Dynamic Transport Model Research Articles

Enhanced Diffusion of Polycyclic Aromatic Hydrocarbons in Artificial and Natural Aqueous Solutions

Uptake of hydrophobic organic compounds into organisms is often limited by the diffusive transport through a thin boundary layer. Therefore, a microscale diffusion technique was applied to determine the diffusive mass transfer of 12 polycyclic aromatic hydrocarbons through water, air, surfactant solutions, humic acid solutions, aqueous soil and horse manure extracts, digestive fluid of a deposit-feeding worm, and root exudates from willow plants. In most cases the diffusive mass transfer of PAHs was much higher through the tested media than through water, and the enhancement factors increased with increasing hydrophobicity of the PAHs. The diffusive flux of benzo[a]pyrene was for instance enhanced 74 times through gut fluid of a deposit-feeding worm when compared to water. These findings demonstrate that a wide variety of dissolved organic carbon (DOC) at environmental levels can enhance diffusive mass transfer in various transport scenarios. The diffusive uptake of PAHs into sediment dwelling organisms is particularly efficient within the gut and at direct contract with the sediment matrix. Bioremediation might be enhanced bythe addition of auxiliary agents that enhance diffusive mass transfer. Enhanced diffusion needs also to be considered in dynamic transport models and for the operation and calibration of passive sampling techniques.

Baryon number and electric charge fluctuations in Pb+Pb collisions at relativistic energies

Event-by-event fluctuations of the net baryon number and electric charge in nucleus-nucleus collisions are studied in Pb+Pb at SPS energies within the Hadron-String Dynamics (HSD) transport model. We reveal an important role of the fluctuations in the number of target nucleon participants. They strongly influence all measured fluctuations even in the samples of events with rather rigid centrality trigger. This fact can be used to check different scenarios of nucleus-nucleus collisions by measuring the multiplicity fluctuations as a function of collision centrality in fixed kinematical regions of the projectile and target hemispheres. The HSD results for the event-by-event fluctuations of electric charge in central collisions at $20A,30A,40A,80A$, and $158A$ GeV are in a good agreement with the NA49 experimental data and considerably larger than expected in a quark-gluon plasma. This demonstrates that the distortions of the initial fluctuations by the hadronization phase and, in particular, by the final resonance decays dominate the observable fluctuations.

Empirical and Modeling Evidence of the Long-Range Atmospheric Transport of Decabromodiphenyl Ether

Understanding of the long-range atmospheric transport (LRT) behavior of decabromodiphenyl ether (BDE-209) is still limited. Most existing model-based approaches to assessing an organic chemical's potential for LRT have assumed invariant environmental conditions, even though many factors impacting on the atmospheric residence time are known to vary considerably over a variety of time scales. Model estimates of LRT also suffer from limited evaluation against observational evidence. Such evidence was sought from dated sediment cores taken from lakes along a latitudinal transect in North America. BDE-209 was generally detected only in recent sediment horizons, and sedimentation fluxes were found to decline exponentially with latitude. The empirical half-distance (EHD) for BDE-209 derived from surface flux data is approximately half that of the sigmaPCBs. A dynamic multimedia fate and transport model provides further insight into the temporal variability of processes that control LRT for BDE-209 and PCBs. The variability of precipitation, and in particular, the occurrence of time periods without precipitation coinciding with strong winds, influences the LRT potential of chemicals that combine a sufficiently long atmospheric half-life with very low volatility. Likewise, the forest filter effect may be important for a wider range of chemicals than believed previously, because models assuming constant precipitation fail to account for the impact of differences in dry deposition on days without rain. Chemicals that are both sorbed to particles and potentially persistent in the atmosphere, such as BDE-209, may have a larger potential for LRT than anticipated on the basis of earlier model evaluations. Still, the EHDs illustrate that the model seems to underestimate atmospheric loss processes of potential significance to BDE-209, illustrating the need to critically compare predictions of LRT against observations. Processes that need to be understood better in order to improve predictions of LRT for BDE-209 include particle dry deposition, precipitation scavenging, and photolysis in the sorbed state.

