Solubility and Activity Coefficients of Atmospheric Surfactants in Aqueous Solution Evaluated Using COSMOtherm

Fatty acids (CH3(CH2)(n-2)COOH) and their salts are an important class of atmospheric surfactants. Here, we use COSMOtherm to predict solubility and activity coefficients for C-2-C-12 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 the 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 Kohler calculations and cloud microphysics.Peer reviewe

Technical note : Estimating aqueous solubilities and activity coefficients of mono- and alpha,omega-dicarboxylic acids using COSMOtherm

We have used the COSMOtherm program to estimate activity coefficients and solubilities of mono- and alpha, omega-dicarboxylic acids and water in binary acid-water systems. The deviation from ideality was found to be larger in the systems containing larger acids than in the systems containing smaller acids. COnductor-like Screening MOdel for Real Solvents (COSMO-RS) underestimates experimental monocarboxylic acid activity coefficients by less than a factor of 2, but experimental water activity coefficients are underestimated more especially at high acid mole fractions. We found a better agreement between COSMOtherm-estimated and experimental activity coefficients of monocarboxylic acids when the water clustering with a carboxylic acid and itself was taken into account using the dimerization, aggregation, and reaction extension (COSMO-RS-DARE) of COSMOtherm. COSMO-RS-DARE is not fully predictive, but fit parameters found here for water-water and acid-water clustering interactions can be used to estimate thermodynamic properties of monocarboxylic acids in other aqueous solvents, such as salt solutions. For the dicarboxylic acids, COSMO-RS is sufficient for predicting aqueous solubility and activity coefficients, and no fitting to experimental values is needed. This is highly beneficial for applications to atmospheric systems, as these data are typically not available for a wide range of mixing states realized in the atmosphere, due to a lack of either feasibility of the experiments or sample availability. Based on effective equilibrium constants of different clustering reactions in the binary solutions, acid dimer formation is more dominant in systems containing larger dicarboxylic acids (C-5-C-8), while for monocarboxylic acids (C-1-C-6) and smaller dicarboxylic acids (C-2-C-4), hydrate formation is more favorable, especially in dilute solutions.Peer reviewe

Cloud condensation nuclei activity of six pollenkitts and the influence of their surface activity

The role of surfactants in governing water interactions of atmospheric aerosols has been a recurring topic in cloud microphysics for more than two decades. Studies of detailed surface thermodynamics are limited by the availability of aerosol samples for experimental analysis and incomplete validation of various proposed Kohler model frameworks for complex mixtures representative of atmospheric aerosol. Pollenkitt is a viscous material that coats grains of pollen and plays important roles in pollen dispersion and plant reproduction. Previous work suggests that it may also be an important contributor to pollen water uptake and cloud condensation nuclei (CCN) activity. The chemical composition of pollenkitt varies between species but has been found to comprise complex organic mixtures including oxygenated, lipid, and aliphatic functionalities. This mix of functionalities suggests that pollenkitt may display aqueous surface activity, which could significantly impact pollen interactions with atmospheric water. Here, we study the surface activity of pollenkitt from six different species and its influence on pollenkitt hygroscopicity. We measure cloud droplet activation and concentration-dependent surface tension of pollenkitt and its mixtures with ammonium sulfate salt. Experiments are compared to predictions from several thermodynamic models, taking aqueous surface tension reduction and surfactant surface partitioning into account in various ways. We find a clear reduction of surface tension by pollenkitt in aqueous solution and evidence for impact of both surface tension and surface partitioning mechanisms on cloud droplet activation potential and hygroscopicity of pollenkitt particles. In addition, we find indications of complex nonideal solution effects in a systematic and consistent dependency of pollenkitt hygroscopicity on particle size. The impact of pollenkitt surface activity on cloud microphysics is different from what is observed in previous work for simple atmospheric surfactants and more resembles recent observations for complex primary and secondary organic aerosol, adding new insight to our understanding of the multifaceted role of surfactants in governing aerosol-water interactions. We illustrate how the explicit characterization of pollenkitt contributions provides the basis for modeling water uptake and cloud formation of pollen and their fragments over a wide range of atmospheric conditions.Peer reviewe

Can COSMOTherm Predict a Salting in Effect?

