Simple Dispersion Model Research Articles

STEMS-Air: A simple GIS-based air pollution dispersion model for city-wide exposure assessment

Current methods of air pollution modelling do not readily meet the needs of air pollution mapping for short-term (i.e. daily) exposure studies. The main limiting factor is that for those few models that couple with a GIS there are insufficient tools for directly mapping air pollution both at high spatial resolution and over large areas (e.g. city wide). A simple GIS-based air pollution model (STEMS-Air) has been developed for PM 10 to meet these needs with the option to choose different exposure averaging periods (e.g. daily and annual). STEMS-Air uses the grid-based FOCALSUM function in ArcGIS in conjunction with a fine grid of emission sources and basic information on meteorology to implement a simple Gaussian plume model of air pollution dispersion. STEMS-Air was developed and validated in London, UK, using data on concentrations of PM 10 from routinely available monitoring data. Results from the validation study show that STEMS-Air performs well in predicting both daily (at four sites) and annual (at 30 sites) concentrations of PM 10. For daily modelling, STEMS-Air achieved r 2 values in the range 0.19–0.43 (p < 0.001) based solely on traffic-related emissions and r 2 values in the range 0.41–0.63 (p < 0.001) when adding information on ‘background’ levels of PM 10. For annual modelling of PM 10, the model returned r 2 in the range 0.67–0.77 (P < 0.001) when compared with monitored concentrations. The model can thus be used for rapid production of daily or annual city-wide air pollution maps either as a screening process in urban air quality planning and management, or as the basis for health risk assessment and epidemiological studies.

Porous medium acoustics of wave-induced vorticity diffusion

A theory for attenuation and dispersion of elastic waves due to wave-induced generation of vorticity at pore-scale heterogeneities in a macroscopically homogeneous porous medium is developed. The diffusive part of the vorticity field associated with a viscous wave in the pore space—the so-called slow shear wave—is linked to the porous medium acoustics through incorporation of the fluid strain rate tensor of a Newtonian fluid in the poroelastic constitutive relations. The method of statistical smoothing is then used to derive dynamic-equivalent elastic wave velocities accounting for the conversion scattering process into the diffusive slow shear wave in the presence of randomly distributed pore-scale heterogeneities. The result is a simple model for wave attenuation and dispersion associated with the transition from viscosity- to inertia-dominated flow regime.

Estimation of southern resident killer whale exposure to exhaust emissions from whale-watching vessels and potential adverse health effects and toxicity thresholds

Southern resident killer whales in British Columbia and Washington are exposed to heavy vessel traffic. This study investigates their exposure to exhaust gases from whale-watching vessels by using a simple dispersion model incorporating data on whale and vessel behavior, atmospheric conditions, and output of airborne pollutants from the whale-watching fleet based on emissions data from regulatory agencies.Our findings suggest that current whale-watching guidelines are usually effective in limiting pollutant exposure to levels at or just below those at which measurable adverse health effects would be expected in killer whales. However, safe pollutant levels are exceeded under worst-case conditions and certain average-case conditions. To reduce killer whale exposure to exhaust we recommend: vessels position on the downwind side of whales, a maximum of 20 whale-watching vessels should be within 800m at any given time, viewing periods should be limited, and current whale-watch guidelines and laws should be enforced.

Tracer concentration profiles measured in central London as part of the REPARTEE campaign

Abstract. There have been relatively few tracer experiments carried out that have looked at vertical plume spread in urban areas. In this paper we present results from two tracer (cyclic perfluorocarbon) experiments carried out in 2006 and 2007 in central London centred on the BT Tower as part of the REPARTEE (Regent's Park and Tower Environmental Experiment) campaign. The height of the tower gives a unique opportunity to study vertical dispersion profiles and transport times in central London. Vertical gradients are contrasted with the relevant Pasquill stability classes. Estimation of lateral advection and vertical mixing times are made and compared with previous measurements. Data are then compared with a simple operational dispersion model and contrasted with data taken in central London as part of the DAPPLE campaign. This correlates dosage with non-dimensionalised distance from source. Such analyses illustrate the feasibility of the use of these empirical correlations over these prescribed distances in central London.

