ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-12-2691-2012 Liquid-liquid phase separation and morphology of internally mixed dicarboxylic acids/ammonium sulfate/water particles Song M. 1 Marcolli C. 1 Krieger U. K. 1 Zuend A. 2 Peter T. 1 Institute for Atmospheric and Climate Science, ETH Zurich, 8092, Zurich, Switzerland Department of Chemical Engineering, California Institute of Technology, Pasadena, 91125, California, USA 13 03 2012 12 5 2691 2712 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/12/2691/2012/acp-12-2691-2012.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/12/2691/2012/acp-12-2691-2012.pdf

Knowledge of the physical state and morphology of internally mixed organic/inorganic aerosol particles is still largely uncertain. To obtain more detailed information on liquid-liquid phase separation (LLPS) and morphology of the particles, we investigated complex mixtures of atmospherically relevant dicarboxylic acids containing 5, 6, and 7 carbon atoms (C5, C6 and C7) having oxygen-to-carbon atomic ratios (O:C) of 0.80, 0.67, and 0.57, respectively, mixed with ammonium sulfate (AS). With micrometer-sized particles of C5/AS/H<sub>2</sub>O, C6/AS/H<sub>2</sub>O and C7/AS/H<sub>2</sub>O as model systems deposited on a hydrophobically coated substrate, laboratory experiments were conducted for various organic-to-inorganic dry mass ratios (OIR) using optical microscopy and Raman spectroscopy. When exposed to cycles of relative humidity (RH), each system showed significantly different phase transitions. While the C5/AS/H<sub>2</sub>O particles showed no LLPS with OIR = 2:1, 1:1 and 1:4 down to 20% RH, the C6/AS/H<sub>2</sub>O and C7/AS/H<sub>2</sub>O particles exhibit LLPS upon drying at RH 50 to 85% and ~90%, respectively, via spinodal decomposition, growth of a second phase from the particle surface or nucleation-and-growth mechanisms depending on the OIR. This suggests that LLPS commonly occurs within the range of O:C < 0.7 in tropospheric organic/inorganic aerosols. To support the comparison and interpretation of the experimentally observed phase transitions, thermodynamic equilibrium calculations were performed with the AIOMFAC model. For the C7/AS/H<sub>2</sub>O and C6/AS/H<sub>2</sub>O systems, the calculated phase diagrams agree well with the observations while for the C5/AS/H<sub>2</sub>O system LLPS is predicted by the model at RH below 60% and higher AS concentration, but was not observed in the experiments. Both core-shell structures and partially engulfed structures were observed for the investigated particles, suggesting that such morphologies might also exist in tropospheric aerosols.

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