Dispersive Transport Model Research Articles

Fractional dispersion in a sand bed river

Random walk models of fluvial bed load transport use probability distributions to describe the distance a grain travels during an episode of transport and the time it rests after deposition. These models typically employ probability distributions with finite first and second moments, reflecting an underlying assumption that all the factors that influence sediment transport tend to combine in such a way that the length of a step or the duration of a rest can be characterized by a mean value surrounded by a specific amount of variability. The observation that many transport systems exhibit apparent scale‐dependent behavior and non‐Fickian dispersion suggests that this assumption is not always valid. We revisit a nearly 50 year old tracer experiment in which the tracer plume exhibits the hallmarks of dispersive transport described by a step length distribution with a divergent second moment and no characteristic dispersive size. The governing equation of this type of random walk contains fractional‐order derivatives. We use the data from the experiment to test two versions of a fractional‐order model of dispersive fluvial bed load transport. The first version uses a heavy‐tailed particle step length distribution with a divergent second moment to reproduce the anomalously high fraction of tracer mass observed in the downstream tail of the spatial distribution. The second version adds a feature that partitions mass into a detectable mobile phase and an undetectable, immobile phase. This two‐phase transport model predicts other features observed in the data: a decrease in the amount of detected tracer mass over the course of the experiment and enhanced particle retention near the source. The fractional‐order models match the observed plume shape and growth rates better than prior attempts with classical models.

Model of dispersive transport across sharp interfaces between porous materials

Recent laboratory experiments on solute migration in composite porous columns have shown an asymmetry in the solute arrival time upon reversal of the flow direction, which is not explained by current paradigms of transport. In this work, we propose a definition for the solute flux across sharp interfaces and explore the underlying microscopic particle dynamics by applying Monte Carlo simulation. Our results are consistent with previous experimental findings and explain the observed transport asymmetry. An interpretation of the proposed physical mechanism in terms of a flux rectification is also provided. The approach is quite general and can be extended to other situations involving transport across sharp interfaces.

Chronic Residential Exposure to Particulate Matter Air Pollution and Systemic Inflammatory Markers

BackgroundLong-term exposure to urban air pollution may accelerate atherogenesis, but mechanisms are still unclear. The induction of a low-grade systemic inflammatory state is a plausible mechanistic pathway. Objectives: We analyzed the association of residential long-term exposure to particulate matter (PM) and high traffic with systemic inflammatory markers.MethodsWe used baseline data from the German Heinz Nixdorf Recall Study, a population-based, prospective cohort study of 4,814 participants that started in 2000. Fine PM [aerodynamic diameter ≤ 2.5 μm (PM2.5)] exposure based on a small-scale dispersion and chemistry transport model was assigned to each home address. We calculated distances between residences and major roads. Long-term exposure to air pollution (annual PM2.5 and distance to high traffic) and concentration of inflammatory markers [high-sensitivity C-reactive protein (hs-CRP) and fibrinogen] on the day of the baseline visit were analyzed with sex-stratified multiple linear regression, controlling for individual-level risk factors.ResultsIn the adjusted analysis, a cross-sectional exposure difference of 3.91 μg/m3 in PM2.5 (interdecile range) was associated with increases in hs-CRP of 23.9% [95% confidence interval (CI), 4.1 to 47.4%] and fibrinogen of 3.9% (95% CI, 0.3 to 7.7%) in men, whereas we found no association in women. Chronic traffic exposure was not associated with inflammatory markers. Short-term exposures to air pollutants and temperature did not influence the results markedly.ConclusionsOur study indicates that long-term residential exposure to high levels of PM2.5 is associated with systemic inflammatory markers in men. This might provide a link between air pollution and coronary atherosclerosis.

