurrence of LLPS and the O : C of the SOA particles. When LLPS was observed, the two liquid phases were present up to similar to 100% RH. This result can help more accurately predict the CCN properties of organic aerosol particles.