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  • 1.
    Hede, Thomas
    Stockholm University, Faculty of Science, Department of Meteorology .
    Beyond Köhler theory: Molecular dynamics simulations as a tool for atmospheric science2013Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    In this thesis, the results from molecular dynamics (MD) simulations of nanoaerosol clusters are discussed. The connecting link of these studies is the Köhler theory, which is the theory of condensational growth and activation of cloud droplets to form clouds. By investigating parameters such as the surface tension, state of mixture and morphology of nanoaerosol particles, conclusions can be drawn to improve the Köhler theory to include the effects of organic compounds previously unaccounted for.

    For the terrestrial environment, the simulations show that the natural surfactant cis-pinonic acid, an oxidation product evaporated from boreal trees, spontaneously accumulates at the surface of nanoaerosol clusters and thereby reduces the surface tension. The surface tension depression is related to the concentration of the surfactant and the size of the clusters. Surface tension is an important parameter of the Köhler theory. A decrease of the surface tension can lower the critical water vapour supersaturation needed for cloud droplet activation, giving rise to more, but smaller cloud droplets (Twomey effect) which in turn could change the optical properties of the cloud. It was also shown that the three organic surfactants, being model compounds for so called Humic-like substances (HULIS) have the ability to form aggregates inside the nanoaerosol clusters. These HULIS aggregates can also promote the solubilization of hydrophobic organic carbon in the form of fluoranthene, enabling soot taking part in cloud drop formation.

    Dissolved intermediately surface-active free amino acids were shown to be of some relevance for cloud formation over remote marine areas. The MD simulations showed differences between the interacting forces for spherical and planar interfaces of amino acids solutions.

    This thesis has emphasized the surface-active properties of organic compounds, including model HULIS and amino acids and their effect on surface tension and molecular orientation including aggregate formation in nanoaerosol clusters and their activation to form droplets. This thesis shows that the Köhler equation does not fully satisfactory describe the condensational growth of nano-sized droplets containing organic surfactants. Different approaches are suggested as revisions of the Köhler theory.

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  • 2.
    Hede, Thomas
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Leck, Caroline
    Stockholm University, Faculty of Science, Department of Meteorology .
    Sun, Lu
    Royal Inst Technol, Sch Biotechnol, Dept Theoret Chem, S-10691 Stockholm, Sweden.
    Tu, Yaoquan
    Royal Inst Technol, Sch Biotechnol, Dept Theoret Chem, S-10691 Stockholm, Sweden.
    Ågren, Hans
    Royal Inst Technol, Sch Biotechnol, Dept Theoret Chem, S-10691 Stockholm, Sweden.
    A theoretical study revealing the promotion of light absorbing carbon particles solubilization by natural surfactants in nanosized water droplets2013In: Atmospheric Science Letters, ISSN 1530-261X, E-ISSN 1530-261X, Vol. 14, no 2, p. 86-90Article in journal (Refereed)
    Abstract [en]

    Many identified effects of atmospheric aerosol particles on climate come from pollutants. The effects of light absorbing carbon particles (soot) are amongst the most uncertain and they are also considered to cause climate warming on the same order of magnitude as anthropogenic carbon dioxide. This study contributes to the understanding of the potential for transformation of the surface character of soot from hydrophobic to hydrophilic, which in clouds promotes a buil-up of water-soluble material. We use molecular dynamics simulations to show how natural surfactants facilitate solubilization of fluoranthene, which we use as a model compound for soot, in nanoaerosol water clusters.

  • 3.
    Hede, Thomas
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Li, Xin
    Leck, Caroline
    Stockholm University, Faculty of Science, Department of Meteorology .
    Tu, Yaoquan
    Ågren, Hans
    Model HULIS compounds in nanoaerosol clusters - investigations of surface tension and aggregate formation using molecular dynamics simulations2011In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 11, no 13, p. 6549-6557Article in journal (Refereed)
    Abstract [en]

    Cloud condensation nuclei act as cores for water vapour condensation, and their composition and chemical properties may enhance or depress the ability for droplet growth. In this study we use molecular dynamics simulations to show that model humic-like substances (HULIS) in systems containing 10 000 water molecules mimic experimental data well referring to reduction of surface tension. The model HULIS compounds investigated in this study are cis-pinonic acid (CPA), pinic acid (PAD) and pinonaldehyde (PAL). The structural properties examined show the ability for the model HULIS compounds to aggregate inside the nanoaerosol clusters.

