Fatty acids (CH 3 (CH 2) n−2 COOH) and their salts are an important class of atmospheric surfactants. Here, we use COSMOtherm to predict solubility and activity coefficients for CC fatty acids with even number of carbon atoms and their sodium salts in binary water solutions and also in ternary water-inorganic salt solutions. COSMOtherm is a continuum solvent model implementation which can calculate properties of complex systems using quantum chemistry and thermodynamics. Calculated solubility values of the organic acids in pure water are in good agreement with reported experimental values. The comparison of the COSMOtherm derived Setschenow constants for ternary solutions comprising NaCl with the corresponding experimental values from literature shows that COSMOtherm overpredicts the salting out effect in all cases except for the solutions of acetic acid. The calculated activity and mean activity coefficients of fatty acids and fatty acid sodium salts, respectively, show deviation of the systems from ideal solution. The computed mean activity coefficients of the fatty acid salts in binary systems are in better agreement with experimentally derived values for the organic salts with longer aliphatic chain (C 8-C 10). The deviation of the solutions from ideality could lead to biased estimations of cloud condensation nuclei number concentrations if not considered in Köhler calculations and cloud microphysics.

The formation of multicomponent aerosol particles from precursor solution droplets often involves segregation and surface enrichment of the different solutes, resulting in non-homogeneous particle structures and diverse morphologies. In particular,...

Abstract. Glyoxal (CHOCHO) and methylglyoxal (CH3C(O)CHO) are well-known components of atmospheric particles and their properties can impact atmospheric chemistry and cloud formation. To get information on their hydration states in aqueous solutions and how they are affected by the addition of inorganic salts (sodium chloride (NaCl) and sodium sulfate (Na2SO4)), we applied carbon and oxygen K-edge X-ray absorption spectroscopy (XAS) in transmission mode. The recorded C K-edge spectra show that glyoxal is completely hydrated in the dilute aqueous solutions, in line with previous studies. For methylglyoxal, supported by quantum chemical calculations we identified not only C–H, C=O and C–OH bonds, but also fingerprints of C–OH(CH2) and C=C bonds. The relatively low intensity of C=O transitions implies that the monohydrated form of methylglyoxal is not favored in the solutions. Instead, the spectral intensity is stronger in regions where products of aldol condensation and enol tautomers of the monohydrates contribute. The addition of salts was found to introduce only very minor changes to absorption energies and relative intensities of the observed absorption features, indicating that XAS in the near-edge region is not very sensitive to these intermolecular organic–inorganic interactions at the studied concentrations. The identified structures of glyoxal and methylglyoxal in an aqueous environment support the uptake of these compounds to the aerosol phase in the presence of water and their contribution to secondary organic aerosol formation.

<p><strong>Abstract.</strong> Glyoxal (CHOCHO) and methylglyoxal (CH<sub>3</sub>C(O)CHO) are well-known components of atmospheric particles and their properties can impact atmospheric chemistry and cloud formation. To get information on their hydration states in aqueous solutions and how they are affected by addition of inorganic salts (sodium chloride (NaCl) and sodium sulfate (Na<sub>2</sub>SO<sub>4</sub>)), we applied carbon and oxygen K-edge X-ray absorption spectroscopy (XAS) in transmission mode. The recorded C K-edge spectra show that glyoxal is completely hydrated in the dilute aqueous solutions, in line with previous studies. For methylglyoxal, we identified, supported by quantum chemical calculations, not only C-H, C=O and C-OH bonds, but also fingerprints of C-OH(CH<sub>2</sub>) and C=C bonds. This implies the presence of both mono- and dihydrated forms of methylglyoxal, as well as products of aldol condensation, and enol tautomers of the monohydrates. The addition of salts was found to introduce only very minor changes to absorption energies and relative intensities of the observed absorption features, indicating that the organic-inorganic interactions at the studied concentrations are not strong enough to affect the spectra in this work. The identified structures of glyoxal and methylglyoxal in aqueous environment support the uptake of these compounds to the aerosol phase in the presence of water and their contribution on secondary organic aerosol formation.</p>

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