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

Abstract

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 semivolatile 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.

A thermodynamic framework for bulk–surface partitioning in finite-volume mixed organic–inorganic aerosol particles and cloud droplets

“…The AIOMFAC model is a state-of-the-art thermodynamic mixing model capable of accurately predicting the chemical activity of species in atmospheric aerosol systems in a thermodynamically rigorous and consistent manner. The model has been updated numerous times to include many atmospherically relevant inorganic cations, anions, and organic functional groups, so that this model can represent a large number of mixed organic-inorganic aerosol systems, including systems with tens to thousands of components (Zuend et al, 2008(Zuend et al, , 2011Yin et al, 2022).…”

Role of Carbon Dioxide, Ammonia, and Organic Acids in Buffering Atmospheric Acidity: The Distinct Contribution in Clouds and Aerosols

“…As carbonic acid is a weak acid with a p K a of ∼6.4 and giving the high mixing ratios of CO 2 in the atmosphere, it was considered to strongly “buffer” the atmospheric water. − Especially, the pH of “pure” rainwater of ∼5.6 is determined by CO 2 , which just falls into the buffering pH ranges of H 2 CO 3 (6.4 ± 1), seemingly to support the strong buffering effect of CO 2 on rain.…”

“…For example, CO 2 determines the pH of “pure” raindrops (∼5.6, namely, the pH when the water is in equilibrium with gas-phase CO 2 mixing ratios of ∼350 ppm; see detailed processes in ref ), while its influence is often neglected in aerosol acidity calculations. In addition, while some studies ,− suggest that carbonates and organic weak acids (e.g., formic acid, acetic acid, and oxalic acid) can “buffer” the aerosol acidity, the recently proposed multiphase buffer theory suggests that this effect is usually negligible compared with the ammonia multiphase buffering. , In comparison, while the importance of ammonia in buffering the aerosols was well illustrated recently, its role in fogs/clouds is less understood. How and to what extent these species influence atmospheric acidity need to be clarified.…”

Mixed Solvent Electrolyte Solutions: A Review and Calculations with the eSAFT-VR Mie Equation of State

“…This model has been applied to VLE, LLE, and SLE of mixed solvent systems, as well as water activity and salt MIAC for an extensive collection of water with various alcohols and for multiple salts and offers an excellent aqueous electrolyte representation up to high ionic strength. In subsequent publications, this model was extended to other temperatures and more ions. − Another model belonging to the UNIFAC family, an extension of the model by Kikic et al, also used for aerosol particles, is X-UNIFAC. − This model uses the relative permittivity of the solvent mixture, calculated with a volume fraction mixing rule.…”