CoZMo-POP 2 – A fugacity-based dynamic multi-compartmental mass balance model of the fate of persistent organic pollutants

CoZMo-POP 2, a dynamic multimedia fate and transport model (MFTM) for describing the long-term fate of persistent organic pollutants (POPs) in a coastal environment or the drainage basin of a large lake is described. The modelled environment, consisting of up to 19 compartments, includes the forest, soils and fresh water bodies of the drainage basin, and a variable number of sequentially arranged marine water units, representing estuarine, coastal, open and deep water environments. Two sediment compartments in each water basin allow to account for the possibility of the simultaneous occurrence of eroding and accumulating bottoms in a water basin. As the movement of POPs in the environment is closely associated with the movement of air, water and organic matter, the model constructs complete steady state mass budgets for air, water and particulate organic carbon (POC) between the model compartments from the environmental parameters supplied by the user. This assures that these input parameters, which can be stored in a database, are all mutually compatible and internally consistent. The CoZMo-POP 2 model takes into account seasonably variable wind speeds, temperatures, canopy developments and OH radical concentrations, and allows for the definition of time-variant emission scenarios. CoZMo-POP 2 is a flexible, dynamic MFTM that can be used for detailed investigation of a particular fate process using a generic environmental scenario, as well as for simulations of the behaviour of a particular POP in a particular environment. All of the model equations, which are expressed in fugacity notation, are provided.

Thermal diffusion characteristics of atmosphere-particle two phase flow in dust storm

A model, coupling metrological dynamic model MM5 and dust transport model, is developed for the atmosphere-particle two phases flow of dust storm. The simulations of the dust storm events in north China with a geographic information database are performed using the model, and represent an overview of dust transport pathways and particles concentration distribution over the north China. The comparison between computations and practical observations shows that the simulations succeed in description of dust storm evolvement and particle transport behavior. Based on the computations and analysis, the characteristics of particle transport, especially well-concerning the factor of the particle thermal diffusion, are studied. A new definition of mass transfer Grd is put forward to discover the internal principle of particle thermal diffusion at various atmospheric layers. Several phenomena, such as thermal diffusion item QT–Grd distribution, and relationships, Particle Grd probability function, are obtained. The investigation indicates particle thermal diffusion can be not ignored in mesoscale atmospheric-particle multiphase flow.

Modelling atmospheric transport of α-hexachlorocyclohexane in the Northern Hemispherewith a 3-D dynamical model: DEHM-POP

Abstract. The Danish Eulerian Hemispheric Model (DEHM) is a 3-D dynamical atmospheric transport model originally developed to describe the atmospheric transport of sulphur into the Arctic. A new version of the model, DEHM-POP, developed to study the atmospheric transport and environmental fate of persistent organic pollutants (POPs) is presented. During environmental cycling, POPs can be deposited and re-emitted several times before reaching a final destination. A description of the exchange processes between the land/ocean surfaces and the atmosphere is included in the model to account for this multi-hop transport. The α-isomer of the pesticide hexachlorocyclohexane (α-HCH) is used as tracer in the model development. The structure of the model and processes included are described in detail. The results from a model simulation showing the atmospheric transport for the years 1991 to 1998 are presented and evaluated against measurements. The annual averaged atmospheric concentration of α-HCH for the 1990s is well described by the model; however, the shorter-term average concentration for most of the stations is not well captured. This indicates that the present simple surface description needs to be refined to get a better description of the air-surface exchange processes of POPs.

Solid–liquid transport in a modified co-rotating twin-screw extruder—dynamic simulator and experimental validations

This work presents a dynamic transport model of a solid–liquid media through a twin-screw extruder (TSE). The application under consideration is the solid–liquid extraction of solute from raw plant substrate. Dynamic experiments are performed and compared with the simulated results for step functions on the solid feed rate and on the screw rotating speed. Despite some imperfections, results allow to validate the simulator.

Dynamic Trip Generation for a Medium-Sized Urban Community

Traditional transportation modeling activities in medium-sized urban communities follow the four-step planning process: trip generation, trip distribution, mode split, and traffic assignment. These models forecast daily traffic volumes on major roadways of the communities to support infrastructure investment decisions. Recently, researchers have focused on dynamic traffic-assignment models, which provide time as a measure in the trip-modeling process, to support incident management and intelligent transportation system–based decisions. As a methodology to support the dynamic traffic-assignment models, development of a dynamic trip-generation model for a medium-sized urban community is considered. An overview of the need for dynamic trip-generation data is presented, and a methodology and a data-collection effort to develop a dynamic trip-generation model are discussed. Results are included of a validation study performed on the model. It is concluded that the models developed in the effort can provide dynamic trip-generation data and represent a first step toward making the dynamic transportation model a viable resource.

