24 Aug 2021

24 Aug 2021

Review status: this preprint is currently under review for the journal ACP.

Extension of the AIOMFAC model by iodine and carbonate species: applications for aerosol acidity and cloud droplet activation

Hang Yin1, Jing Dou2, Liviana Klein2, Ulrich K. Krieger2, Alison Bain3, Brandon J. Wallace3, Thomas C. Preston1,3, and Andreas Zuend1 Hang Yin et al.
  • 1Department of Atmospheric and Oceanic Sciences, McGill University, Montreal, Quebec, Canada
  • 2Institute for Atmospheric and Climate Science, ETH Zurich, Switzerland
  • 3Department of Chemistry, McGill University, Montreal, Quebec, Canada

Abstract. Iodine and carbonate species are important components in marine and dust aerosols, respectively. The non-ideal interactions between these species and other inorganic and organic compounds within aqueous particle phases affect hygroscopicity, acidity, and gas–particle partitioning of semi-volatile components. In this work, we present an extended version of the Aerosol Inorganic–Organic Mixtures Functional groups Activity Coefficients (AIOMFAC) model by incorporating the ions I, IO3, HCO3, CO32−, OH, and CO2(aq) as new species. First, AIOMFAC ion interaction parameters for aqueous solutions were determined based on available thermodynamic data, such as water activity, mean molal activity coefficients, solubility, and vapor–liquid equilibrium measurements. Second, the interaction parameters for the new ions and various organic functional groups were optimized based on experimental data or, where data are scarce, alternative estimation methods such as multiple linear regression or a simple substitution by analogy approach. Additional bulk water activity and electrodynamic balance measurements were carried out to augment the database for the AIOMFAC parameter fit. While not optimal, we show that the use of alternative parameter estimation methods enables physically sound predictions and offers the benefit of a more broadly applicable model. Our implementation of the aqueous carbonate/bicarbonate/CO2(aq) system accounts for the associated temperature-dependent dissociation equilibria explicitly and enables closed- or open-system computations with respect to carbon dioxide equilibration with the gas phase. We discuss different numerical approaches for solving the coupled equilibrium conditions and highlight critical considerations when extremely acidic or basic mixtures are encountered.

The fitted AIOMFAC model performance for inorganic aqueous systems is considered excellent over the whole range of mixture compositions where reference data are available. Moreover, the model provides physically meaningful predictions of water activity under highly concentrated conditions. For organic–inorganic mixtures involving new species, the model–measurement agreement is found to be good in most cases, especially at equilibrium relative humidities above ~70 %; reasons for deviations are discussed. Several applications of the extended model are shown and discussed, including: the effects of ignoring the auto-dissociation of water in carbonate systems, the effects of mixing bisulfate and bicarbonate compounds in closed- or open-system scenarios on pH and solution speciation, and the prediction of critical cloud condensation nucleus activation of NaI or Na2CO3 particles mixed with suberic acid.

Hang Yin et al.

Status: open (until 10 Oct 2021)

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Hang Yin et al.


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Short summary
Iodine and carbonate species are important components in marine and dust aerosols, respectively. We introduce an extended version of the AIOMFAC thermodynamic mixing model, which includes the ions I, IO3, HCO3, CO32−, OH, and CO2(aq) as new species, and we discuss two methods for solving the carbonate dissociation equilibria numerically. We also present new experimental water activity data for aqueous iodide and iodate systems.