A simple model for squirt-flow dispersion and attenuation in fluid-saturated granular rocks

A major cause of seismic attenuation in fluid-saturated rocks is the flow of the pore fluid induced by the passing wave. At sonic and ultrasonic frequencies, attenuation appears to be dominated by the local (pore-scale) flow between pores of different shapes and orientations. A simple squirt flow model is developed in which all of the parameters can be independently measured or estimated from measurements. The pore space of the rock is assumed to consist of stiff porosity and compliant (or soft) pores present at grain contacts. The effect of isotropically distributed compliant pores is modeled by considering pressure relaxation in a disk-shaped gap between adjacent grains. This derivation gives the complex and frequency-dependent effective bulk and shear moduli of a rock, in which the compliant pores are liquid saturated and stiff pores are dry. The resulting squirt model is consistent with Gassmann’s and Mavko–Jizba equations at low and high frequencies, respectively. The magnitude of attenuation and dispersion given by the model is directly related to the variation of dry bulk modulus with pressure and is relatively independent of fluid properties.

Urban tracer dispersion experiment in London (DAPPLE) 2003: field study and comparison with empirical prediction

AbstractThe DAPPLE (Dispersion of Air Pollutants and their Penetration into the Local Environment) project seeks to characterise near‐field urban atmospheric dispersion using a multidisciplinary approach. In this paper we report on the first tracer dispersion experiment carried out in May 2003. Results of concurrent meteorological measurements are presented. Variations of receptor tracer concentration with time are presented. Meteorological observations suggest that in‐street channelling and flow‐switching at intersections take place. A comparison between roof top and surface measurements suggest that rapid vertical mixing occurs, and a comparison between a simple dispersion model and maximum concentrations observed are presented. Copyright © 2010 Royal Meteorological Society

CityFlux perfluorocarbon tracer experiments

Abstract. In June 2006, two perfluorocarbon tracer experiments were conducted in central Manchester UK as part of the CityFlux campaign. The main aim was to investigate vertical dispersion in an urban area during convective conditions, but dispersion mechanisms within the street network were also studied. Paired receptors were used in most cases where one receptor was located at ground level and one at roof level. One receptor was located on the roof of Portland Tower which is an 80 m high building in central Manchester. Source receptor distances in the two experiments varied between 120 and 600 m. The results reveal that maximum concentration was sometimes found at roof level rather than at ground level implying the effectiveness of convective forces on dispersion. The degree of vertical dispersion was found to be dependent on source receptor distance as well as on building height in proximity to the release site. Evidence of flow channelling in a street canyon was also found. Both a Gaussian profile and a street network model were applied and the results show that the urban topography may lead to highly effective flow channelling which therefore may be a very important dispersion mechanism should the right meteorological conditions prevail. The experimental results from this campaign have also been compared with a simple urban dispersion model that was developed during the DAPPLE framework and show good agreement with this. The results presented here are some of the first published regarding vertical dispersion. More tracer experiments are needed in order to further characterise vertical concentration profiles and their dependence on, for instance, atmospheric stability. The impact of urban topography on pollutant dispersion is important to focus on in future tracer experiments in order to improve performance of models as well as for our understanding of the relationship between air quality and public health.

Urban tracer dispersion experiments during the second DAPPLE field campaign in London 2004

As part of the DAPPLE programme two large scale urban tracer experiments using multiple simultaneous releases of cyclic perfluoroalkanes from fixed location point sources was performed. The receptor concentrations along with relevant meteorological parameters measured are compared with a three screening dispersion models in order to best predict the decay of pollution sources with respect to distance. It is shown here that the simple dispersion models tested here can provide a reasonable upper bound estimate of the maximum concentrations measured with an empirical model derived from field observations and wind tunnel studies providing the best estimate. An indoor receptor was also used to assess indoor concentrations and their pertinence to commonly used evacuation procedures.

Uncertainty propagation in puff-based dispersion models using polynomial chaos

Atmospheric dispersion is a complex nonlinear physical process with numerous uncertainties in model parameters, inputs, source parameters, initial and boundary conditions. Accurate propagation of these uncertainties through the dispersion models is crucial for a reliable prediction of the probability distribution of the states and assessment of risk. A simple three-dimensional Gaussian puff-based dispersion model is used as a test case to study the effect of uncertainties in the model parameters and initial conditions on the output concentration. A polynomial chaos based approach is used to numerically investigate the evolution of the model output uncertainties due to initial condition and parametric uncertainties. The polynomial chaos solution is found to be an accurate approximation to ground truth, established by Monte Carlo simulation, while offering an efficient computational approach for large nonlinear systems with a relatively small number of uncertainties.