Characterisation of a DNAPL source zone in a porous aquifer using the Partitioning Interwell Tracer Test and an inverse modelling approach

In this paper, we discuss the results of a Partitioning Interwell Tracer Test (PITT) performed in a large scale experiment with a well-defined TCE spill, and present a novel combined analytical–numerical inverse modelling approach using measured concentration profiles within a TCE plume to predict the distribution of the DNAPL in a virtual vertical plane of the source. The proposed inverse modelling approach assumes local thermodynamic equilibrium of the distribution of TCE between the NAPL phase and the aqueous phase and no decay or sorption of the dissolved TCE concentrations downstream of the spill area. The analytical part of the inverse modelling approach contains two steps. As a first step, the location of the contaminant in a virtual vertical plane of a porous medium is fixed by using measured concentration profiles and considering the dissolution of the organic phase under equilibrium conditions. In the second step, the volume of contaminant entrapped in the source cells is estimated. A multiphase advective–dispersive transport model is used in the final step to adjust the volumes quantified in the second step. The predictions are highly dependent on the quantity and quality of the data in space and time. From the PITT-breakthrough curves measured at the pumping well, a mean TCE saturation in the sweep zone of 0.0004 was derived, which is very low compared to that determined at the local scale. In a second analysis, tracer breakthrough curves available at measuring points placed closely downstream and upstream of the presumed source zone, were used to explain why the globally obtained DNAPL saturation was very low compared to the “real”, locally evaluated TCE saturations in the source zone. This was principally caused by the overall travel time compared to the short travel time of the tracers in the source zone. Another reason is that due to bypassing, only part of the volume of tracer injected had been in contact and had eventually interacted with the DNAPL. Furthermore, the quantified TCE volume was nearly 30% higher than the spilled volume; this agrees with the conclusions from other studies emphasizing that calculated volumes can overestimate the measured volumes, particularly in the case of an inhomogeneous distribution of the DNAPL within the soil. A good agreement of the measured and inversed concentration profiles was obtained, highlighting that it is possible to determine the length-averaged distribution of a residual pollution source from dissolved concentration profiles measured downstream of the source zone. The numerically obtained non-uniform distribution of DNAPL entrapped in the vertical plane of the source zone was experimentally confirmed by the TCE saturation values derived from PITT-breakthrough curves at measuring points located 0.75 m downstream of the source zone. However, the sensitivity study showed that the inverse modelling approach provided a rather non-unique solution. More data available may reduce the number of possible representations of the estimated source zone.

Mechanism of positive charge generation in the bulk of HfAlO/SiO 2 stack

We have investigated electrical stress-induced positive charge buildup in a hafnium aluminate (HfAlO)/silicon dioxide (SiO 2) dielectric stack (equivalent oxide thickness = 2.63 nm) in metal–oxide–semiconductor (MOS) capacitor structures with negative bias on the TaN gate. Various mechanisms of positive charge generation in the dielectric have been theoretically studied. Although, anode hole injection (AHI) and valence band hole tunneling are energetically favorable in the stress voltage range studied, the measurement results can be best explained by the dispersive proton transport model.

Residential Exposure to Urban Air Pollution, Ankle–Brachial Index, and Peripheral Arterial Disease

Long-term exposure to particulate air pollution has been associated with increased cardiovascular disease. Biologic pathways for this association are not fully understood. We examined the association of urban air pollution with atherosclerosis of the peripheral vascular bed, using baseline data (2000-2003) from 4348 participants in a population-based cohort study in the German Ruhr Area. Levels of annual fine particulate matter (PM2.5) exposure, derived from a dispersion and chemistry transport model, were assigned to the participants' home addresses. Residential traffic exposure was assessed by the distance between residence and major roads (federal and state highways). Using multiple regression analyses and controlling for individual level risk factors, we examined the association of PM2.5 and traffic with the ankle-brachial index and prevalence of peripheral arterial disease, defined as an index of less than 0.9 or a history of treatment for peripheral artery disease. Living within 101-200, 51-100, and 50 m of a major road was associated with an adjusted absolute decrease in ankle-brachial index of -0.015 (95% confidence interval [CI] = -0.030 to 0.0), -0.002 (-0.021 to 0.016) and -0.024 (-0.047 to -0.001), respectively. Stronger associations were seen in women, whereas no clear association was found in men. Individuals living within 50 m of a major road had an OR of 1.77 (1.01-2.1) for peripheral arterial disease compared with those living more than 200 m away. Associations with PM2.5 were inconsistent. This study adds to the evidence that long-term residential exposure to traffic is associated with atherosclerosis.

