The phase state of atmospheric aerosol particles impacts the climate system through its effects on heterogeneous chemistry, particle optical properties, and water uptake. Liquid-liquid phase separation in aerosol particles is a phase transition that requires the formation of an interface. At sufficiently small particle sizes, liquid-liquid phase separation is inhibited due to the activation energy required to form the interface separating the two liquid phases. This inhibition is observed in cryo-transmission electron microscopy studies in model systems consisting of one to hundreds of organic compounds, salt, and water. To investigate how the relative humidity at which phase separation occurs (separation relative humidity) depends on particle size, we have developed a flash freeze flow tube to vitrify aerosol particles at fixed relative humidity. Our results indicate that the separation relative humidity for submicron particles is significantly lower than for larger droplets. We have also investigated the role of soluble organic compounds on the morphology and separation relative humidity of droplets that are 10s of micrometers in diameter composed of salt, a liquid-liquid phase separating organic, and a soluble organic compound. The presence of soluble organic compounds can change the morphology of the particles and at sufficiently high concentrations, can shut off the liquid-liquid phase separation. I will discuss the implications of these results for aerosol phase transitions and atmospheric processes.

Separation relative humidity of systems that undergo liquid-liquid phase separation: Effects of particle size and the presence of soluble organic compounds

ACS Spring Meeting 2021

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