The physical state of the tropospheric aerosol is still largely unknown despite its importance for cloud formation, multiphase and heterogeneous chemistry in and on aerosol particles, and for the aerosol’s radiative properties. Especially the high organic fraction (of largely unknown composition) of the atmospheric particulate matter makes it difficult to predict its physical state.
We have developed a thermodynamic activity coefficient model called AIOMFAC (Aerosol Inorganic-Organic Mixtures Functional groups Activity Coefficients) that is able to calculate vapor-liquid, solid-liquid, and liquid-liquid equilibria in atmospheric aerosols composed of a wide range of inorganic salts and organic compounds. In the present project, we will assess and generalize AIOMFAC. The main focus will be on aqueous organic systems at low temperatures containing the functionalities typically found in tropospheric aerosol particles, namely alkyl, hydroxyl, carboxyl, carbonyl, ether, aromatic, aldehyde, and vinyl groups. A second focus is the generalization of the model to low temperatures by the development of suitable parameterizations for electrolyte solutions containing the important inorganic constituents of aerosols such as ammonium sulfate and nitrate and sodium chloride.
In the experimental part of the project, we will study systems with liquid-liquid phase separations that are confined within the supersaturated region of the inorganic salt with a Raman microscope during humidity cycles. The investigation of micrometer sized droplets allows determining liquid-liquid phase boundaries and metastability phase diagrams that are not accessible by bulk methods. Moreover, we will investigate whether the crystallization of the salt is preceded or even triggered by a liquid-liquid phase separation and how gas/particle partitioning is influenced in the presence of a liquid-liquid phase separation.
With the further development of AIOMFAC it will be possible to model the phases of the systems that we investigate experimentally and to predict their phase states based on their chemical composition. Thus, the experimental investigations together with the AIOMFAC model allow to estimate the occurrence probability of liquid-liquid phase separations in tropospheric aerosols, and more generally their hygroscopicity, reactivity, and gas/particle partitioning of semi-volatile species.