Reliability of ultra thin ZrO 2 films on strained-Si

Ultra thin high- k zirconium oxide (equivalent oxide thickness ∼1.57 nm) films have been deposited on strained-Si/relaxed-Si 0.8Ge 0.2 heterolayers using zirconium tetra- tert-butoxide (ZTB) as an organometallic source at low temperature (<200 °C) by plasma enhanced chemical vapour deposition (PECVD) technique in a microwave (700 W, 2.45 GHz) plasma cavity discharge system at a pressure of 66.67 Pa. The trapping/detrapping behavior of charge carriers in ultra thin ZrO 2 gate dielectric during constant current (CCS) and voltage stressing (CVS) has been investigated. Stress induced leakage current (SILC) through ZrO 2 is modeled by taking into account the inelastic trap-assisted tunneling (ITAT) mechanism via traps located below the conduction band of ZrO 2 layer. Trap generation rate and trap cross-section are extracted. A capture cross-section in the range of ∼10 −19 cm 2 as compared to ∼10 −16 cm 2 in SiO 2 has been observed. The trapping charge density, Q ot and charge centroid, X t are also empirically modeled. The time dependence of defect density variation is calculated within the dispersive transport model, assuming that these defects are produced during random hopping transport of positively charge species in the insulating layer. Dielectric breakdown and reliability of the dielectric films have been studied using constant voltage stressing. A high time-dependent dielectric breakdown (TDDB, t bd > 1500 s) is observed under high constant voltage stress.

A fractional Poisson process in a model of dispersive charge transport in semiconductors

A Monte Carlo algorithm for the solution of an integro-differential equation with a frac- tional derivative is described, this method is based on appl ication of a model of a fractional Poisson process. The algorithm is used for the solution of the equati on of dispersive transport in disordered semiconductors. The transient photocurrent calculated by the Monte Carlo method is in good agree- ment with experimental data.

Electrical properties of SiO 2/TiO 2 high- k gate dielectric stack

The trapping/detrapping behavior of charge carriers in ultrathin SiO 2/TiO 2 stacked gate dielectric during constant current (CCS) and voltage stressing (CVS) has been investigated. Titanium tetrakis iso-propoxides (TTIP) was used as the organometallic source for the deposition of ultra-thin TiO 2 films at low temperature (<200 °C) on strained-Si/relaxed-Si 0.8Ge 0.2 heterolayers by plasma-enhanced chemical vapor deposition (PECVD) in a microwave (700 W, 2.45 GHz) plasma cavity discharge system at a pressure of 66.67 Pa. Stress-induced leakage current (SILC) through SiO 2/TiO 2 stacked gate dielectric is modeled by taking into account the inelastic trap-assisted tunneling (ITAT) mechanism via traps located below the conduction band of TiO 2 layer. The increase in the gate current density observed during CVS from room temperature up to 125 oC has been analyzed and modeled considering both the buildup of charges in the layer as well as the SILC contribution. Trap generation rate and trap cross-section are extracted. A capture cross-section in the range of ∼10 −19 cm 2 as compared to ∼10 −16 cm 2 in SiO 2 has been observed. A temperature-dependent trap generation rate and defects have also been investigated using time-dependent current density variation during CVS. The time dependence of defect density variation is calculated within the dispersive transport model, assuming that these defects are produced during random hopping transport of positively charge species in the insulating high- k stacked layers. SILC generation kinetics, i.e. defect generation probability under different injected fluences for various high-constant stress voltages in both polarities have been studied. An empirical relation between trap generation probability and applied stress voltage for various injected fluences has been developed.