Solids Flow Models for Gas - Flowing Solids - Fixed Bed Contactors

Three solids flow models for gas – flowing solids – fixed bed contactors are analyzed. They all presume axial dispersion in the dynamic, freely flowing zone, but they differ in the interpretation of the stagnant zone. The models have been examined and the model parameters have been optimized on the basis of two types of tracer experiments. One provides step response curves for flowing solids at the exit and the other presents the response curves of the static flowing solids holdup. The model which assumes axial dispersion and exchange between dynamic and two active static zones, most accurately describes the solids flow pattern. A simpler model which presumes exchange between dynamic and one static zone can be used if there is no need for a precise description of the behavior of stagnant particles. The most simple axial dispersion model is not realistic, as it does not explain stagnancy at all, which was experimentally observed for the gas – flowing solids – fixed bed contactors.

Extraction of the Wideband Dielectric Properties of a Material Layer Using Measured Natural Frequencies

A new experimental technique for determining the complex permittivity of a dielectric material is introduced. A conductor-backed dielectric sheet is illuminated by a transient plane wave in a free-space measurement range and the temporal reflected field is analyzed as a natural mode series. By comparing the natural frequencies extracted from the response to those found theoretically using a simple dispersion model, the permittivity of the sheet may be determined over a broad band of frequencies. Since the natural frequencies are not affected by the waveform of the incident field, they remain a robust descriptor of the permittivity even after the requisite calibration steps. Examples of the low-loss dielectrics polyvinyl chloride, acrylic, and garolite are examined.

UAV navigation by an expert system for contaminant mapping with a genetic algorithm

Source characterization for an unknown contaminant release can be achieved by inverting an atmospheric transport and dispersion model given concentration observations from a moderately dense spatial array at one or more times. Achieving the required observation density over large geographic regions can, however, be prohibitively expensive if fixed sensors are employed. Mobile sensors provide a cost-saving alternative, with unmanned aerial vehicles (UAVs) being particularly well suited for these large-area problems because of their relatively high speed. The challenge then becomes to devise a set of navigation rules by which the aircraft can determine the route which most expeditiously acquires the required concentration observations. This task involves physical reasoning based on the wind vector, ongoing concentration observations, and current estimates of source position to plan leg length and direction. Each flight leg is planned based on data from all of the prior legs, the flight plan adapts to the observations as they are taken. Expert system navigation systems are developed for two situations: instantaneous (puff) and continuous (plume) releases. Of the two the puff poses the greater challenge because it provides a moving target rather than a quasi-steady concentration pattern. Thus, this rule-based navigation system must guide the UAV to an intercept for each pass through the puff rather than just sweeping across the contaminant field at multiple downwind distances as suffices with a plume. The navigation systems are tested in a virtual world consisting of a single fixed wind and concentration sensor, a UAV with wind and concentration sensing capability, a uniform wind at a significant fraction of the UAV airspeed, and a simple Gaussian dispersion model. The resulting concentration data is used to characterize the source strength and location by using a genetic algorithm to tune the variables until the model output matches the observations. Tests conducted using randomized source locations indicate that these UAV navigation systems are sophisticated enough to successfully acquire the necessary concentration data in the majority of the cases. The success rate is greatly improved by using an ensemble of non-communicating UAVs and taking the median of the resulting source variables. This process eliminates the outliers that result from occasional navigational failures.

Improving spatial concentration estimates for nitrogen oxides using a hybrid meteorological dispersion/land use regression model in Los Angeles, CA and Seattle, WA

Predictions from a simple line source dispersion model, Caline3, were included as a covariate in a land use regression (LUR) model for NO X /NO 2 in Los Angeles, CA and Seattle, WA. The Caline3 model prediction assumed a unit emission factor for all roadway segments (1.0 g/vehicle-mile). The NO X and/or NO 2 measurements for LA and Seattle were obtained from a comprehensive measurement campaign that is part of the Multi-Ethnic Study of Atherosclerosis Air Pollution Study (MESA Air). The measurement campaigns in both cities were approximately 2 weeks in duration employing approximately 145 measurement sites in Greater LA and 26 sites in Seattle. The best “standard” LUR model (obtained without the inclusion of the Caline3 predictions) in LA had R 2 values of 0.53 for NO X and 0.74 for NO 2. The leave-one-out cross-validated R 2 values for NO X and NO 2 were 0.45 and 0.71, respectively. The equivalent “standard” NO 2 model for Seattle had an R 2 of 0.72 and a leave-one-out cross-validated R 2 of 0.63. When the Caline3 variable was included in the LA hybrid model, the R 2 values were 0.71 and 0.79 for NO X and NO 2, respectively. The corresponding cross-validated R 2 values were 0.66 and 0.77, for NOX and NO2, respectively. In Seattle, the inclusion of the Caline3 variable resulted in a NO 2 model with an R 2 of 0.81 and a corresponding cross-validated R 2 of 0.67. In LA, hybrid model performance was not affected by excluding roadways with annual average daily traffic volumes (AADT) < 100,000. When the Caline3 predictions for heavy-duty trucks and lighter-duty vehicles were modelled as separate terms, the estimated fleet average NO X emission factors were 8.9 (SE = 0.7) and 0.16 (SE = 0.12) grams NO X /vehicle mile for heavy-duty and lighter-duty vehicles, respectively. These values are consistent with fleet average emission factors computed for LA with EMFAC 2007.

