Computation of liquid-liquid equilibria and phase stabilities: implications for RH-dependent gas/particle partitioning of organic-inorganic aerosols

摘要

Semivolatile organic and inorganic aerosol speciespartition between the gas and aerosol particle phases to maintainthermodynamic equilibrium. Liquid-liquid phase separation into anorganic-rich and an aqueous electrolyte phase can occur in the aerosol as aresult of the salting-out effect. Such liquid-liquid equilibria (LLE) affectthe gas/particle partitioning of the different semivolatile compounds andmight significantly alter both particle mass and composition as compared to aone-phase particle. We present a new liquid-liquid equilibrium andgas/particle partitioning model, using as a basis the group-contributionmodel AIOMFAC (Zuend et al., 2008). This model allows the reliablecomputation of the liquid-liquid coexistence curve (binodal), correspondingtie-lines, the limit of stability/metastability (spinodal), and furtherthermodynamic properties of multicomponent systems. Calculations for ternary andmulticomponent alcohol/polyol-water-salt mixtures suggest that LLE are aprevalent feature of organic-inorganic aerosol systems. A six-componentpolyol-water-ammonium sulphate system is used to simulate effects ofrelative humidity (RH) and the presence of liquid-liquid phase separation onthe gas/particle partitioning. RH, salt concentration, and hydrophilicity(water-solubility) are identified as key features in defining the region of amiscibility gap and govern the extent to which compound partitioning isaffected by changes in RH. The model predicts that liquid-liquid phaseseparation can lead to either an increase or decrease in total particulatemass, depending on the overall composition of a system and the particle watercontent, which is related to the hydrophilicity of the different organic andinorganic compounds. Neglecting non-ideality and liquid-liquid phaseseparations by assuming an ideal mixture leads to an overestimation of thetotal particulate mass by up to 30% for the composition and RH rangeconsidered in the six-component system simulation. For simplifiedpartitioning parametrizations, we suggest a modified definition of theeffective saturation concentration, C_j*, by including water andother inorganics in the absorbing phase. Such a C_j* definitionreduces the RH-dependency of the gas/particle partitioning of semivolatileorganics in organic-inorganic aerosols by an order of magnitude as comparedto the currently accepted definition, which considers the organic speciesonly.