On the construction, comparison, and variability of airsheds for interpreting semivolatile organic compounds in passively sampled air

Air mass origin as determined by back trajectories often aids in explaining some of the short-term variability in the atmospheric concentrations of semivolatile organic contaminants. Airsheds, constructed by amalgamating large numbers of back trajectories, capture average air mass origins over longer time periods and thus have found use in interpreting air concentrations obtained by passive air samplers. To explore some of their key characteristics, airsheds for 54 locations on Earth were constructed and compared for roundness, seasonality, and interannual variability. To avoid the so-called "pole problem" and to simplify the calculation of roundness, a "geodesic grid" was used to bin the back-trajectory end points. Departures from roundness were seen to occur at all latitudes and to correlate significantly with local slope but no strong relationship between latitude and roundness was revealed. Seasonality and interannual variability vary widely enough to imply that static models of transport are not sufficient to describe the proximity of an area to potential sources of contaminants. For interpreting an air measurement an airshed should be generated specifically for the deployment time of the sampler, especially when investigating long-term trends. Samples taken in a single season may not represent the average annual atmosphere, and samples taken in linear, as opposed to round, airsheds may not represent the average atmosphere in the area. Simple methods are proposed to ascertain the significance of an airshed or individual cell. It is recommended that when establishing potential contaminant source regions only end points with departure heights of less than ～700 m be considered.     

Chromium speciation in coal and biomass co-combustion products

Chromium speciation is vital for the toxicity of products resulting from co-combustion of coal and biomass. Therefore, understanding of formation processes has been studied using a combination of X-ray absorption fine structure (XAFS) spectroscopy and thermodynamic equilibrium calculations. The influence of cofiring on Cr speciation is very dependent on the type of fuel. Cr(VI) contents in the investigated fly ash samples from coal and cofiring average around 7% of the total chromium. An exception is cofiring 7-28% wood for which ashes exhibited Cr(VI) concentrations of 12-16% of the total chromium. Measurements are in line with thermodynamic predictions: RE factors of Cr around 1 are in line with volatile Cr only above 1400 °C; lower Cr(VI) concentrations with lower oxygen content and Cr(III) dissolved in aluminosilicate glass. Stability of Cr(VI) below 700 °C does not correlate with Cr(VI) concentrations found in the combustion products. It is indicated that Cr(VI) formation is a high-temperature process dependent on Cr evaporation (mode of occurrence in fuel, promoted by organic association), oxidation (local oxygen content), and formation of solid chromates (promoted by presence of free lime (CaO) in the ash). CaCrO(4)(s) is a probable chemical form but, given different leachable fractions (varying from 25 to 100%), different forms of Cr(VI) must be present. Clay-bound Cr is likely to dissolve in the aluminosilicate glass phase during melting of the clay.     

Effectiveness of nitrogen incorporation to enhance the photoelectrochemical activity of nanostructured TiO2:NH3 versus H2-N2 annealing

Highly ordered TiO(2) nanohole layers were synthesized by anodic oxidation of titanium foils using ethylene glycol and ammonium fluoride as the electrolyte. The effectiveness of different methods, namely annealing at 500?°C in NH(3) and in H(2) diluted in N(2), to incorporate nitrogen into TiO(2) and thus extend its photoelectrochemical (PEC) activity to the visible range was studied. The intra-gap levels introduced by both processes were identified by means of XPS and PL measurements. Water splitting experiments demonstrated that annealing in H(2) improved the photocatalytic activity of pure TiO(2), while annealing in ammonia led to a decrease in the PEC performance.     

