Size Dependence of Organic/Inorganic Aerosol Morphology and the Role of Oxidation and Aromaticity of the Organic Component

Atmospheric aerosol particles can exist in a phase separated state depending on their chemical components and size as well as relative humidity and temperature conditions, although the effects of these conditions on particle morphology are not well constrained. Whether a system is phase separated has implications for new particle growth, cloud condensation nucleus formation, ice nucleation, and heterogeneous chemistry. In this study, the identity of the organic species in a binary organic-inorganic phase separating system was found to influence the phase separation properties of these internally mixed aerosol particles. Aerosol particles composed of a carboxylic acid, ammonium sulfate, and water were generated, dried, and characterized using transmission electron microscopy, which revealed the influence of aromaticity and the oxidation of the organic component on the size dependence of phase separation. Organic oxidation was defined by the elemental oxygen to carbon ratio (O:C ratio) and lipophilicity (characterized by the negative distribution coefficient at pH 5.5, −log D_5.5) for each compound. The span of particle diameters at which both phase-separated and homogeneous particles are found, or the transition region, was evaluated using a logistic regression to calculate the diameters at which there is a 20%, 50%, and 80% probability of a phase separated morphology. The seven systems containing aliphatic organic species showed a positive correlation between the particle diameters in the transition region and organic oxidation, while the four systems containing aromatic organic species showed transition regions with a tentative negative correlation until phase separation was arrested. Overall, these results show how the aromaticity and oxygen content of organic species can determine the morphologies of environmentally relevant aerosol particles and may provide improved constraints for particle-resolved atmospheric models.