An Examination of the Fluxes of Nitrogen and Phosphorus in Temperate and Tropical Estuaries: Current Estimates and Uncertainties

In a number of regions of the world, enhanced flows of nitrogen (N) and phosphorus (P) from land to sea are of major concern because of the observable deterioration in the quality of many nearshore marine waters. Estuaries receive N and P from river and other runoff, from waste discharges, from the atmosphere and ocean and from exchange with coastal groundwaters (which in all likelihood results in a net input to the estuary). For rivers that do not discharge directly onto the continental shelf, seaward fluxes of N and P will be modified by within-estuary transformations of reactive species, the burial of particulate N and P in sediments (sub/intertidal, saltmarsh, mangrove) and the loss of gaseous N and P species by bacterial reduction.Driven by a desire to understand the effects of changing N and P loads on water quality, and to gain insights into the true modification of their fluxes within estuaries, much effort has been expended on providing quantitative estimates of the sources and sinks of these constituents. Yet, accurate and precise estimates on a global scale remain elusive. Riverine inputs of total N and P are calculated to be 35–64 and 22Mta−1, respectively. These inputs are dominated by particulate species, and because of this, are likely to be imprecise as overall sediment fluxes are disproportionately influenced by infrequent, poorly sampled, high flow events. Direct aeolian inputs of N to estuaries (P inputs are minor), at a minimum of 1–4Mta−1, are small but significant, although again good estimates are hampered by the apparent importance of infrequent, and thus under-sampled, deposition events. Indirect atmospheric inputs via deposition onto and runoff from catchments may be highly significant, at least in environments bounding the North Atlantic Ocean. Groundwater inputs are generally unknown, but, for N, may be 5–10Mta−1 (no data on P). Information on the global inputs of N and P from waste discharges and mariculture do not appear to be available. Denitrification, estimated to beca .33Mta−1, may account for 52–94% of the currently estimated total N inputs; in contrast, the loss of P via venting of gaseous phosphine is unknown. The burial of N and P in sediments is about 7% and 30% of their total inputs, respectively. Nevertheless, reliable information on the modifying role of estuarine sediments appears far from complete.Globally, the inputs of N and P to the marine environment from all sources are expected to increase over the next few decades. The resulting effects of these increases on the marine environment, including any influences due to estuarine processing, may be partly assessed through the use of dynamic transport and transformation estuarine models for N and P. A further important development in this respect will be the linking of complementary models (e.g. catchment/river/estuarine/coastal zone) and their coupling to strategic large scale observations.

Platinum catalysed aqueous alcohol oxidation: model-based investigation of reaction conditions and catalyst design

A dynamic transport model is derived to describe the platinum catalysed aqueous alcohol oxidation, considering a single spherical catalyst particle surrounded by a stagnant liquid film. The transport model is based on a heterogeneous kinetic model with mass transfer and intra particle diffusion resistances. The developed model uses the kinetic model of Markusse et al. (Catal. Today 66 (2001) 191) and is validated with the experimental kinetic data of methyl α- D-glucopyranoside (MGP) oxidation obtained by Vleeming et al. (Ind. Eng. Chem. Res.). The model is used to investigate the effect of process conditions, catalyst and particle properties, and transport parameters on the performance of the catalyst for alcohol oxidation. It is found that the electron conductivity of the catalyst support affects the rate of MGP oxidation especially at low bulk liquid oxygen concentrations, while at high bulk liquid oxygen concentrations, the catalyst support conductivity does not play a role. A major cause of catalyst deactivation is over-oxidation under oxygen rich conditions. This can be reversed by applying redox-cycle operation, an alternating exposure of the catalyst to oxidative and reductive environments. The advantages of redox-cycle application are demonstrated using the developed model. For reactions of negative order, such as MGP oxidation, at high bulk oxygen concentrations (C O 2,L >0.3 mol/m 3) , concentrating the active catalytic material in a layer buried some distance from the surface (core) gives considerable better performance than the conventional “egg shell” design of shallow deposition near the surface or uniform distribution. However, at low bulk oxygen concentrations (C O 2,L ⩽0.3 mol/m 3) , the performance of the uniform or egg shell distribution catalyst is superior to the core catalyst.

