Singularitäten konvexer Flächen

Ohne Zusammenfassung     

Photon tunneling contributions to extinction for laboratory grown hexagonal columns

A new treatment predicting the extinction and absorption properties of ice particles is evaluated in this study using laboratory measurements of the extinction efficiency, Q_ext. In this treatment, the degree of ‘photon tunneling’ for ice crystals is unspecified, and laboratory measurements of Q_ext were used in conjunction with this scheme to quantify the significance of this process by determining a tunneling factor, denoted t_f. The term tunneling here refers to the interaction of a particle with radiation outside its area cross-section. A t_f of 1.0 corresponds to tunneling exhibited by ice spheres as predicted by Mie theory, while a t_f of 0 indicates no tunneling.

The laboratory work entailed Fourier transform infrared spectroscopy (FTIR) for optical depth measurements in an ice cloud grown in a chamber, over a wavelength range of 2–18 μm. From these measurements, the extinction efficiency Q_ext as a function of wavelength was determined. Ice particle size spectra were measured in the cloud chamber, and were used to predict Q_ext using the radiation scheme noted above and also using a new implementation of T-matrix, which is based on the exact geometry of a ‘pristine’ hexagonal ice crystal, without approximating the crystal as a spheroid.

Results show that t_f values determined from the laboratory measurements and the new radiation scheme are qualitatively in agreement with t_f values based on fundamental theory. Mean Q_ext errors (relative to measured Q_ext) over all wavelengths sampled were 3.0% when using a constant optimized t_f in the radiation scheme, and 2.3% when using a t_f scheme based on complex angular momentum theory. Moreover, Q_ext as predicted from T-matrix over the wavelength interval 8–12 μm is also in excellent agreement with the measured Q_ext. A single wavelength calculation at 14 μm was performed using the finite difference time domain (FDTD) and T-matrix methods, both of which agreed precisely with the measured Q_ext value. This validates the integrity of T-matrix, FDTD, the new radiation scheme, and the laboratory measurements for the corresponding range of wavelengths and size parameters. Collectively, these results indicate the tunneling contributions predicted for solid hexagonal columns are realistic.     

Comparison of operator- and computer-controlled scanning electron microscopy of particles from different atmospheric aerosol types

Analytical and Bioanalytical Chemistry - Individual aerosol particles from an urban background site in Mainz (Germany), a traffic hotspot site in Essen (Germany), the free troposphere in the Swiss...     

The Effect of pH and Biogenic Ligands on the Weathering of Chrysotile Asbestos: The Pivotal Role of Tetrahedral Fe in Dissolution Kinetics and Radical Formation

Chrysotile asbestos is a soil pollutant in many countries. It is a carcinogenic mineral, partly due to its surface chemistry. In chrysotile, Fe^II and Fe^III substitute Mg octahedra (Fe[6]), and Fe^III substitutes Si tetrahedra (Fe[4]). Fe on fiber surfaces can generate hydroxyl radicals (HO^.) in Fenton reactions, which damage biomolecules. To better understand chrysotile weathering in soils, net Mg and Si dissolution rates over the pH range 3.0–11.5 were determined in the presence and absence of biogenic ligands. Also, HO^. generation and Fe bulk speciation of pristine and weathered fibers were examined by EPR and Mössbauer spectroscopy. Dissolution rates were increased by ligands and inversely related to pH with complete inhibition at cement pH (11.5). Surface-exposed Mg layers readily dissolved at low pH, but only after days at neutral pH. On longer timescales, the slow dissolution of Si layers became rate-determining. In the absence of ligands, Fe[6] precipitated as Fenton-inactive Fe phases, whereas Fe[4] (7 % of bulk Fe) remained redox-active throughout two-week experiments and at pH 7.5 generated 50±10 % of the HO^. yield of Fe[6] at pristine fiber surfaces. Ligand-promoted dissolution of Fe[4] (and potentially Al[4]) labilized exposed Si layers. This increased Si and Mg dissolution rates and lowered HO^. generation to near-background level. It is concluded that Fe[4] surface species control long-term HO^. generation and dissolution rates of chrysotile at natural soil pH.     

Hydrogenative Depolymerization of End-of-Life Poly-(Bisphenol A Carbonate) Catalyzed by a Ruthenium-MACHO-Complex

The valorization of waste to valuable chemicals can contribute to a more resource-efficient and circular chemistry. In this regard, the selective degradation of end-of-life polymers/plastics to produce useful chemical building blocks can be a promising target. We have investigated the hydrogenative depolymerization of end-of-life poly(bisphenol A carbonate). Applying catalytic amounts of the commercial available Ruthenium-MACHO-BH complex the end-of-life polycarbonate was converted to bisphenol A and methanol. Importantly, bisphenol A can be reprocessed for the manufacture of new poly-(bisphenol A carbonate) and methanol can be utilized as energy storage material.