Abstract
Abstract. This study presents a new, improved parameterisation of the temperature dependence of activity coefficients in the AIOMFAC (Aerosol Inorganic–Organic Mixtures Functional groups Activity Coefficients) model applicable for aqueous as well as water-free organic solutions. For electrolyte-free organic and organic–water mixtures the AIOMFAC model uses a group-contribution approach based on UNIFAC (UNIversal quasi-chemical Functional-group Activity Coefficients). This group-contribution approach explicitly accounts for interactions among organic functional groups and between organic functional groups and water. The previous AIOMFAC version uses a simple parameterisation of the temperature dependence of activity coefficients, aimed to be applicable in the temperature range from ~ 275 to ~ 400 K. With the goal to improve the description of a wide variety of organic compounds found in atmospheric aerosols, we extend the AIOMFAC parameterisation for the functional groups carboxyl, hydroxyl, ketone, aldehyde, ether, ester, alkyl, aromatic carbon-alcohol, and aromatic hydrocarbon to atmospherically relevant low temperatures. To this end we introduce a new parameterisation for the temperature dependence. The improved temperature dependence parameterisation is derived from classical thermodynamic theory by describing effects from changes in molar enthalpy and heat capacity of a multi-component system. Thermodynamic equilibrium data of aqueous organic and water-free organic mixtures from the literature are carefully assessed and complemented with new measurements to establish a comprehensive database, covering a wide temperature range (~ 190 to ~ 440 K) for many of the functional group combinations considered. Different experimental data types and their processing for the estimation of AIOMFAC model parameters are discussed. The new AIOMFAC parameterisation for the temperature dependence of activity coefficients from low to high temperatures shows an overall improvement of 28% in comparison to the previous model version, when both versions are compared to our database of experimentally determined activity coefficients and related thermodynamic data. When comparing the previous and new AIOMFAC model parameterisations to the subsets of experimental data with all temperatures below 274 K or all temperatures above 322 K (i.e. outside a 25 K margin of the reference temperature of 298 K), applying the new parameterisation leads to 37% improvement in each of the two temperature ranges considered. The new parameterisation of AIOMFAC agrees well with a large number of experimental data sets. Larger model–measurement discrepancies were found particularly for some of the systems containing multi-functional organic compounds. The affected systems were typically also poorly represented at room temperature and further improvements will be necessary to achieve better performance of AIOMFAC in these cases (assuming the experimental data are reliable). The performance of the AIOMFAC parameterisation is typically better for systems containing relatively small organic compounds and larger deviations may occur in mixtures where molecules of high structural complexity such as highly oxygenated compounds or molecules of high molecular mass (e.g. oligomers) prevail. Nevertheless, the new parameterisation enables the calculation of activity coefficients for a wide variety of different aqueous/water-free organic solutions down to the low temperatures present in the upper troposphere.
Highlights
Atmospheric aerosols are complex mixtures of inorganic and organic components
As in the UNIFAC model, in the short range (SR) part of AIOMFAC, activity coefficients of a mixture component j are in general expressed as the sum of contributions of a combinatorial part, which accounts for the size and shape of the molecule, and the residual part, which reflects the residual contribution from intermolecular interactions (Fredenslund et al, 1975; Marcolli and Peter, 2005; Zuend et al, 2008)
We compare the model performance of the new AIOMFAC-P3 version, with AIOMFAC-P1 based on overall quantitative measures followed by a discussion of a selection of aqueous organic mixtures and water-free organic mixtures
Summary
Atmospheric aerosols are complex mixtures of inorganic and organic components. A large variety of organic compounds account for a significant fraction of the tropospheric aerosol composition. UNIFAC thermodynamic model calculations for component activity coefficients at infinite dilution are sometimes not in agreement with the experimental data This can be understood since most VLE measurements were performed for liquid mole fractions between 0.02 to 0.98 and, do not provide specific information for the highly dilute regions (Compernolle and Müller, 2014). A small uncertainty in aw of about 0.025 can change the corresponding homogeneous nucleation rate coefficients by 6 orders of magnitude and may significantly affect predictions of the onset of ice crystal formation in cloud microphysical models (Knopf and Rigg, 2011; Alpert et al, 2011) This shows the need for an improved UNIFAC (and AIOMFAC) parameterisation at low temperatures. The new AIOMFAC parameterisation introduced in this work leads to substantial improvements in activity coefficient calculations at temperatures significantly higher than room temperature, which is of interest for applications in other fields of science and engineering, such as distillation
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