Erratum: Sequential decay distortion of Goldhaber model widths for spectator fragments [Phys. Rev. C65, 051602(R) (2002)

Momentum widths of the primary fragments and observed final fragments have been investigated within the framework of an Antisymmetrized Molecular Dynamics transport model code (AMD-V) with a sequential decay afterburner (GEMINI). It is found that the secondary evaporation effects cause the values of a reduced momentum width, $\sigma_0$, derived from momentum widths of the final fragments to be significantly less than those appropriate to the primary fragment but close to those observed in many experiments. Therefore, a new interpretation for experiemental momentum widths of projectile-like fragments is presented.

Simulation of Saturation Effects in Electroabsorption Modulators

Saturation phenomena have a deep impact on the performance of electroabsorption (EA) modulators. It is important to develop algorithms which as well describe the phenomenon of electroabsorption as they include nonlinear carrier dependent effects, band filling and reduction of the band gap. Thus, a dynamical transport model has to be coupled to the density matrix equation for excitons. The density matrix equation for interband processes determines quantum optical effects, while a drift-diffusion model (DDM) adequately describes transport phenomena in the device. Within this approach quantum transport is considered by a correction of the classical potential, the Bohm potential. To demonstrate the presented algorithm, waveguide EA-modulators including strained lattice and full Fermi statistics are investigated.

A dynamic transportation model with multiple criteria and multiple constraint levels

In this paper, a dynamic transportation model with multiple criteria and multiple constraints levels (DMC 2) is formulated by using the framework of MC 2 linear programming. An algorithm is developed to solve such DMC 2 transportation problems. In this algorithm, dynamic programming ideology is adopted to find the optimal subpolicies and optimal policy for a given DMC 2 transportation problem. Then the MC 2-simplex method is applied to locate the set of all potential solutions over possible changes of the objective coefficient parameter and the supply and demand parameter for the DMC 2 transportation problem. A numerical example is illustrated to demonstrate the applicability of the algorithm in solving the DMC 2 transportation problems.

Kinetics of THM Species in Finished Drinking Water

The U.S. Safe Drinking Water Act and its amendments impose restrictive rules for filtration of surface water and maximum contaminant levels for total trihalomethanes (THMs) in distribution systems. Extension of these regulations can potentially address the possibility of limiting each individual THM compound since the health risks associated with these compounds are not all alike. Current models of THMs are predicated on empirical reaction kinetic equations derived from linear and nonlinear multiregression analysis. This paper applies nonlinear optimization to model the kinetics of THM species under representative extreme conditions. The proposed approach combines site-specific trends with stoichiometric expressions based on average representative bromine content factors. The model was tested and validated using data from finished (desalinated) water collected in the United Arab Emirates. Reasonable agreements between model-predicted and measured values of different species were consistently obtained, indicating the applicability of the approach with a full dynamic water-quality transport model. This model can lead to improved management of water quality and assist utilities in attempting to optimize chlorine disinfection practices in water distribution systems.

Response of the Osmotic Tensiometer to Varying Temperatures Modeling and Experimental Validation

In an osmotic tensiometer, the hydrostatic pressure of a confined polymer solution is measured to infer the soil water potential. Essential in the operation is the osmotic potential of the polymer solution. Because the osmotic potential depends on the temperature, a significant and undesired pressure lag may develop when the osmotic tensiometer is used under conditions of varying temperature. To understand and eliminate the pressure lag, a dynamic transport model was derived that describes the time response of the osmotic tensiometer while assuming complete temperature equilibration and a resistance to water transfer that is fully located within the filter. Essential elements of the model are the filter conductance and the sensitivity of the pressure transducer, for which submodels are set up. The transport model is validated with measurements on an osmotic tensiometer that uses an inorganic membrane as filter. The experiments were done for instrument‐limited conditions by placing the osmotic tensiometer in free water. For these conditions, the model describes measurements with reasonable accuracy. The new transport model clarifies the behavior of the osmotic tensiometer for changing temperatures and can be used to design an osmotic tensiometer with a lower pressure lag.

