Abstract
Abstract. We present a new and considerably extended parameterization of the thermodynamic activity coefficient model AIOMFAC (Aerosol Inorganic-Organic Mixtures Functional groups Activity Coefficients) at room temperature. AIOMFAC combines a Pitzer-like electrolyte solution model with a UNIFAC-based group-contribution approach and explicitly accounts for interactions between organic functional groups and inorganic ions. Such interactions constitute the salt-effect, may cause liquid-liquid phase separation, and affect the gas-particle partitioning of aerosols. The previous AIOMFAC version was parameterized for alkyl and hydroxyl functional groups of alcohols and polyols. With the goal to describe a wide variety of organic compounds found in atmospheric aerosols, we extend here the parameterization of AIOMFAC to include the functional groups carboxyl, hydroxyl, ketone, aldehyde, ether, ester, alkenyl, alkyl, aromatic carbon-alcohol, and aromatic hydrocarbon. Thermodynamic equilibrium data of organic-inorganic systems from the literature are critically assessed and complemented with new measurements to establish a comprehensive database. The database is used to determine simultaneously the AIOMFAC parameters describing interactions of organic functional groups with the ions H+, Li+, Na+, K+, NH4+, Mg2+, Ca2+, Cl−, Br−, NO3−, HSO4−, and SO42−. Detailed descriptions of different types of thermodynamic data, such as vapor-liquid, solid-liquid, and liquid-liquid equilibria, and their use for the model parameterization are provided. Issues regarding deficiencies of the database, types and uncertainties of experimental data, and limitations of the model, are discussed. The challenging parameter optimization problem is solved with a novel combination of powerful global minimization algorithms. A number of exemplary calculations for systems containing atmospherically relevant aerosol components are shown. Amongst others, we discuss aqueous mixtures of ammonium sulfate with dicarboxylic acids and with levoglucosan. Overall, the new parameterization of AIOMFAC agrees well with a large number of experimental datasets. However, due to various reasons, for certain mixtures important deviations can occur. The new parameterization makes AIOMFAC a versatile thermodynamic tool. It enables the calculation of activity coefficients of thousands of different organic compounds in organic-inorganic mixtures of numerous components. Models based on AIOMFAC can be used to compute deliquescence relative humidities, liquid-liquid phase separations, and gas-particle partitioning of multicomponent mixtures of relevance for atmospheric chemistry or in other scientific fields.
Highlights
Thermodynamic models are key tools to gain insight into the non-ideal behavior of organic-inorganic mixtures
Gas-particle partitioning of water and semivolatile organic and inorganic compounds is determined by thermodynamic equilibrium between the gaseous and condensed phases (Pankow, 1994, 2003; Hallquist et al, 2009; Zuend et al, 2010) and by the kinetics of exchange processes such as gas phase diffusion (Marcolli et al, 2004b)
AIOMFAC is based on the group-contribution model LIFAC (Yan et al, 1999) – yet modified in many respects to better represent relevant species, reference states, and the relative humidity range of the atmosphere. This is described in our previous work (Zuend et al, 2008), where we we have considered cations H+, Li+, Na+, K+, NH+4, Mg2+, and Ca2+, anions Cl−, Br−, NO−3, HSO−4, and SO24− and a wide range of alcohols/polyols composed of the alkyl (CHn, n = 0, 1, 2, 3) and hydroxyl (OH) functional groups for a first parameterization of organic-inorganic interactions
Summary
Thermodynamic models are key tools to gain insight into the non-ideal behavior of organic-inorganic mixtures. Interactions between ions and neutral organic molecules may have a crucial impact on the dissolution behavior and phase state of a system, commonly known as the salt-effect: Increasing the concentration of a strong electrolyte in a mixture may lead to “salting-out” of relatively nonpolar organics, i.e., the dissolved ions drive the organic compounds out of the mixed phase – either to the gas phase or into a different, organic-rich liquid phase, initiating or modifying a liquid-liquid phase separation and a new equilibrium state This well-known property of electrolytes is used in chemical and biochemical process engineering to separate aqueous organic mixtures (liquid-liquid extraction, two-phase partitioning) and to shift azeotropes in distillation processes, with large-scale applications in the petrochemical industry, in seawater desalination plants, and water purification systems. With respect to tropospheric aerosols, recent modeling studies (Zuend et al, 2010) and experiments (Smith et al, 2011; Bertram et al, 2011) on the phase state of idealized laboratory organic-inorganic aerosol mixtures suggest that ambient aerosols likely exhibit liquid-liquid phase separation at relative humidities (RH) 85 %
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