TY - THES A1 - Köhler, Ralf T1 - Phasen- und Transportverhalten von Triacontansubmonolagen an der Grenzfläche zwischen Luft und Siliziumoxid/Silizium Y1 - 2006 CY - Potsdam ER - TY - JOUR A1 - Köhler, Ralf A1 - Doench, Ingo A1 - Ott, Patrick A1 - Laschewsky, André A1 - Fery, Andreas A1 - Krastev, Rumen T1 - Neutron reflectometry study of swelling of polyelectrolyte multilayers in water vapors : influence of charge density of the polycation N2 - We studied the swelling of polyelectrolyte (PE) multilayers (PEM) in water (H2O) vapors. The PEM were made from polyanion poly(styrene sulfonate) (PSS) and polycation poly(diallyldimethylammonium chloride)-N-methyl-N-vinylacetamide (pDADMAC-NMVA). While PSS is a fully charged polyanion, pDADMAC-NMVA is a random copolymer made of charged pDADMAC and uncharged NMVA monomer units. Variation of the relative amount of these two units allows for controlling the charge density of pDADMAC-NMVA. The degree of swelling was studied as it function of the relative humidity in the experimental chamber (respectively water concentration in the gas phase) for PEM prepared from PSS and pDADMAC-NMVA with their different charge densities - 100%, 89% and 75%. The films were prepared by means of spraying technique and consisted of six PE couples-PSS/pDADMAC-NMVA. Neutron reflectometry was applied as main tool to observe the swelling process. The technique allows to obtain in a single experiment information about film thickness and amount of water in the film. The experiments were complemented with AFM measurements to obtain the thickness of the films. It was found that the Film thickness increases when the charge density of the polycation decreases. The swelling of the PEM increases with the relative humidity and it depends on the charge density of pDADMAC-NMVA. The swelling behavior is 2-fold, splitting up in a charge dependent mode with relatively little volume increase, and a second mode With high volume expansion, which is independent from charge density of PEM. The "swelling transition" occurs for all samples at a relative humidity about 60% and a volume increase of ca. 20%. The results were interpreted according to the Flory-Huggins theory which assumes a phase separation in PEM network at higher water contents. Y1 - 2009 UR - http://pubs.acs.org/journal/langd5 U6 - https://doi.org/10.1021/La901508w SN - 0743-7463 ER - TY - JOUR A1 - Köhler, Yvonne A1 - Luther, Eva Maria A1 - Meyer, Sören A1 - Schwerdtle, Tanja A1 - Dringen, Ralf T1 - Uptake and toxicity of arsenite and arsenate in cultured brain astrocytes JF - Journal of trace elements in medicine and biology N2 - Inorganic arsenicals are environmental toxins that have been connected with neuropathies and impaired cognitive functions. To investigate whether such substances accumulate in brain astrocytes and affect their viability and glutathione metabolism, we have exposed cultured primary astrocytes to arsenite or arsenate. Both arsenicals compromised the cell viability of astrocytes in a time- and concentration-dependent manner. However, the early onset of cell toxicity in arsenite-treated astrocytes revealed the higher toxic potential of arsenite compared with arsenate. The concentrations of arsenite and arsenate that caused within 24 h half-maximal release of the cytosolic enzyme lactate dehydrogenase were around 0.3 mM and 10 mM, respectively. The cellular arsenic contents of astrocytes increased rapidly upon exposure to arsenite or arsenate and reached after 4 h of incubation almost constant steady state levels. These levels were about 3-times higher in astrocytes that had been exposed to a given concentration of arsenite compared with the respective arsenate condition. Analysis of the intracellular arsenic species revealed that almost exclusively arsenite was present in viable astrocytes that had been exposed to either arsenate or arsenite. The emerging toxicity of arsenite 4 h after exposure was accompanied by a loss in cellular total glutathione and by an increase in the cellular glutathione disulfide content. These data suggest that the high arsenite content of astrocytes that had been exposed to inorganic arsenicals causes an increase in the ratio of glutathione disulfide to glutathione which contributes to the toxic potential of these substances. KW - Arsenic KW - Astrocytes KW - GSH KW - Metabolism KW - Toxicity Y1 - 2014 U6 - https://doi.org/10.1016/j.jtemb.2014.04.007 SN - 0946-672X VL - 28 IS - 3 SP - 328 EP - 337 PB - Elsevier CY - Jena ER - TY - JOUR A1 - Köhler, Raphael H. A1 - Handorf, Dörthe A1 - Jaiser, Ralf A1 - Dethloff, Klaus A1 - Zängl, Günther A1 - Majewski, Detlev A1 - Rex, Markus T1 - Improved circulation in the Northern hemisphere by adjusting gravity wave drag parameterizations in seasonal experiments with ICON-NWP JF - Earth and Space Science : ESS N2 - The stratosphere is one of the main potential sources for subseasonal to seasonal predictability in midlatitudes in winter. The ability of an atmospheric model to realistically simulate the stratospheric dynamics is essential in order to move forward in the field of seasonal predictions in midlatitudes. Earlier studies with the ICOsahedral Nonhydrostatic atmospheric model (ICON) point out that stratospheric westerlies in ICON are underestimated. This is the first extensive study on the evaluation of Northern Hemisphere stratospheric winter circulation with ICON in numerical weather prediction (NWP) mode. Seasonal experiments with the default setup are able to reproduce the basic climatology of the stratospheric polar vortex. However, westerlies are too weak and major stratospheric warmings too frequent in ICON. Both a reduction of the nonorographic, and a reduction of the orographic gravity wave and wake drag lead to a strengthening of the stratospheric vortex and a bias reduction, in particular in January. However, the effect of the nonorographic gravity wave drag scheme on the stratosphere is stronger. Stratosphere-troposphere coupling is intensified and more realistic due to a reduced gravity wave drag. Furthermore, an adjustment of the subgrid-scale orographic drag parameterization leads to a significant error reduction in the mean sea level pressure. As a result of these findings, we present our current suggested improved setup for seasonal experiments with ICON-NWP.
Plain Language Summary Although seasonal forecasts for midlatitudes have the potential to be highly beneficial to the public sector, they are still characterized by a large amount of uncertainty. Exact simulations of the circulation in the stratosphere can help to improve tropospheric predictability on seasonal time scales. For this reason, we investigate how well the new German atmospheric model is able to simulate the stratospheric circulation. The model reproduces the basic behavior of the Northern Hemisphere stratospheric polar vortex, but the westerly circulation in winter is underestimated. The stratospheric circulation is influenced by gravity waves that exert drag on the flow. These processes are only partly physically represented in the model, but are very important and are hence parameterized. By adjusting the parameterizations for the gravity wave drag, the stratospheric polar vortex is strengthened, thereby yielding a more realistic stratospheric circulation. In addition, the altered parameterizations improve the simulated surface pressure pattern. Based upon this, we present our current suggested improved model setup for seasonal experiments. Y1 - 2021 U6 - https://doi.org/10.1029/2021EA001676 SN - 2333-5084 VL - 8 IS - 3 PB - American Geophysical Union CY - Malden, Mass. ER -