Observations and implications of liquid–liquid phase separation at highrelative humidities in secondary organic material produced by α-pinene ozonolysis without inorganic salts

摘要

Particles consisting of secondary organic material (SOM) are abundant in theatmosphere. To predict the role of these particles in climate, visibility andatmospheric chemistry, information on particle phase state (i.e., singleliquid, two liquids and solid) is needed. This paper focuses on the phasestate of SOM particles free of inorganic salts produced by the ozonolysis of-pinene. Phase transitions were investigated in the laboratory usingoptical microscopy and theoretically using a thermodynamic model at 290 Kand for relative humidities ranging from    0.5 to 100 %. In thelaboratory studies, a single phase was observed from 0 to 95 % relativehumidity (RH) while two liquid phases were observed above 95 % RH. Forincreasing RH, the mechanism of liquid–liquid phase separation (LLPS) wasspinodal decomposition. The RH range over which two liquid phases wereobserved did not depend on the direction of RH change. In the modelingstudies, the SOM took up very little water and was a single organic-richphase at low RH values. At high RH, the SOM underwent LLPS to form anorganic-rich phase and a water-rich phase, consistent with the laboratorystudies. The presence of LLPS at high RH values can have consequences for thecloud condensation nuclei (CCN) activity of SOM particles. In the simulatedKöhler curves for SOM particles, two local maxima were observed.Depending on the composition of the SOM, the first or second maximum candetermine the critical supersaturation for activation. Recently researchershave observed inconsistencies between measured CCN properties of SOMparticles and hygroscopic growth measured below water saturation (i.e.,hygroscopic parameters measured below water saturation were inconsistent withhygroscopic parameters measured above water saturation). The work presentedhere illustrates that such inconsistencies are expected for systems with LLPSwhen the water uptake at subsaturated conditions represents thehygroscopicity of an organic-rich phase while the barrier for CCN activationcan be determined by the second maximum in the Köhler curve when theparticles are water rich.