Person:

Qin, Yi Ming

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 homogenous 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 to 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 homogenous phase to the RH of NaCl crystallization. The SRH values for the submicron particles are >5% RH lower than those reported in the literature of 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.

The influence of relative humidity (RH) on the condensational growth of organic aerosol particles remains incompletely understood. Herein, the RH dependence was investigated via a series of experiments for α-pinene ozonolysis in a continuously mixed flow chamber in which recurring cycles of particle growth occurred every 7 to 8 h at a given RH. In 5 h, the mean increase in the particle mode diameter was 15 nm at 0% RH and 110 nm at 75% RH. The corresponding particle growth coefficients, representing a combination of the thermodynamic driving force and the kinetic resistance to mass transfer, increased from 0.35 to 2.3 nm2 s–1. The chemical composition, characterized by O:C and H:C atomic ratios of 0.52 and 1.48, respectively, and determined by mass spectrometry, did not depend on RH. The Model for Simulating Aerosol Interactions and Chemistry (MOSAIC) was applied to reproduce the observed size- and RH-dependent particle growth by optimizing the diffusivities Db within the particles of the condensing molecules. The Db values increased from 5 α–1 × 10–16 at 0% RH to 2 α–1 × 10–12 cm–2 s–1 at 75% RH for mass accommodation coefficients α of 0.1 to 1.0, highlighting the importance of particle-phase properties in modeling the growth of atmospheric aerosol particles.