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Computation of Liquid-Liquid Equilibria and Phase Stabilities: Implications for RH-Dependent Gas/Particle Partitioning of Organic-Inorganic Aerosols

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Computation of Liquid-Liquid Equilibria and Phase Stabilities: Implications for RH-Dependent Gas/Particle Partitioning of Organic-Inorganic Aerosols Atmos. Chem. Phys., 10, 7795–7820, 2010 www.atmos-chem-phys.net/10/7795/2010/ Atmospheric doi:10.5194/acp-10-7795-2010 Chemistry © Author(s) 2010. CC Attribution 3.0 License. and Physics Computation of liquid-liquid equilibria and phase stabilities: implications for RH-dependent gas/particle partitioning of organic-inorganic aerosols A. Zuend1,2, C. Marcolli2, T. Peter2, and J. H. Seinfeld1 1Department of Chemical Engineering, California Institute of Technology, 91125, Pasadena, California, USA 2Institute for Atmospheric and Climate Science, ETH Zurich, 8092, Zurich, Switzerland Received: 4 May 2010 – Published in Atmos. Chem. Phys. Discuss.: 12 May 2010 Revised: 17 August 2010 – Accepted: 19 August 2010 – Published: 24 August 2010 Abstract. Semivolatile organic and inorganic aerosol species tem and the particle water content, which is related to the partition between the gas and aerosol particle phases to hydrophilicity of the different organic and inorganic com- maintain thermodynamic equilibrium. Liquid-liquid phase pounds. Neglecting non-ideality and liquid-liquid phase sep- separation into an organic-rich and an aqueous electrolyte arations by assuming an ideal mixture leads to an overestima- phase can occur in the aerosol as a result of the salting- tion of the total particulate mass by up to 30% for the compo- out effect. Such liquid-liquid equilibria (LLE) affect the sition and RH range considered in the six-component system gas/particle partitioning of the different semivolatile com- simulation. For simplified partitioning parametrizations, we pounds and might significantly alter both particle mass and suggest a modified definition of the effective saturation con- ∗ composition as compared to a one-phase particle. We present centration, Cj , by including water and other inorganics in ∗ a new liquid-liquid equilibrium and gas/particle partitioning the absorbing phase. Such a Cj definition reduces the RH- model, using as a basis the group-contribution model AIOM- dependency of the gas/particle partitioning of semivolatile FAC (Zuend et al., 2008). This model allows the reliable organics in organic-inorganic aerosols by an order of magni- computation of the liquid-liquid coexistence curve (binodal), tude as compared to the currently accepted definition, which corresponding tie-lines, the limit of stability/metastability considers the organic species only. (spinodal), and further thermodynamic properties of mul- ticomponent systems. Calculations for ternary and multi- component alcohol/polyol-water-salt mixtures suggest that 1 Introduction LLE are a prevalent feature of organic-inorganic aerosol sys- tems. A six-component polyol-water-ammonium sulphate Primary aerosol particles, directly emitted to the atmosphere, system is used to simulate effects of relative humidity (RH) and secondary aerosol, formed by gas-to-particle conversion, and the presence of liquid-liquid phase separation on the undergo chemical and physical processing during their life- gas/particle partitioning. RH, salt concentration, and hy- time, resulting in a mixture of organic and inorganic aerosol drophilicity (water-solubility) are identified as key features species (Maria et al., 2004; Kanakidou et al., 2005; Aumont in defining the region of a miscibility gap and govern the ex- et al., 2005). Single particle measurements suggest that or- tent to which compound partitioning is affected by changes ganic and inorganic species are internally mixed in particu- in RH. The model predicts that liquid-liquid phase separa- late matter (PM), except for air masses close to local sources tion can lead to either an increase or decrease in total partic- (Murphy and Thomson, 1997; Middlebrook et al., 1998; Lee ulate mass, depending on the overall composition of a sys- et al., 2002; Murphy et al., 2006). Internal mixing of organ- ics and inorganics is established by gas phase diffusion (Mar- colli et al., 2004b) and partitioning between the gas phase and Correspondence to: A. Zuend particle phases driving the system towards thermodynamic ([email protected]) equilibrium. Published by Copernicus Publications on behalf of the European Geosciences Union. 