pH Dependence of Liquid–Liquid Phase Separation\nin Organic Aerosol

Abstract

Atmospheric\naerosol particles influence climate through their direct\nand indirect effects. These impacts depend in part on the morphology\nof the particles, which is determined by their composition. The effect\nof pH on morphology was investigated using particles composed of 3-methylglutaric\nacid and ammonium sulfate by manipulating the starting pH of the bulk\nsolution through the addition of aqueous sodium hydroxide. Efflorescence,\ndeliquescence, phase separation, and mixing transitions were observed\nwith optical microscopy. Due to changes in its protonation states,\nthe solubility of the organic component increases with increasing\npH, which shifts the location of the separation relative humidity\n(SRH) from 78.7% for the fully protonated acid to 63.9% for the fully\ndeprotonated acid. Surprisingly, this shift in the SRH leads to hysteresis\nbetween the SRH and the mixing relative humidity (MRH). Particle pH\nhas the greatest effect on phase transitions that require nucleus\nformation, that is, efflorescence and SRH.