EC-16. The Change in Separation Relative Humidity (SRH) for Submicron Aerosol Particles

Abstract
Liquid-liquid phase separation (LLPS), the process in which dissimilar liquid components demix, is an important process in the biological, materials, and atmospheric sciences. The presence of LLPS in atmospheric aerosol particles can alter their physical and chemical properties, ultimately influencing their effects on climate. Aerosol particles in the submicron range can possess unique properties. For instance, particles with the smallest diameters in the submicron range do not undergo the process of LLPS. In aqueous aerosol particles, LLPS is tracked as a function of particle water activity, which is equivalent to relative humidity (RH) at equilibrium. We probed three organic/inorganic submicron aerosol systems with transmission electron microscopy (TEM) prepared using a flash freeze flow tube to vitrify particles for imaging. Our findings indicate that SRH for submicron aerosol particles is lower (by 10% RH or more) than the SRH measured for micron-sized droplets. The decrease in SRH for submicron aerosol particles indicates a shift in the binodal curve of the phase diagram. Additionally, we found that the smallest diameter particles remain homogeneous throughout the phase separation process when phase separation follows a nucleation and growth mechanism. These findings show that it may be necessary to update the representation of phase transitions in aerosol particles. Using SRH values observed in droplets tens to hundreds of microns in diameter for submicron particles will overestimate actual SRH values. Not accounting for the lower SRH values in submicron particles will result in assuming more particles are phase separated than in actuality. Correct values for SRH in submicron particles will allow us to more accurately model the chemical and physical properties of aerosol particles such as optical properties and heterogeneous chemistry processes which can be influenced by phase separated morphologies.