We have used COSMO-RS, a method combining quantum chemistry with statistical thermodynamics, to compute Setschenow constants (K-S) for a large array of organic solutes and salts. These comprise both atmospherically relevant solute-salt combinations, as well as systems for which experimental data are available. In agreement with previous studies on single salts, the Setschenow constants predicted by COSMO-RS (as implemented in the COSMOTherm program) are generally too large compared to experiments. COSMOTherm overpredicts salting out (positive K-S), and/or underpredicts salting in (negative K-S). For ammonium and sodium salts, K-S values are larger for oxalates and sulfates, and smaller for chlorides and bromides. For chloride and bromide salts, K-S values usually increase with decreasing size of the cation, along the series Pr4N+ <Et4N+ <Me4N+Peer reviewe

Is Decreased Xylem Sap Surface Tension Associated With Embolism and Loss of Xylem Hydraulic Conductivity in Pathogen-Infected Norway Spruce Saplings?

Increased abiotic stress along with increasing temperatures, dry periods and forest disturbances may favor biotic stressors such as simultaneous invasion of bark beetle and ophiostomatoid fungi. It is not fully understood how tree desiccation is associated with colonization of sapwood by fungi. A decrease in xylem sap surface tension (sigma(xylem)) as a result of infection has been hypothesized to cause xylem embolism by lowering the threshold for air-seeding at the pits between conduits and disruptions in tree water transport. However, this hypothesis has not yet been tested. We investigated tree water relations by measuring the stem xylem hydraulic conductivity (K-stem), sigma(xylem), stem relative water content (RWCstem), and water potential (psi(stem)), and canopy conductance (g(canopy)), as well as the compound composition in xylem sap in Norway spruce (Picea abies) saplings. We conducted our measurements at the later stage ofEndoconidiophora polonicainfection when visible symptoms had occurred in xylem. Saplings of two clones (44 trees altogether) were allocated to treatments of inoculated, wounded control and intact control trees in a greenhouse. The saplings were destructively sampled every second week during summer 2016. sigma(xylem), K(stem)and RWC(stem)decreased following the inoculation, which may indicate that decreased sigma(xylem)resulted in increased embolism. g(canopy)did not differ between treatments indicating that stomata responded to psi(stem)rather than to embolism formation. Concentrations of quinic acid, myo-inositol, sucrose and alkylphenol increased in the xylem sap of inoculated trees. Myo-inositol concentrations also correlated negatively with sigma(xylem)and K-stem. Our study is a preliminary investigation of the role of sigma(xylem)inE. polonicainfected trees based on previous hypotheses. The results suggest thatE. polonicainfection can lead to a simultaneous decrease in xylem sap surface tension and a decline in tree hydraulic conductivity, thus hampering tree water transport.Peer reviewe

Is Decreased Xylem Sap Surface Tension Associated With Embolism and Loss of Xylem Hydraulic Conductivity in Pathogen-Infected Norway Spruce Saplings?