  • 4. Li, Xin
    et al.
    Hede, Thomas
    Stockholm University, Faculty of Science, Department of Meteorology .
    Tu, Yaoquan
    Leck, Caroline
    Stockholm University, Faculty of Science, Department of Meteorology .
    Ågren, Hans
    Cloud droplet activation mechanisms of amino acid aerosol particles: insight from molecular dynamics simulationsManuscript (preprint) (Other academic)
    Abstract [en]

    Atmospheric amino acids constitute a large fraction of water-soluble organic nitrogen compounds in aerosol particles, and have been confirmed as effective cloud condensation nuclei materials in laboratory experiments. We here present a molecular dynamics study of six amino acids with different structures and chemical properties that are relevant to the remote marine atmospheric aerosol-cloud system, with the aim to investigate the detailed mechanism of their induced changes in surface activity and surface tension, which are important properties for cloud drop activation. Distributions and orientations of the amino acid molecules are studied; these L-amino acids are serine, glycine, alanine, valine, methionine and phenylalanine and are categorized as hydrophilic and hydrophobic according to their affinities to water. The results suggest that the presence of surface-concentrated hydrophobic amino acid molecules give rise to enhanced Lennard-Jones repulsion, which in turn results in decreased surface tension of a planar interface but an increased surface tension of the spherical interface of droplets with diameters below 10 nm. The observed surface tension perturbation for the different amino acids under study not only serves as benchmark for future studies of more complex systems, but also shows that hydrophobic amino acids are surface active. The molecular dynamics simulations used in this study reproduce experimental results of surface tension measurements for planar interfaces and the method is therefore applicable for spherical interfaces of nano-size for which experimental measurements are not possible to conduct.

  • 5. Li, Xin
    et al.
    Hede, Thomas
    Stockholm University, Faculty of Science, Department of Meteorology .
    Tu, Yaoquan
    Leck, Caroline
    Stockholm University, Faculty of Science, Department of Meteorology .
    Ågren, Hans
    Cloud droplet activation mechanisms of amino acid aerosol particles: insight from molecular dynamics simulations2013In: Tellus. Series B, Chemical and physical meteorology, ISSN 0280-6509, E-ISSN 1600-0889, Vol. 65, p. 65-Article in journal (Refereed)
    Abstract [en]

    Atmospheric amino acids constitute a large fraction of water-soluble organic nitrogen compounds in aerosol particles, and have been confirmed as effective cloud condensation nuclei (CCN) materials in laboratory experiments. We present a molecular dynamics (MD) study of six amino acids with different structures and chemical properties that are relevant to the remote marine atmospheric aerosol-cloud system, with the aim of investigating the detailed mechanism of their induced changes in surface activity and surface tension, which are important properties for cloud drop activation. Distributions and orientations of the amino acid molecules are studied; these L-amino acids are serine (SER), glycine (GLY), alanine (ALA), valine (VAL), methionine (MET) and phenylalanine (PHE) and are categorised as hydrophilic and amphiphilic according to their affinities to water. The results suggest that the presence of surface-concentrated amphiphilic amino acid molecules give rise to enhanced Lennard-Jones repulsion, which in turn results in decreased surface tension of a planar interface and an increased surface tension of the spherical interface of droplets with diameters below 10 nm. The observed surface tension perturbation for the different amino acids under study not only serves as benchmark for future studies of more complex systems, but also shows that amphiphilic amino acids are surface active. The MD simulations used in this study reproduce experimental results of surface tension measurements for planar interfaces and the method is therefore applicable for spherical interfaces of nano-size for which experimental measurements are not possible to conduct.

  • 6. Li, Xin
    et al.
    Hede, Thomas
    Stockholm University, Faculty of Science, Department of Meteorology .
    Tu, Yaoquan
    Leck, Caroline
    Stockholm University, Faculty of Science, Department of Meteorology .
    Ågren, Hans
    Glycine in aerosol water droplets: a critical assessment of Kohler theory by predicting surface tension from molecular dynamics simulations2011In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 11, no 2, p. 519-527Article in journal (Refereed)
    Abstract [en]

    Aerosol particles in the atmosphere are important participants in the formation of cloud droplets and have significant impact on cloud albedo and global climate. According to the Kohler theory which describes the nucleation and the equilibrium growth of cloud droplets, the surface tension of an aerosol droplet is one of the most important factors that determine the critical supersaturation of droplet activation. In this paper, with specific interest to remote marine aerosol, we predict the surface tension of aerosol droplets by performing molecular dynamics simulations on two model systems, the pure water droplets and glycine in water droplets. The curvature dependence of the surface tension is interpolated by a quadratic polynomial over the nano-sized droplets and the limiting case of a planar interface, so that the so-called Aitken mode particles which are critical for droplet formation could be covered and the Kohler equation could be improved by incorporating surface tension corrections.

  • 7. Li, Xin
    et al.
    Hede, Thomas
    Stockholm University, Faculty of Science, Department of Meteorology .
    Tu, Yaoquan
    Leck, Caroline
    Stockholm University, Faculty of Science, Department of Meteorology .
    Ågren, Hans
    Surface-Active cis-Pinonic Acid in Atmospheric Droplets: A Molecular Dynamics Study2010In: Journal of Physical Chemistry Letters, ISSN 1948-7185, E-ISSN 1948-7185, Vol. 1, no 4, p. 769-773Article in journal (Refereed)
    Abstract [en]

    Water vapor in the atmosphere can condensate and form cloud droplets when there is a certain amount of humidity and a presence of cloud condensation nuclei, and organic solutes called surfactants can significantly lower the surface tension of water-one of the parameters determining cloud droplet population. We here present a molecular dynamics study of the behavior of cis-pinonic acid, a commonly found organic compound in cloud condensation nuclei, and its effect on the surface tension of water clusters. Specifically, the decrease in surface tension is found to depend on not only the concentration of the organic compound but also the droplet size due to the spontaneous assembly of the surfactant molecules on the droplet surface. This leads to the conclusion that the partitioning of the surfactant between the bulk and surface plays an important role in the behavior of atmospheric aerosol particles and thus in their availability for cloud formation.

