| 1. |
- Almeida, Joao, et al.
(författare)
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Molecular understanding of sulphuric acid-amine particle nucleation in the atmosphere
- 2013
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Ingår i: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 502:7471, s. 359-
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Tidskriftsartikel (refereegranskat)abstract
- Nucleation of aerosol particles from trace atmospheric vapours is thought to provide up to half of global cloud condensation nuclei(1). Aerosols can cause a net cooling of climate by scattering sunlight and by leading to smaller but more numerous cloud droplets, which makes clouds brighter and extends their lifetimes(2). Atmospheric aerosols derived from human activities are thought to have compensated for a large fraction of the warming caused by greenhouse gases(2). However, despite its importance for climate, atmospheric nucleation is poorly understood. Recently, it has been shown that sulphuric acid and ammonia cannot explain particle formation rates observed in the lower atmosphere(3). It is thought that amines may enhance nucleation(4-16), but until now there has been no direct evidence for amine ternary nucleation under atmospheric conditions. Here we use the CLOUD (Cosmics Leaving OUtdoor Droplets) chamber at CERN and find that dimethylamine above three parts per trillion by volume can enhance particle formation rates more than 1,000-fold compared with ammonia, sufficient to account for the particle formation rates observed in the atmosphere. Molecular analysis of the clusters reveals that the faster nucleation is explained by a base-stabilization mechanism involving acid-amine pairs, which strongly decrease evaporation. The ion-induced contribution is generally small, reflecting the high stability of sulphuric acid-dimethylamine clusters and indicating that galactic cosmic rays exert only a small influence on their formation, except at low overall formation rates. Our experimental measurements are well reproduced by a dynamical model based on quantum chemical calculations of binding energies of molecular clusters, without any fitted parameters. These results show that, in regions of the atmosphere near amine sources, both amines and sulphur dioxide should be considered when assessing the impact of anthropogenic activities on particle formation.
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| 2. |
- Ekström, Sanna, 1980-
(författare)
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The influence of biogenic organic compounds on cloud formation
- 2010
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Aerosols and clouds provide the largest uncertainty in the atmospheric radiation budget. The main focus of this thesis was to investigate the ability of organic compounds in aerosol particles to form clouds, and more specifically those emitted by living organisms. The cloud forming properties of the highly water-soluble methyltetrols and polyols, which are compounds produced by plants and fungi that are common in aerosol, were studied. All compounds and their salt mixtures have a moderate potential to serve as cloud condensation nuclei (CCN). They are thus not likely to have a significant global impact on cloudiness. The potential presence of surfactants released by microorganisms was investigated for aerosols sampled at different locations. Very low surface tension values were measured for these aerosol extracts (30 mN/m), which implies that these aerosols have good CCN properties and indicate the presence of biosurfactants. Their occurrence in aerosols still needs to be confirmed directly by chemical identification. Reactions of organic compounds in sulfate salt solutions exposed to UV-light were studied and found to produce surface active compounds. Thus, mixed sulfate/organic aerosol could have more favourable CCN properties after exposure to light than when kept in the dark. The surface active compounds were proposed to be long-chained organosulfates with hydrophilic and hydrophobic parts, similar to other amphiphilic surfactants. Mixtures of salt and strong surfactants formed by bacteria were studied using two different techniques for determining their CCN properties. There were inconsistencies between the two methods which could be accounted for by surface partitioning. The studied mixtures were determined to be good potential CCN material in both techniques. All these aspects require further investigation, but if the impact of strong biogenic surfactants on cloud formation is confirmed, a new link between living organisms and climate would be identified.
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| 3. |
- Kammermann, Lukas, et al.
(författare)
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Subarctic atmospheric aerosol composition: 3. Measured and modeled properties of cloud condensation nuclei
- 2010
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Ingår i: Journal of Geophysical Research. - 2156-2202. ; 115
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Tidskriftsartikel (refereegranskat)abstract
- Aerosol particles can modify cloud properties by acting as cloud condensation nuclei (CCN). Predicting CCN properties is still a challenge and not properly incorporated in current climate models. Atmospheric particle number size distributions, hygroscopic growth factors, and polydisperse CCN number concentrations were measured at the remote subarctic Stordalen mire, 200 km north of the Arctic Circle in northern Sweden. The CCN number concentration was highly variable, largely driven by variations in the total number of sufficiently large particles, though the variability of chemical composition was increasingly important for decreasing supersaturation. The hygroscopicity of particles measured by a hygroscopicity tandem differential mobility analyzer (HTDMA) was in agreement with large critical diameters observed for CCN activation (kappa approximate to 0.07-0.21 for D = 50-200 nm). Size distribution and time- and size-resolved HTDMA data were used to predict CCN number concentrations. Agreement of predictions with measured CCN within +/- 11% was achieved using parameterized Kohler theory and assuming a surface tension of pure water. The sensitivity of CCN predictions to various simplifying assumptions was further explored: We found that (1) ignoring particle mixing state did not affect CCN predictions, (2) averaging the HTDMA data in time with retaining the size dependence did not introduce a substantial bias, while individual predictions became more uncertain, and (3) predictions involving the hygroscopicity parameter recommended in literature for continental sites (kappa approximate to 0.3 +/- 0.1) resulted in a significant prediction bias. Future modeling studies should therefore at least aim at using averaged, size-resolved, site-specific hygroscopicity or chemical composition data for predictions of CCN number concentrations.
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| 4. |
- Zieger, Paul, et al.
(författare)
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Influence of water uptake on the aerosol particle light scattering coefficients of the Central European aerosol
- 2014
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Ingår i: Tellus. Series B, Chemical and physical meteorology. - : Stockholm University Press. - 0280-6509 .- 1600-0889. ; 66, s. 22716-
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Tidskriftsartikel (refereegranskat)abstract
- The influence of aerosol water uptake on the aerosol particle light scattering was examined at the regional continental research site Melpitz, Germany. The scattering enhancement factor f(RH), defined as the aerosol particle scattering coefficient at a certain relative humidity (RH) divided by its dry value, was measured using a humidified nephelometer. The chemical composition and other microphysical properties were measured in parallel. f(RH) showed a strong variation, e.g. with values between 1.2 and 3.6 at RH = 85% and lambda = 550 nm. The chemical composition was found to be the main factor determining the magnitude of f(RH), since the magnitude of f(RH) clearly correlated with the inorganic mass fraction measured by an aerosol mass spectrometer (AMS). Hysteresis within the recorded humidograms was observed and explained by long-range transported sea salt. A closure study using Mie theory showed the consistency of the measured parameters.
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