Equilibrium Morphology of Mixed Organic/Inorganic/Aqueous Aerosol Droplets: Investigating the Effect of Relative Humidity and Surfactants

There is considerable uncertainty regarding the phase, morphology, and composition of atmospheric aerosol. In particular, it is important to understand the microphysical structure of mixed inorganic/organic aerosol given that the structure can influence the surface composition, the role of heterogeneous chemistry, gas-particle partitioning of semivolatile organics, and water uptake in the sub- and supersaturated regimes. We present here a thermodynamic model that predicts the equilibrium morphology of mixed inorganic/organic aerosol. The model uses an iterative process to calculate the total surface free energy of all possible morphologies when two immiscible droplets are brought into contact, with the configuration with the lowest total surface free energy representing the final equilibrium structure. Sensitivity tests and validation experiments were performed by investigating the decane/NaCl/aqueous system. The addition of a water-soluble surfactant was found to promote spreading of decane on the aqueous droplet. This was confirmed by laboratory experiments, although the importance of considering the relative volumes of the aqueous and organic phases was found to play a significant role in determining the equilibrium structure. Decreasing the relative humidity (RH) of the surrounding gas phase was found to decrease the spreading of decane on the aqueous droplet, leading to thicker organic lenses on smaller aqueous droplets. We conclude that a core-shell structure is not always predicted to be the thermodynamically favored state of aerosol containing distinct hydrophobic and hydrophilic phases. Gaining a more reliable picture of the microphysical structure of aerosol is crucial to be able to model aerosol behavior and properties in the atmosphere, particularly when aerosol is dominated by internal mixtures of inorganic and organic components and when the organic is present in a (subcooled) liquid state.