New Equipment for Laboratory Studies of Heterogeneously Catalyzed Reactive Distillation

AbstractIn this paper, novel equipment for laboratory studies of heterogeneously catalyzed reactive distillation is described. The so called D+R tray equipment consists of a combination of conventional bubble cap trays with sections that are filled with catalyst and through which the liquid flows when it passes from one distillation tray down to the next. It is designed so that the catalyst can be easily changed, and its amount can be flexibly chosen. In the present study, D+R tray equipment built in glass in the 50 mm diameter scale was investigated. The pressure drop is only slightly higher than that of a standard distillation tray alone and it allows the same fluid dynamic capacity. In order to characterize the properties of the equipment as a reactor, residence time distribution (RTD) measurements were carried out. For this purpose, the equipment was modified so that only the reactive section could be studied. Still, the RTD of the catalytic bed cannot be investigated directly. It was therefore determined based on accompanying computational fluid dynamics simulations. The RTD of the catalytic bed can be described well by a simple dispersion model with a Bodenstein number of 60–65, showing that it basically has plug flow characteristics.

Liquid Expanded Monolayers of Lipids As Model Systems to Understand the Anionic Hofmeister Series: 1. A Tale of Models

In this work, we use Langmuir monolayers of dipalmitoyl phosphatidylcholine (DPPC) as model systems to enhance the understanding of specific anion effects in physicochemical and biological systems. The 298 K isotherms (equation of state, EOS) of DPPC over solutions of a range of sodium salts depend strongly on the type and concentration of the salt in the subphase. We focus in particular on the liquid expanded phase region of the DPPC EOS and assume that the deviation of the isotherms over electrolyte solutions from that over pure water is due entirely to the charging of the lipid monolayer by the ions. We then examine the ability of a range of phenomenological continuum models to explain the pressure increase in the presence of electrolytes. The important finding is that insoluble lipid monolayers allow the discrimination between possible modes of ion-lipid interaction. Chemical binding models, simple or modified, cannot fit the range of data presented in this work. Both dispersion interaction and partitioning models fit most of the experimental isotherms and provide unique values for dispersion coefficients or ionic partitioning constants, respectively, even though the nature of these models is completely different (the former concentrates on the potential of mean force that acts on an ion in the double layer, while the latter concentrates on the treatment of interactions at the interface). Surprisingly, the respective fitting parameters are very highly correlated, reflecting, we believe, the effect of ion size on ionic properties and interactions. With sodium fluoride (NaF) as the subphase electrolyte, it is demonstrated that sodium exhibits a weak complexation-type interaction with the zwitterionic lipids. The simple dispersion and partitioning models cannot account for the NaF results, highlighting the need for more complex salt-lipid interaction models that account both for sodium binding and anion partitioning. This realization sets the stage for the companion paper.

A simple urban dispersion model tested with tracer data from Oklahoma City and Manhattan

A simple urban dispersion model is tested that is based on the Gaussian plume model and modifications to the Briggs urban dispersion curves. An initial dispersion coefficient ( σ o) of 40 m is assumed to apply in built-up downtown areas, and the stability is assumed to be slightly unstable during the day and slightly stable during the night. Observations from tracer experiments during the Joint Urban 2003 (JU2003) field study in Oklahoma City and the Madison Square Garden 2005 (MSG05) field study in Manhattan are used for model testing. The tracer SF 6 was released during JU2003 near ground level in the downtown area and concentrations were observed at over 100 locations within 4 km from the source. Six perfluorocarbon tracer (PFT) gases were released near ground level during MSG05 and sampled by about 20 samplers at the surface and on building roofs. The evaluations compare concentrations normalized by source release rate, C/ Q, for each sampler location and each tracer release, where data were used only if both the observed and predicted concentrations exceeded threshold levels. At JU2003, for all samplers and release times, the fractional mean bias (FB) is about 0.2 during the day (20% mean underprediction) and 0.0 during the night. About 45 –50% of the predictions are within a factor of two (FAC2) of the observations day and night at JU2003. The maximum observed C/ Q is about two times the maximum predicted C/ Q both day and night. At MSG05, for all PFTs, surface samplers, and release times, FB is 0.14 and FAC2 is about 45%. The overall 60 min-averaged maximum C/ Q is underpredicted by about 40% for the surface samplers and is overpredicted by about 25% for the building-roof samplers.

