Influence of Nonpolar Amino Acids on Phase Transitions of Atmospheric Aerosol Particles

Abstract

Abstract The fate of aerosol particles in the atmosphere depends, in part, on their composition, morphology, and phase transitions. Studies of aerosol phase transitions have focused on aerosol particles with organic compounds consisting of C, H, and O atoms, although a substantial fraction of organic compounds contains heteroatoms such as sulfur and nitrogen. In this study, nonpolar amino acids were used to study the effect of organonitrogen compounds on phase transitions of droplets using optical microscopy. The ternary internal mixtures used were composed of ammonium sulfate (AS), a nonpolar amino acid, and either 2-methylglutaric acid (2MGA) or 1,2,6-hexanetriol (126HT) (model oxidized organic compounds). The morphology, phase transitions, and spatially resolved chemical compositions were characterized using optical and Raman microscopies. As the amino acid concentration in the organic component increases, a decrease in the separation relative humidity was observed. For some systems, the amino acid crystallizes, while the oxidized organic compounds and ammonium sulfate undergo liquid–liquid phase separation, yielding a three-phase system. Trends in the efflorescence relative humidity depend on the solubility of the amino acids. These studies are important in understanding the impact of organic–organic interactions as well as organonitrogen compounds on the phase transitions of aerosol particles, which have consequences for heterogeneous chemistry and cloud droplet nucleation in the atmosphere.