Prediction of activity coefficients in liquid aerosol particles containing organic compounds, dissolved inorganic salts, and water—Part 1: Organic compounds and water by consideration of short- and long-range effects using X-UNIFAC.1

The semi-empirical group contribution method (GCM) of Kikic et al. [Chem. Eng. Sci. 46 (1991) 2775–2780] for estimating activity coefficient (ζ) values of neutral organic compounds and water in solutions composed of organic compounds, dissolved inorganic salts, and water is adapted for application to atmospheric particulate matter (PM). It is assumed that ζ values are determined by a combination of short- and long-range interactions. The ζ expression involves conventional UNIFAC terms and a Debye–Hückel term, with the former computed using group–group interaction parameters. Organic–organic interaction parameters are assigned the values from the UNIFAC-LLE model of Magnussen et al. [Ind. Eng. Chem. Process Design Develop. 20 (1981) 331–339]. Forty interaction parameters (ion–solvent group and anion–cation) were obtained from Kikic et al. [Chem. Eng. Sci. 46 (1991) 2775–2780], Achard et al. [Fluid Phase Equilibria 98 (1994) 71–89], and Ming and Russell [Am. Inst. Chem. Eng. J. 48 (2002) 1331–1348]. Twenty additional interaction parameters (ion–solvent group) are estimated based on 879 UNIQUAC-fitted ζ values for organic compounds and water. The fitted ζ values are based on liquid–liquid equilibrium (LLE) data for a range of ternary and quaternary organic/inorganic salt/water mixtures at 293–308 K. The UNIQUAC fits are analogous to those described by Fredenslund et al. [Vapor–Liquid Equilibria Using UNIFAC: A Group-Contribution Method, Elsevier Scientific Publishing, New York, 1977]. The LLE mixture compositions range from primarily organic solutions to primarily aqueous solutions with maximum ionic strengths of ∼5 mol kg−1. The groups characteristic of organic compounds found in atmospheric PM considered here include: CH3–, –CH2–, -CH|-, -C||-, –OH, –CH2CO–, and –COOH. These are: single bonded carbon with three, two, one, and zero hydrogens, respectively, hydroxyl, –CH2-carbonyl, and carboxyl, respectively. The inorganic salts represented in the mixture data include NaCl, NaNO3, Na2SO4, (NH4)2SO4, and CaCl2 so that the cations and anions considered include Na+, NH4+, and Ca2+, and Cl−, NO3-, and SO42-, respectively. The method predicts the UNIQUAC-fitted ζ values for all datasets with an average error of ∼20%. In an application of the method, ζ values are predicted in an aerosol PM phase containing four oxidation products (from α-pinene/O3) and water, without and with 1 and 2 mol kg−1 dissolved (NH4)2SO4. The presence of the dissolved salt can cause significant increases in the ζ values of the oxidation products considered, reflecting a potential “salting-out” effect for (NH4)2SO4 on the oxidation products considered. Results indicate an important role played by dissolved salts in affecting the thermodynamic properties of atmospheric organic PM and the utility of X-UNIFAC.1 as a tool for evaluating those effects.