Steady Groundwater Transport of Highway Deicing Agent Constituents from an Infiltration Basin

The highway deicing agent groundwater plume from an infiltration basin in the Plymouth-Carver Aquifer near State Route 25 in southeastern Massachusetts is modeled and measured in order to assess the impact of the basin on groundwater transport processes. The advective transport model superimposes the existing steady models of axisymmetric basin hydraulics of Zlotnik and Ledder in 1992 and the two-dimensional ambient flow of Gelhar and Wilson in 1974. The basin component incorporates a surface source of finite-radius into a Hankel transform model for unconfined aquifers, whereas the ambient component varies linearly in the horizontal and vertical directions. Contaminant streamlines describe the resulting groundwater plume, and highway deicing agent constituents provide useful tracers. A simple vertical dispersion model quantifies the spread of contaminants across the bottom of the plume. Deicing agent constituent data calibrate a 17 m basin radius, a 45 m plume width, a bottom streamline 6–10 m below the w...

Near source deposition of vehicle generated fugitive dust on vegetation and buildings: Model development and theory

This paper describes the development of a simple quasi-2D Eulerian atmospheric dispersion model that accounts for dry deposition of fugitive dust onto vegetation and buildings. The focus of this work is on the effects of atmospheric surface layer parameterizations on deposition in the “impact zone” near unpaved roads where horizontal advection of a dust cloud through roughness is important. A wind model for computing average and turbulent wind fields is presented for flow within and above a roughness canopy. The canopy model has been developed to capture the most essential transport and deposition physics while minimizing the number of difficult-to-obtain input parameters. The deposition model is based on a bulk sink term in the transport equation that lumps the various dry deposition physical process. Wind field, turbulence and deposition results are presented for a range of atmospheric stabilities and roughnesses. The canopy model produces results in which deposition within a canopy is enhanced under certain initial, atmospheric and roughness conditions, while under other conditions much less deposition occurs. The primary limitation of the model is the ability to accurately determine (typically using experimental data) the vegetative deposition parameter (clearance frequency). To understand the clearance frequency better, a dimensionless parameter called the deposition effectiveness is identified that can be used to estimate deposition in the canopy. In general, the model captures the essential physics of near source dust transport and provides a tool that can efficiently simulate site-specific conditions in practical situations.

A study of pollutant concentration variability in an urban street under low wind speeds

AbstractThe short time‐scale variability in pollutant concentrations in an urban street under very low wind speed conditions and short source–receptor distance has been investigated using the inert tracer sulfur hexafluoride (SF6) as a continuous point‐source (release times ≥ 5 min), and fast detection using separation by gas chromatography coupled with a μ‐electron capture detector (ECD). The results are complex but can be broadly interpreted in terms of horizontal wind speed and direction coherence. Comparisons with a simple dispersion model suggest that observed time‐averaged maximum concentrations approach predicted values, whilst instantaneous maximum concentrations vary greatly and would therefore be difficult to predict. Copyright © 2008 Royal Meteorological Society

The transient buoyancy driven motion of bubbles across a two-dimensional quiescent domain

The transient buoyancy driven motion of two-dimensional bubbles across a domain bounded by two horizontal walls is studied by direct numerical simulations. The bubbles are initially released next to the lower wall and as they rise, they disperse. Eventually all the bubbles collect at the top wall. The goal of the study is to examine how a simple one-dimensional model for the averaged void fraction captures the unsteady bubble motion. By using void fraction dependent velocities, where the exact dependency is obtained from simulations of homogeneous bubbly flows, the overall dispersion of the bubbles is predicted. Significant differences remain, however. We suggest that bubble dispersion by the bubble induced liquid velocity must be included, and by using a simple model for the bubble dispersion we show improved agreement.