Dielectric Relaxation and Defect Generation Under Pulsed and Constant Voltage Stressing of Ultrathin TiO2 Films on Strained-Si/Si0.8Ge0.2

Ultra-thin high-k TiO2 (~11 nm) films have been deposited on strained-Si/relaxed-SiGe heterolayers using microwave plasma below 200ºC using organo-metallic precursor titanium isopropoxide (TTIP). Dielectric relaxation and oxide reliability studies have been performed with constant voltage stressing. During constant voltage stressing of MIS capacitors, an increase in time-dependent gate current is observed followed by occurrence of current fluctuations. The amplitude of fluctuations increases with increasing stress voltage. A comparative study on the time dependent defect density variation, ΔNhigh-k is performed within a dispersive transport model, assuming that the defects are produced during random hopping transport of defects in the insulating layer. Effects of transient response and dielectric relaxation have been studied under high pulse voltage stress. Stress induced trap charge density and its spatial distribution have also been investigated.

Demonstration of a global modeling methodology to determine the relative importance of local and long-distance sources

A global three-dimensional (3D) transport–dispersion model was used to simulate Krypton-85 ( 85Kr) background concentrations at five sampling locations along the US east coast during 1982–1983. The samplers were established to monitor the 85Kr plume downwind of the Savannah river plant (SRP), a nuclear fuel reprocessing facility. The samplers were located 300–1000 km downwind of the SRP. In the original analyses of the measurements, a constant background concentration, representing an upper-limit and different for each sampling station, was subtracted from the measurements to obtain the part of the measurement representing the SRP plume. The use of a 3D global model, which includes all major 85Kr sources worldwide, was able to reproduce the day-to-day concentration background variations at the sampling locations with correlation coefficients of 0.36–0.46. These 3D model background predictions, without including the nearby SRP source, were then subtracted from the measured concentrations at each sampler, the result representing the portion of the measurement that can be attributed to emissions from the SRP. The revised plume estimates were a factor of 1.3–2.4 times higher than from the old method using a constant background subtraction. The greatest differences in the SRP plume estimates occurred at the most distant sampling stations.

Transport of sulfadiazine in soil columns — Experiments and modelling approaches

Antibiotics, such as sulfadiazine, reach agricultural soils directly through manure of grazing livestock or indirectly through the spreading of manure or sewage sludge on the field. Knowledge about the fate of antibiotics in soils is crucial for assessing the environmental risk of these compounds, including possible transport to the groundwater. Transport of 14C-labelled sulfadiazine was investigated in disturbed soil columns at a constant flow rate of 0.26 cm h − 1 near saturation. Sulfadiazine was applied in different concentrations for either a short or a long pulse duration. Breakthrough curves of sulfadiazine and the non-reactive tracer chloride were measured. At the end of the leaching period the soil concentration profiles were determined. The peak maxima of the breakthrough curves were delayed by a factor of 2 to 5 compared to chloride and the decreasing limbs are characterized by an extended tailing. However, the maximum relative concentrations differed as well as the eluted mass fractions, ranging from 18 to 83% after 500 h of leaching. To identify relevant sorption processes, breakthrough curves of sulfadiazine were fitted with a convective–dispersive transport model, considering different sorption concepts with one, two and three sorption sites. Breakthrough curves can be fitted best with a three-site sorption model, which includes two reversible kinetic and one irreversible sorption site. However, the simulated soil concentration profiles did not match the observations for all of the used models. Despite this incomplete process description, the obtained results have implications for the transport behavior of sulfadiazine in the field. Its leaching may be enhanced if it is frequently applied at higher concentrations.

