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Surface active compounds (surfactants) found in atmospheric aerosols can decrease droplet surface tension as they adsorb to the droplet surfaces simultaneously depleting the droplet bulk. These processes may influence the activation properties of aerosols into cloud droplets and investigation of their role in cloud microphysics has been ongoing for decades. In this study, we have used six different approaches documented in the literature to represent surface activity in Köhler calculations predicting cloud droplet activation properties for particles consisting of one of three different moderately surface active organics mixed with ammonium sulphate in different ratios. We find that the different models predict comparable activation properties at small organic mass fractions in the dry particles for all three moderately surface active organics tested, even with large differences in the predicted degree of bulk-to-surface partitioning of the surface active component. However, differences between the models regarding both the predicted critical diameter and supersaturation for the same dry particle size increase with the organic fraction in the particles. Comparison with available experimental data shows that assuming complete bulk-to-surface partitioning of the organic component (total depletion of the bulk) along the full droplet growth curve does not adequately represent the activation properties of particles with high moderate surfactant mass fractions. Accounting for the surface tension depression mitigates some of the effect. Models that include the possibility for partial bulk-to-surface partitioning yield comparable results to the experimental data, even at high organic mass fractions in the particles. The study highlights the need for using thermodynamically consistent model frameworks to treat surface activity of atmospheric aerosols and for firm experimental validation of model predictions across a wide range of states relevant to the atmosphere. [ABSTRACT FROM AUTHOR]

The atmospheric reaction of OH radicals with dimethyl disulfide, CH3SSCH3, proceeds primarily via OH addition forming CH3S and CH3SOH as reactive intermediates, and to a lesser extent via H-abstraction resulting in the peroxy radical CH3SSCH2OO in the presence of O2. The latter undergoes a fast two-step isomerization process leading to HOOCH2SSCHO. CH3S and CH3SOH are both converted to SO2 and CH3O2 with near unity yields under atmospheric conditions. [ABSTRACT FROM AUTHOR]

We have used the COSMO therm program to estimate activity coefficients and solubilities of mono- and α , ω -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 COSMO therm -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 COSMO therm. 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. [ABSTRACT FROM AUTHOR]

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 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, we identified, supported by quantum chemical calculations, not only C-H, C=O and C-OH bonds, but also fingerprints of C-OH(CH2) 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. [ABSTRACT FROM AUTHOR]

We study the adsorption of water onto deposited inorganic sodium chloride and organic malonic acid and sucrose nanoparticles at ambient water pressures corresponding to relative humidities (RH) from 0 to 16 %. To obtain information about water uptake at conditions where not accessible with typical aerosol instrumentation, we use surface-sensitive ambient pressure X-ray photoelectron spectroscopy (APXPS), which has a detection sensitivity from parts per thousand. Our results show that water is already adsorbed on sodium chloride particles at RH well below deliquescence, and that the chemical environment on the particle surface is changing with increasing humidity. While the sucrose particles exhibit only very modest changes on the surface at these relative humidities, the chemical composition and environment of malonic acid particle surfaces is clearly affected. Our observations indicate that water uptake by inorganic and organic aerosol particles could already have an impact on atmospheric chemistry at low relative humidities. We also conclude that the APXPS technique is a viable tool for studying chemical changes on the surfaces of atmospherically relevant aerosol particles which are not accessible with typical online mass- and volume-based methods. [ABSTRACT FROM AUTHOR]

We have used COSMO-RS (the COnductor-like Screening MOdel for Real Solvents), as implemented in the COSMOTherm program, to compute the saturation vapor pressures at 298 K of two photo-oxidation products of isoprene: the dihydroxy dihydroperoxide C5H12O6, and the dihydroperoxy hydroxy aldehyde, C5H10O6. The predicted saturation vapor pressures were significantly higher (by up to a factor of 1000) than recent experimental results, very likely due to the overestimation of the effects of intramolecular hydrogen bonds, which tend to increase saturation vapor pressures by stabilizing molecules in the gas phase relative to the liquid. Modifying the hydrogen bond enthalpy parameter used by COSMOTherm can improve the agreement with experimental results - however the optimal parameter value is likely to be system-specific. Alternatively, vapor pressure predictions can be substantially improved (to within a factor of 10 of the experimental values for the two systems studied here) by selecting only conformers with a minimum number of intramolecular hydrogen bonds. The computed saturation vapor pressures were very sensitive to the details of the conformational sampling approach, with the default scheme implemented in the COSMOconf program proving insufficient for the task, for example by predicting significant differences between enantiomers, which should have identical physical properties. Even after exhaustive conformational sampling, COSMOTherm predicts significant differences in saturation vapor pressures between both structural isomers and diastereomers. For C5H12O6, predicted differences in psat between structural isomers are up to two orders of magnitude, and differences between stereoisomers up to a factor of 20 - though these differences are very likely exaggerated by the overestimation of the effect of intramolecular H-bonds. For C5H10O6, predicted differences between (stereo)isomers are below a factor of 3. In future studies of saturation vapor pressures of polyfunctional atmospheric oxidation products using COSMOTherm, we recommend first performing thorough conformational sampling, and subsequently selecting conformers with a minimal number of intramolecular H-bonds. [ABSTRACT FROM AUTHOR]

Pollenkitt is a viscous material that coats grains of pollen and plays important roles in pollen dispersion and plant reproduction. It may also be an important contributor to pollen water uptake and 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. The 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 impact on pollenkitt hygroscopicity. We measure cloud 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 activation potential and hygroscopicity. In addition, we find indication of significant impact of complex non-ideal solution effects in systematic and consistent size dependency of pollenkitt hygroscopicity. [ABSTRACT FROM AUTHOR]