Fluorescence Aerosol Flow Tube Spectroscopy to Detect Liquid–Liquid Phase Separation

The phase behavior of atmospheric aerosol particles influences processes like gas-particle partitioning, solar light scattering, and cloud formation, ultimately affecting atmospheric air quality and climate. An important aspect of this phase behavior is whether an individual particle exists in a single homogeneous phase or undergoes liquid–liquid phase separation (LLPS). Herein, fluorescence aerosol flow tube (F-AFT) spectroscopy is developed to characterize LLPS in aerosolized submicron particles of 100–200 nm. A solvatochromic fluorescent probe molecule is incorporated into the particles. The link between its fluorescence emission and the local particle-phase chemical environment is used to determine the separation relative humidity (SRH) at which LLPS occurs. The LLPS behaviors of mixed organic/inorganic particles composed of polyethylene glycol (PEG), ammonium sulfate (AS), and sodium chloride (NaCl) are characterized. PEG/AS particles undergo LLPS at SRH values that vary with PEG composition. By comparison, PEG/NaCl particles continue as a single homogeneous phase to the RH of NaCl crystallization. The SRH values for the submicron particles are lower by >5% RH than those reported in the literature for supermicron particles deposited to substrate surfaces. Possible reasons for the differences are discussed, including kinetic and thermodynamic effects of system size and foreign substrate as well as observation time in the experimental apparatus.