Characterization of oxide thin films using optical techniques

Thin films of oxide materials are playing a growing role as critical elements in optoelectronic devices and nanoscale devices. In this work, thin films of some typical oxides such as WO 3, Ga 2O 3 and SrTiO 3 were investigated. We present measurements of those films, using various optical techniques like photoconductivity transients over a wide time range and photo-Hall measurements. Analysis of the photo-Hall and photoconductivity data permits the determination of the contribution to the photoconductivity made by the carrier mobility and concentration. A model for dispersive carrier transport was proposed to explain the relaxation of the photoconductivity in oxide thin films. In addition, photoluminescence characterization was used to study microstructures and energy band in oxide thin films. The broad emission from oxide host, consisting of several band peaks, was likely due to a recombination process with several possible paths. The dependence of the luminescent intensity on the annealing atmosphere was associated with the presence of oxygen vacancies. It is suggested that our optical analysis efforts have improved the understanding of oxide thin films, and this should lead to the necessary advancements in a variety of devices.

Influence of flow rate and organic carbon content on benzene transport in a sandy soil

AbstractThe influence of organic carbon content at various flow rates on the transport of aqueous benzene in a sandy soil was investigated. Column experiments were conducted for a sandy soil with different carbon contents (0%, 0·5%, and 2% by weight) and flow rates (0·25, 0·625, and 1·25 ml min−1) to observe breakthrough curves of KCl and benzene using a step application. Three types of convection–dispersion transport model (equilibrium, reversible two‐site, and reversible–irreversible sorption) were used to determine the appropriate model that best described the observed benzene transport. The modelling results revealed that the reversible–irreversible transport model was suitable for describing the transport and sorption behaviour of benzene in the sandy soil studied. Retardation and irreversible sorption coefficients increased with increasing carbon content, since the increment of carbon content resulted in the enhancement of sorption capacity for benzene. With increasing flow rate, decreased retardation of aqueous benzene was observed due to less reaction time; however, the model parameter for irreversible sorption increased unrealistically. This was attributed to the interference of velocity change on the irreversible sorption coefficient during parameter estimation. In order to avoid this problem, we introduced a Damköhler number (Da1), which took into account the velocity changes in estimating the irreversible sorption coefficient. A reasonable relationship was then found between Da1 and flow rate, which showed a decrease of Da1 with the increase in flow rate. Copyright © 2006 John Wiley &amp; Sons, Ltd.

Discrete hopping model of exciton transport in disordered media

A model of dispersive exciton transport has been developed for a medium with exciton energy levels randomly distributed in both space and energy scale. For a boxcar density of states of excitons an analytical solution is given describing the exciton density as a function of time and the proximity to the exciton quenching interfaces. The model parameters, such as exciton lifetime, effective number of exciton energy levels within inhomogeneously broadened density of states, and hopping distance and hopping rate constants, could be determined using time-resolved photoluminescence data. The developed model is verified via comparison with experimental data on the time-dependent photoluminescence decay of the conjugated polymer MEH-PPV and on the spectrum of internal quantum efficiency of the heterojunction photovoltaic device based on the MEH-PPV/TiO2 nanostructure.

The memory of volcanic waters: Shallow magma degassing revealed by halogen monitoring in thermal springs of La Soufrière volcano (Guadeloupe, Lesser Antilles)

The halogen contents of thermal waters collected since 1979 at La Soufrière volcano (Guadeloupe, Lesser Antilles) are interpreted as a retarded record of magma degassing pulses dispersed into the hydrothermal system. The further the spring is located from the source, the larger the time delay and the older the event recorded in water chemistry. Using advection–dispersion transport models in porous media, we reconstruct the time-series of degassing pulses for the period 1971–1992 and show that it correlates with the seismic records. The 1975–1977 sismo-volcanic crisis at La Soufrière is thereby interpreted as the result of a magma intrusion at shallow depth (∼3 km) which likely began in approximately 1973 and degassed in a pulsatory regime during ∼15 yr. The recent recrudescence of fumarolic and seismic activity could represent the initial stage of new magma injection. Measurement of halogen contents in hydrothermal waters collected around active volcanoes may provide a powerful tool for detection of the initial stages of magma intrusions.