Increased abiotic stress along with increasing temperatures, dry periods and forest disturbances may favor biotic stressors such as simultaneous invasion of bark beetle and ophiostomatoid fungi. It is not fully understood how tree desiccation is associated with colonization of sapwood by fungi. A decrease in xylem sap surface tension (sigma(xylem)) as a result of infection has been hypothesized to cause xylem embolism by lowering the threshold for air-seeding at the pits between conduits and disruptions in tree water transport. However, this hypothesis has not yet been tested. We investigated tree water relations by measuring the stem xylem hydraulic conductivity (K-stem), sigma(xylem), stem relative water content (RWCstem), and water potential (psi(stem)), and canopy conductance (g(canopy)), as well as the compound composition in xylem sap in Norway spruce (Picea abies) saplings. We conducted our measurements at the later stage ofEndoconidiophora polonicainfection when visible symptoms had occurred in xylem. Saplings of two clones (44 trees altogether) were allocated to treatments of inoculated, wounded control and intact control trees in a greenhouse. The saplings were destructively sampled every second week during summer 2016. sigma(xylem), K(stem)and RWC(stem)decreased following the inoculation, which may indicate that decreased sigma(xylem)resulted in increased embolism. g(canopy)did not differ between treatments indicating that stomata responded to psi(stem)rather than to embolism formation. Concentrations of quinic acid, myo-inositol, sucrose and alkylphenol increased in the xylem sap of inoculated trees. Myo-inositol concentrations also correlated negatively with sigma(xylem)and K-stem. Our study is a preliminary investigation of the role of sigma(xylem)inE. polonicainfected trees based on previous hypotheses. The results suggest thatE. polonicainfection can lead to a simultaneous decrease in xylem sap surface tension and a decline in tree hydraulic conductivity, thus hampering tree water transport.Peer reviewe

Estimates of the organic aerosol volatility in a boreal forest using two independent methods

The volatility distribution of secondary organic aerosols that formed and had undergone aging - i. e., the particle mass fractions of semi-volatile, low-volatility and extremely low volatility organic compounds in the particle phase - was characterized in a boreal forest environment of Hyytiala, southern Finland. This was done by interpreting field measurements using a volatility tandem differential mobility analyzer (VTDMA) with a kinetic evaporation model. The field measurements were performed during April and May 2014. On average, 40% of the organics in particles were semi-volatile, 34% were low-volatility organics and 26% were extremely low volatility organics. The model was, however, very sensitive to the vaporization enthalpies assumed for the organics (Delta H-VAP). The best agreement between the observed and modeled temperature dependence of the evaporation was obtained when effective vaporization enthalpy values of 80 kJ mol(-1) were assumed. There are several potential reasons for the low effective enthalpy value, including molecular decomposition or dissociation that might occur in the particle phase upon heating, mixture effects and compound-dependent uncertainties in the mass accommodation coefficient. In addition to the VTDMA-based analysis, semi-volatile and low-volatility organic mass fractions were independently determined by applying positive matrix factorization (PMF) to high-resolution aerosol mass spectrometer (HR-AMS) data. The factor separation was based on the oxygenation levels of organics, specifically the relative abundance of mass ions at m/z 43 (f43) and m/z 44 (f44). The mass fractions of these two organic groups were compared against the VTDMA-based results. In general, the best agreement between the VTDMA results and the PMF-derived mass fractions of organics was obtained when Delta H-VAP D 80 kJ mol(-1) was set for all organic groups in the model, with a linear correlation coefficient of around 0.4. However, this still indicates that only about 16% (R-2)of the variation can be explained by the linear regression between the results from these two methods. The prospect of determining of extremely low volatility organic aerosols (ELVOAs) from AMS data using the PMF analysis should be assessed in future studies.Peer reviewe

Strong Even/Odd Pattern in the Computed Gas-Phase Stability of Dicarboxylic Acid Dimers : Implications for Condensation Thermodynamics

The physical properties of small straight-chain dicarboxylic acids are well known to exhibit even/odd alternations with respect to the carbon chain length. For example, odd numbered diacids have lower melting points and higher saturation vapor pressures than adjacent even numbered diacids. This alternation has previously been explained in terms of solid-state properties, such as higher torsional strain of odd number diacids. Using quantum chemical methods, we demonstrate an additional contribution to this alternation in properties resulting from gas-phase dimer formation. Due to a combination of hydrogen bond strength and torsional strain, dimer formation in the gas phase occurs efficiently for glutaric acid (CS) and pimelic acid (C7) but is unfavorable for succinic acid (C4) and adipic acid (C6). Our results indicate that a significant fraction of the total atmospheric gas-phase concentration of glutaric and pimelic acid may consist of dimers.Peer reviewe

Aqueous-phase behavior of glyoxal and methylglyoxal observed with carbon and oxygen K-edge X-ray absorption spectroscopy

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.Peer reviewe