Monolithic ZnTe-based pillar microcavities containing CdTe quantum dots

Micropillars of different diameters have been prepared by focused ion beam milling out of a planar ZnTe-based cavity. The monolithic epitaxial structure, deposited on a GaAs substrate, contains CdTe quantum dots embedded in a ZnTe λ-cavity delimited by two distributed Bragg reflectors (DBRs). The high refractive index material of the DBR structure is ZnTe, while for the low index material a short-period triple MgTe/ZnTe/MgSe superlattice is used. The CdTe quantum dots are formed by a novel Zn-induced formation process and are investigated by scanning transmission electron microscopy. Micro-photoluminescence measurements show discrete optical modes for the pillars, in good agreement with calculations based on a vectorial transfer matrix method. The measured quality factor reaches a value of 3100.     

Spin-wave interference patterns created by spin-torque nano-oscillators for memory and computation

Magnetization dynamics in nanomagnets has attracted broad interest since it was predicted that a dc current flowing through a thin magnetic layer can create spin-wave excitations. These excitations are due to spin momentum transfer, a transfer of spin angular momentum between conduction electrons and the background magnetization, that enables new types of information processing. Here we show how arrays of spin-torque nano-oscillators can create propagating spin-wave interference patterns of use for memory and computation. Memristic transponders distributed on the thin film respond to threshold tunnel magnetoresistance values, thereby allowing spin-wave detection and creating new excitation patterns. We show how groups of transponders create resonant (reverberating) spin-wave interference patterns that may be used for polychronous wave computation and information storage.     

Interface modification to optimize charge separation in cyanine heterojunction photovoltaic devices

We investigate heterojunction photovoltaic devices using the carbocyanine 1,1′-diethyl-3,3,3′,3′-tetramethylcarbocyanine perchlorate (Cy5) as donor and buckminsterfullerene (C₆₀) as acceptor. We find that photocurrent generation occurs at the interface between CY5 and indium tin oxide (ITO) as well as at the organic heterointerface. By analyzing the spectral dependence of the photocurrent as a function of applied voltage, we were able to demonstrate that poly(3,4-ethylenedioxythiophene) (PEDOT) inhibits electron injection from the cyanine into ITO. Since the photocurrent generation at the ITO electrode is opposite to the one generated at the organic heterojuncion, the use of PEDOT leads to increased short-circuit current and open-circuit voltage.     

Performance modeling of non-metallic flat plate solar collectors

This paper extends the current techniques used in the prediction of flat plate solar collector performance for use in the analysis of non-metallic collectors. An analytical model was developed to study the characteristics of these solar collctors which eliminate the need for metals, glass, and special coatings. Using this model, plate efficiency factors are presented for various common non-metallic absorber plate configurations. A parametric study was conducted with emphasis placed on collector plate thermal conductivity and partial transmittance of glazings to long-wave radiation. From the results of this study, it was shown that it is possible to meet or exceed performance levels of conventional metal tube and fin collectors through the use of non-metallic collectors in the low- to medium-temperature range.     

Cooperative MAC Design in Multi-hop Wireless Networks: Part I: When Source and Destination are within the Transmission Range of Each Other

Cooperative communication is regarded as a promising technology in future 5G wireless networks to enhance network performance by exploiting time and/or space diversity via distributed terminals. In this paper, we propose a cooperative medium access protocol which addresses three key aspects of cooperative communications from MAC layer perspective, namely, when to cooperate, whom to cooperate with and how to protect ongoing cooperative transmissions. To further improve the protocol performance in dense networks, three techniques are investigated to avoid potential collision among multiple contending relays. Both analysis and simulation results demonstrate that significant improvement in terms of throughput and packet delivery ratio can be achieved by the proposed cooperative protocol.     

Nondestructive ion trap mass analysis at high pressure

A method for performing nondestructive ion trap mass analysis at high pressures (>1 mTorr) has been developed using image current measurement with constant dipolar excitation. Instead of monitoring the ion secular motion, a harmonic of the ion motion was used for narrow band image current measurement followed by Fourier Transform. The capability of this technique has been demonstrated with mass analysis using a single measurement at pressures of 10 mTorr or higher. Methods for mixture analysis and tandem mass spectrometry have also been developed for nondestructive mass analysis.