In Situ Long‐Term Chloride Transport through a Layered, Nonsaturated Subsoil. 1. Data Set, Interpolation Methodology, and Results

A long‐term data set of Cl− concentrations in an artificial, leaking lake and in the underlying vadose zone was used to assess in situ vertical transport through a tertiary sandy deposit. Since the temporal resolution of the observed Cl− concentrations in the lake was relatively poor, a method was implemented to assess uncertainty on process identification as caused by a poor definition of the boundary condition. For this purpose we performed continuous stochastic simulations of Cl− concentrations in the lake conditioned to the discretely measured Cl− concentrations. This allows one to generate an ensemble of equiprobable time series of top boundary conditions. Each time series was subsequently used in an inverse convective–dispersive (CD) transport model based on the transfer function concept to identify the apparent transport properties of the unsaturated natural porous medium. For each simulation, one optimal parameter set in the least‐square sense is then obtained, and the variance of the optimized parameters reflects the uncertainty due to the poor sampling frequency of Cl− in the lake. For the case study presented, we show that the variance component due to poor sampling of the top boundary condition is small for estimating the transport velocities. This component increases with the hydrodynamic dispersivity. However, for this latter parameter, the sampling variance component was still small compared with the total variability between dispersivity at different depths. Physical interpretation of variability and scale dependency of this latter parameter is studied in more detail in an accompanying paper.

Sorption and desorption of radioselenium on calcareous soil and its solid components studied by batch and column experiments

The effect of different calcareous soil solid components on the sorption and desorption of radioselenium was investigated by using a selective extraction method. The distribution coefficients, the breakthrough curves and the displacement curves were experimentally determined by batch and column experiments. The experiments were carried out at pH 7.8±0.2, T=20±1°C and in the presence of 0.001M CaCl2. It is found that all sorption isotherms are linear, the sorption–desorption hysteresis for untreated and treated soils to remove organic matter, CaCO3 and organic matter plus CaCO3 is obvious; and the retention of radioselenium can probably be attributed to clay and oxides. The contributions of organic matter and CaCO3 to the sorption of radioselenium are significant, and the interaction effects among organic matter, CaCO3 and other components play an important role in the sorption of radioselenium in calcareous soil. The content of interlaminary space of compacted soil plays an important role in the sorption of radioselenium in compacted soil. The presence of CaCO3 retards the transport of radioselenium in soils, CaCO3 is an important trap of radioselenium. It is very difficult for the effluent concentration to reach the influent concentration, although radioselenium is very quickly detected in the effluent solution. The breakthrough curves (BTCs) can be simulated by a one-dimensional convection–dispersion transport model.

Origin and repartition of the oxide fixed charges generated by electrical stress in memory tunnel oxide

A method is proposed to determine the origin and the spatial oxide fixed charge repartition in memory tunnel oxide from Fowler–Nordheim current measurements after electrical stress. The Poisson equation resolution in the dielectric layer is required to account for the nonsymmetric tunneling barrier deformation, resulting from charges generated within the dielectric layer. From current–voltage characteristics measurements and simulations, we have determined the spatial distribution of the oxide fixed charges within the dielectric layer of metal/SiO2/metal structures. In addition, the kinetics of the oxide charge generation can be explained by a dispersive hydrogen transport model.

Lake circulation and sediment transport in Lake Myvatn

Lake circulation and sediment transport in Lake Myvatn have been calculated using AQUASEA, a numerical model developed by Vatnaskil Consulting Engineers. The goal of the modelling was to calculate changes in sediment transport within the lake due to changes in lake bathymetry caused by diatomite mining. The model uses the Galerkin finite element method and consists of a hydrodynamic flow model and a transport-dispersion model. The flow model is based on the shallow water equations and the wave equation. The transport model is based on the conservation of mass for suspended sediment. The model was calibrated against measurements performed during the summer of 1992. These included measurements of water elevation, current velocity, wave height, and concentration of suspended sediment. After calibration, the model was run for different mining scenarios to determine their impact on the sediment transport in the lake.