  • 8. Li, Xin
    et al.
    Leck, Caroline
    Stockholm University, Faculty of Science, Department of Meteorology .
    Sun, Lu
    Hede, Thomas
    Stockholm University, Faculty of Science, Department of Meteorology .
    Tu, Yaoquan
    Ågren, Hans
    Cross-Linked Polysaccharide Assemblies in Marine Gels: An Atomistic Simulation2013In: Journal of Physical Chemistry Letters, ISSN 1948-7185, E-ISSN 1948-7185, Vol. 4, no 16, p. 2637-2642Article in journal (Refereed)
    Abstract [en]

    Marine polymeric gels or colloidal nano- and microgels have been shown to contribute significantly to the primary marine aerosol and cloud condensation nuclei over remote marine areas. A microscopic understanding of such biologically derived matter at the sea air interface is important for future development of global climate models, but unfortunately cannot be obtained from modern characterization techniques. In this contribution, we employ molecular dynamics simulations to reveal the atomistic details of marine polymeric gels represented by anionic polysaccharide assemblies. The ionic bonds formed between polysaccharides and metal ions in seawater as well as the hydrophobic contribution to surface area are investigated in detail, and destabilization of the assemblies upon removal of Ca2+ or acidification is explained. These results provide insight into physicochemical properties of polysaccharide-Ca2+ structures and enable future studies of their roles of in the wetting process of cloud droplet activation.

  • 9. Sun, Lu
    et al.
    Hede, Thomas
    Stockholm University, Faculty of Science, Department of Meteorology .
    Tu, Yaoquan
    Leck, Caroline
    Stockholm University, Faculty of Science, Department of Meteorology .
    Ågren, Hans
    Combined Effect of Glycine and Sea Salt on Aerosol Cloud Droplet Activation Predicted by Molecular Dynamics Simulations2013In: Journal of Physical Chemistry A, ISSN 1089-5639, E-ISSN 1520-5215, Vol. 117, no 41, p. 10746-10752Article in journal (Refereed)
    Abstract [en]

    The present study illustrates the combined effect of organic and inorganic compounds on cloud droplet nucleation and activation processes representative for the marine environment. Amino acids and sea salt are common marine cloud condensation nuclei (CCN) which act as a prerequisite for growth of cloud droplets. The chemical and physical properties of these CCN play a key role for interfacial properties such as surface tension, which is important for the optical properties of clouds and for heterogeneous reactions. However, there is a lack of detailed information and in situ measurements of surface tension of such nanosized droplets. Here we present a study of the combined effect of zwitterionic glycine (ZGLY) and sea salt in nanosized water droplets using molecular dynamics simulations, where particular emphasis is placed on the surface tension for the nanosized droplets. The critical supersaturation is estimated by the Kohler equation. It is found that dissolved sea salt interacts with ZGLY through a water bridge and weakens the hydrogen bonds among ZGLYs, which has a significant effect on both surface tension and water vapor supersaturation. Clusters of glycine mixed with sea salt deliquesce more efficiently and have higher growth factors.

  • 10. Sun, Lu
    et al.
    Li, Xin
    Hede, Thomas
    Stockholm University, Faculty of Science, Department of Meteorology .
    Tu, Yaoquan
    Leck, Caroline
    Stockholm University, Faculty of Science, Department of Meteorology .
    Ågren, Hans
    Molecular Dynamics Simulations of the Surface Tension and Structure of Salt Solutions and Clusters2012In: Journal of Physical Chemistry B, ISSN 1520-6106, E-ISSN 1520-5207, Vol. 116, no 10, p. 3198-3204Article in journal (Refereed)
    Abstract [en]

    Sodium halides, which are abundant in sea salt aerosols, affect the optical properties of aerosols and are active in heterogeneous reactions that cause ozone depletion and acid rain problems. Interfacial properties, including surface tension and halide anion distributions, are crucial issues in the study of the aerosols. We present results from molecular dynamics simulations of water solutions and clusters containing sodium halides with the interatomic interactions described by a conventional force field. The simulations reproduce experimental observations that sodium halides increase the surface tension with respect to pure water and that iodide anions reach the outermost layer of water clusters or solutions. It is found that the van der Waals interactions have an impact on the distribution of the halide anions and that a conventional force field with optimized parameters can model the surface tension of the salt solutions with reasonable accuracy.

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