7796 A. Zuend et al.: Computation of LLE and gas/particle partitioning of aerosols While the most prevalent inorganic aerosol components certainty in describing liquid phase non-ideality, represented are well established (Zhang et al., 2007; Russell et al., 2009), (x) by γj , and related effects, such as liquid-liquid phase sep- the organic fraction can consist of a large number of dif- arations. A third major uncertainty, endemic to modelling ferent compounds (Goldstein and Galbally, 2007), contain- of atmospheric organic aerosols, is the lack of knowledge of ing functional groups such as alkyl, hydroxyl, carboxyl, car- the actual molecular composition of the aerosol phase (the bonyl, and aromatic groups (Rogge et al., 1993; Saxena and “j”s). In this study, we focus on a systematic reduction Hildemann, 1996; Yu et al., 1999; Decesari et al., 2000; of uncertainties related to issue (2) by means of modelling Griffin et al., 2002; Decesari et al., 2006; Russell et al., phase behaviour and gas/particle partitioning of non-ideal 2009). Non-ideal interactions between organic and inor- solutions typical of these present in atmospheric organic- ganic species in the liquid aerosol phase affect water up- inorganic aerosols. take (Saxena et al., 1995; Clegg et al., 2001; Zuend et al., Thermodynamic models are essential for the computation 2008), may induce phase separation (Pankow, 2003; Erdakos of phase diagrams and gas/particle partitioning of organic- and Pankow, 2004; Chang and Pankow, 2006; Marcolli and inorganic mixtures. Such models help to understand and Krieger, 2006), affect efflorescence and deliquescence be- link data from different experimental techniques and to haviour (Choi and Chan, 2002; Chan and Chan, 2003; Pant predict thermodynamic properties for multicomponent mix- et al., 2004; Parsons et al., 2004; Marcolli and Krieger, tures. Models have been developed for calculating the parti- 2006; Ling and Chan, 2008), and influence the gas/particle tioning of organics into an organic aerosol particle (Pankow partitioning of semivolatile compounds (Saxena and Hilde- et al., 2001), as well as for organics and water into an aque- mann, 1997; Turpin et al., 2000; Cocker et al., 2001; Seinfeld ous organic solution (Seinfeld et al., 2001). So far, only et al., 2001; Bowman and Melton, 2004; Chang and Pankow, a few models have been reported that are able to predict 2006). Moreover, liquid and solid phases present in aerosol gas/particle partitioning of mixed organic-inorganic systems particles provide reaction media for heterogeneous and mul- (Pun et al., 2002; Griffin et al., 2003; Chang and Pankow, tiphase chemistry (Ravishankara, 1997; Kalberer et al., 2004; 2006, 2010; Pankow and Chang, 2008). The model of Pun Barsanti and Pankow, 2004; Knopf et al., 2005; Herrmann et al. (2002) uses a so called hydrophobic-hydrophilic ap- et al., 2005; Kroll and Seinfeld, 2008). At least in the lower proach, characterizing the particulate matter in terms of a troposphere, most of the organic aerosol (OA) fraction is hydrophobic organic phase and a hydrophilic aqueous phase, expected to be present in liquid form even at low relative allowing an organic compound to partition only to one of the humidity (RH), because the large number of organic com- two phases. Griffin et al. (2003) modified this hydrophobic- pounds depresses the temperature at which solids form (Mar- hydrophilic approach to allow equilibration of all organic colli et al., 2004a). compounds between both condensed phases and the gas The gas/particle partitioning of a specific compound de- phase. However, in their approach water and salts were not pends on the ambient conditions (temperature, RH), its allowed to partition between both condensed phases. Chang vapour pressure and on the concentrations and vapour pres- and Pankow (2006) introduced a gas/particle partitioning and sures of all other particle-phase species, and the manner in activity coefficient model (XUNIFAC.2) based on the ac- which they interact (non-ideality). These factors determine tivity coefficient model LIFAC (Yan et al., 1999) and the whether an organic compound resides primarily in the gas gas/particle partitioning theory of Pankow (2003), that en- phase, primarily in the particulate phase, or to a significant ables the equilibration of all species between all phases degree in both phases. Following Pankow (2003), at tem- present. perature T , partitioning of compound j can be expressed We present in this work a new liquid-liquid phase sep- in terms of a molar (n) particle/gas equilibrium coefficient aration and gas/particle partitioning model, using as a ba- PM,(n) Kj : sis the group-contribution model AIOMFAC (Zuend et al., 2008). Considering the potential importance of liquid-liquid RT KPM,(n) = , (1) equilibria (LLE), a reliable method to check and account for j ◦ (x) pj γj phase separations has been implemented and tested with a number of ternary alcohol-water-salt solutions at room tem- where R is the universal gas constant, p◦ the pure liquid com- j perature. Motivated by the ability of a thermodynamic model (x) pound vapour pressure, and γj the mole fraction-based ac- to provide additional information regarding the stability of tivity coefficient. Details and assumptions involved in the liquid phases, we introduce a method to compute the spin- derivation of Eq. (1) are given in Sect. 3.1. From Eq. (1), two odal curves of ternary mixtures. The resulting stability di- of the most prominent sources of uncertainties in modelling agrams allow the determination of metastable and unstable the gas/particle partitioning of complex, often multifunc- one-phase regions with respect to composition in the phase tional organic compounds are evident (Hallquist et al., 2009; diagram. Barley and McFiggans, 2010): (1) potentially large errors in ◦ estimating pure compound vapour pressures (pj as a func- tion of T ) in the absence of actual measurements and (2) un- Atmos. Chem. Phys., 10, 7795–7820, 2010 www.atmos-chem-phys.net/10/7795/2010/ A.
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