Phase transitions in multicomponent systems relevant to atmospheric new particle formation

Typically, molecular clusters containing sulfuric acid, water, ammonia, and/or amines initiate particle nucleation, followed by particle growth through condensation of highly oxidized organic molecules. The coexistence of multiple organic and inorganic components in these particles can lead to variations in morphology and phase state. In this study, optical and Raman microscopy were used to study multicomponent model aerosol particles representative of new particle formation and growth. We focused on 2-methylglutaric acid, sulfuric acid, and ammonium sulfate systems with different amines; and dipropylamine, sulfuric acid, and ammonium sulfate systems with various dicarboxylic acids. Liquid-liquid phase separation occurred in many systems, while amine functionality and molecular structure affect morphology and phase state. Systems containing dipropylamine along with sulfuric acid underwent liquid-liquid phase separation in several mixtures that remained homogeneous in its absence. These particles formed an inorganic-rich core surrounded by an organic-rich shell containing both the dicarboxylic acid and dipropylamine, with slight dipropylamine enrichment at the air-water interface. Their deliquescence relative humidity was generally the same or lower, while efflorescence relative humidity was similar to the corresponding dicarboxylic acid/ammonium sulfate systems. These results demonstrate that the chemical identity of atmospheric amines and overall compound activity should be considered when predicting the phase state, morphology, and water uptake of newly formed particles, which is important for their heterogeneous chemistry and cloud droplet formation.