D′ production in heavy-ion collisions

The production of d′ dibaryons in heavy-ion collisions due to the elementary process NN → d′π is considered. The NN → d′π cross section is estimated using the vacuum d′ width Γ d′ ≈ 0.5 MeV extracted from data on the double charge exchange reactions on nuclei. The d′ production rate per single collision of heavy ions is estimated at an incident beam energy of 1 A GeV within the framework of the Quantum Molecular Dynamics transport model. We suggest to analyse the invariant mass spectrum of the NNπ system in order to search for an abundance of events with the invariant mass of the d′ dibaryon. The d′ peak is found to exceed the statistical fluctuations of the background at a 6σ level for 2 × 10 5 · A central collisions of heavy ions with the atomic number A.

A numerical global meteorological sulfur transport model and its application to Arctic air pollution

The paper describes the construction of a dynamic atmospheric sulfur transport model and addresses the issue of long-range atmospheric sulfur transport to the Arctic as an application of the model. The global model includes the dynamics of meteorologial and tracer fields, thermodynamics, cloud processes, turbulent boundary layer mixing, multiple three-dimensional anthropogenic sulfur emission sources, dry and aqueous-phase chemical processes for sulfur, dry deposition and the precipitation scavenging of sulfur. So far, the incomplete description of clouds and precipitation has been a major limitation to the modeling of wet chemical processes on the global scale. One of the main features of our study is an attempt to a realistic representation of the interaction between clouds and chemical reactions. The model includes a detailed sub-grid scale convective and stratiform condensation scheme which includes cloud liquid water content as a predictive variable. It is shown that the model is able to reproduce important dynamic and physical structures in the atmospheric circulation leading to a realistic simulation of the important aspects of the long-range transport of sulfur to the Arctic. Realistic simulation of seasonal variations in atmospheric flow and cloud related processes provides reliable estimates of the sulfur deposition fluxes and reproduces the characteristic annual cycle of sulfur concentrations over the Arctic. Zonally averaged fields for the source and Arctic regions reveal important differences in the long-range transport mechanisms in different seasons. The model represents a powerful tool for further examining the mechanisms of sulfur transport and its impact on the atmosphere.

Self-organized critical directed percolation

We introduce and study a dynamic transport model exhibiting Self-Organized Criticality. The novel concepts of our model are the probabilistic propagation of activity and unbiased random repartition of energy among the active site and its nearest neighbors. For space dimensionality d ≥ 2 we argue that the model is related to ( d + 1)-dimensional directed percolation, with time interpreted as the preferred direction.

Modeling the transport of acidity in soil profiles with front ? A dynamic transport model

A dynamic transport model, FRONT, that describes the downwards transport of acidity in podzolized forest soils is presented. In this model the downward transport of acidity with the soil solution is counteracted by a production of alkalinity through the weathering of primary minerals and delayed by the adsorption of sulfate and hydrogen ions on iron- and aluminium oxides. The heart of the model is a massbalance equation that describes the transport of bulk acidity/alkalinity. The FRONT model was tested on 23 deep soil profiles situated along three transects in west-to-east direction across Sweden. Using a deposition scenario starting at 1910 the model was able to account for the large regional differences in the present depth of the acid front. Assuming a linearly decreasing deposition until 30% of present deposition is reached in 2010 the model was used to simulate a scenario for profiles in different parts of Sweden. The scenarios indicated that the upper parts of soil profiles that are severely acidified today will recover and assume a new steady-state in 2030. However, for soil profiles that have large stores of adsorbed sulfate in the B horizon the simulations indicate that one can expect an increased acidity in the deep soil layers several decades after the deposition has ceased due to downward transport of acidity.

Modelling the fate of organic chemicals in the soil plant environment: Model study of root uptake of pesticides

The fate of organic chemicals in the soil-plant-atmosphere environment and the governing processes were studied with a coupled dynamic soil transport and plant compartment model. Scenarios with applications of pesticides on sand and loam soils with chemical uptake in barley and wheat were used in the model calculations. Root uptake and concentrations in the plant compartments stem, leave and fruit were calculated for the pesticides terbuthylazine, isoproturon and carbofuran. The effectivity of uptake from soils with different soil sorption coefficients had been shown for sand and loam soils. The processes degradation in plant and volatilization from leaves to atmosphere are especially effective for carbofuran and terbuthylazine. Although the concentrations in corn at harvest are lower than the maximum allowed concentrations, the peak concentrations in the course of the vegetation period are significantly higher (factor ≤ 200).