Tropospheric aerosol particles consisting of inorganic salts and organic compounds undergo phase transitions such as deliquescence and efflorescence as a consequence of changes in ambient relative humidity (RH) [Martin, 2000; Marcolli and Krieger, 2006; Martin et al., 2008]. In addition, non-ideal interactions between dissolved inorganic ions, water, and organic compounds may lead to liquid-liquid phase separation (LLPS) [Marcolli and Krieger, 2006; Ciobanu et al., 2009; Zuend et al., 2010; Bertram et al., 2011; Song et al., 2012; Zuend and Seinfeld, 2012]. The physical state and the hygroscopicity of aerosol particles need to be considered for an accurate quantification of atmospheric and climate effects because they influence gas-particle partitioning of semivolatile compounds [e.g.,Zuend et al., 2010; Zuend and Seinfeld, 2012], heterogeneous chemistry [e.g., Anttila et al., 2007; Cosman et al., 2008] and the scattering and absorption of light [e.g., Martin et al., 2004]. The presence or absence of LLPS in aqueous mixtures can be determined in laboratory experiments with model mixtures representing tropospheric aerosols [e.g., Marcolli and Krieger, 2006; Bertram et al., 2011; Song et al., 2012] or computed using a liquid-liquid equilibrium model [Zuend et al., 2010; Zuend and Seinfeld, 2012].
 While ammonium sulfate (AS) is a main constituent of aerosol particles and well suited to represent the inorganic aerosol fraction in model mixtures, selecting appropriate surrogates for the organic fraction is more complex. The fractions of organics (18–70%), sulfate (10–67%) and ammonium (6.9–19%) have been measured by aerosol mass spectrometry at various locations [Zhang et al., 2007]. The organic aerosol fraction typically consists of up to thousands of different compounds and their characterization and quantification is a task that challenges common analytical techniques. Typically only 10–20% of the organic aerosol fraction can be identified on the level of individual substances using gas and liquid chromatography of filter extracts [Decesari et al., 2006; Hallquist et al., 2009]. With this approach, classes representing the organic aerosol have been identified. These comprise oxidized aliphatic substances such as mono- or multifunctional carboxylic acids, alcohols, polyols, sugars, and oxidized aromatic substances such as aromatic acids and aldehydes. Alternative methods are needed to gain a more quantitative overview of organic functionalities. Functional group analysis by proton NMR spectroscopy has identified aliphatic groups bound to an unsaturated carbon, alkoxyl, acetal, alkylic, and aromatic groups as main functionalities representing the organic aerosol fraction [Decesari et al., 2006]. FTIR spectroscopy is able to quantify alcohol, aromatic, aliphatic unsaturated, carbonyl, carboxylic acid, and amine groups [Gilardoni et al., 2009]. An even more general characterization of organic aerosols based on their degree of oxidation is achieved by high-resolution mass spectrometry that allows quantifying O:C and H:C ratios [e.g.,Heald et al., 2010; Ng et al., 2010]. Typical values of organic O:C in ambient samples range from 0.2 to 1.0 [Heald et al., 2010; Takahama et al., 2011]. Recent studies on a limited number of model systems have shown that LLPS in mixed organic/AS/H2O particles commonly occurs for O:C < 0.7 [Bertram et al., 2011; Song et al., 2012]. Hence, the O:C ratio of the organic aerosol fraction may be a good predictor for the presence of LLPS in tropospheric aerosol particles because it reflects the polarity of organic compounds and their miscibility with water and electrolytes. However, the investigated systems so far are small in number and most of them represent the organic fraction by one substance only. To determine whether the O:C ratio is indeed an accurate LLPS predictor for more realistic representations of the organic/inorganic aerosol, the following questions are addressed in this study: (i) Does in addition to the O:C ratio also the specific functional group composition influence the occurrence of LLPS? (ii) Does the number of components influence the occurrence of LLPS in organic/AS/H2O mixtures? (iii) Does the spread of O:C ratios of the individual organic components present in a mixture influence the occurrence of LLPS? To answer these questions, we investigated the presence or absence of LLPS in model systems of aerosol particles, consisting of various organic components, AS, and water.