Ground-level Area Source Research Articles

Turbulent flow and dispersion inside and around elevated walkways

The influence of an elevated walkway on turbulent flow and dispersion inside a street canyon is investigated numerically with large-eddy simulation (LES). Different walkway designs and elevations are examined for a unit-aspect-ratio street canyon and an external wind perpendicular to the canyon axis. In place of a principal vortex extending through the canyon, smaller primary vortices appear above the walkway lid or below the walkway deck, in accord with the effective aspect ratio of the upper and lower regions. The mean flow decelerates below the deck and the turbulence kinetic energy (TKE) increases around the walkway structure for all configurations. Ventilation mechanisms for a pollutant emitted by a ground-level area source are qualitatively altered. Consequently mean concentrations at the pedestrian and walkway levels may increase by up to 20% and 30%, respectively. The velocity and pollutant statistics show a complicated dependence on the walkway design; this dependence is determined in part by interactions between the mean flow and turbulence.

Large-eddy simulation of pollutant dispersion from a ground-level area source over urban street canyons with irreversible chemical reactions

Abstract. In this study, the dispersion of chemically reactive pollutants is calculated by large-eddy simulation (LES) in a neutrally stratified urban canopy layer (UCL) over urban areas. As a pilot attempt, idealized street canyons of unity building-height-to-street-width (aspect) ratio are used. Nitric oxide (NO) is emitted from the ground surface of the first street canyon into the domain doped with ozone (O3). In the absence of ultraviolet radiation, this irreversible chemistry produces nitrogen dioxide (NO2), developing a reactive plume over the rough urban surface. A range of timescales of turbulence and chemistry are utilized to examine the mechanism of turbulent mixing and chemical reactions in the UCL. The Damköhler number (Da) and the reaction rate (r) are analyzed along the vertical direction on the plane normal to the prevailing flow at 10 m after the source. The maximum reaction rate peaks at an elevation where Damköhler number Da is equal or close to unity. Hence, comparable timescales of turbulence and reaction could enhance the chemical reactions in the plume.

Numerical Simulation of Transport of Particles Emitted From Ground-Level Area Source Using Aermod and CFD

Fugitive sources, including animal feeding operations, can contribute significantly to particulate pollution and cause environmental and health problems. Transport of particles emitted from these sources can be simulated using atmospheric dispersion models and computational fluid dynamics (CFD). This study evaluated the capability of AERMOD, the U.S. EPA preferred dispersion model, in modeling transport of particles emitted from area sources by comparing it to CFD. The two models responded similarly to effects of atmospheric stability and wind speed. AERMOD calculated lower downwind particle concentrations than CFD. In addition, predicted particle concentrations at locations downwind of the source remained constant with height in AERMOD but varied with height in CFD. Comparison of convection and diffusion rates simulated in CFD indicated that the assumptions on transport processes in AERMOD are suitable for modeling dispersion for area sources. However, estimated values for eddy diffusivity in crosswind and vertical directions were much higher in AERMOD than in CFD, which may explain the low concentrations predicted by the dispersion model and contribute to the difference in their calculated downwind concentrations and simulated concentration profiles.

Development and evaluation of a ground-level area source analytical dispersion model to predict particulate matter concentration for different particle sizes

The dispersion of particulate matter downwind of a biosolids applied site was modeled incorporating dry deposition. For this purpose, a ground-level area source analytical dispersion model known as particulate matter deposition – PMD was developed. The PMD model includes dry deposition which involves particulate deposition velocities in predicting concentrations. The performance of this model was statistically evaluated after applying it to particulate matter emitting from an agricultural field applied with biosolids. The results of the statistical evaluation are presented for different particle sizes. The statistical evaluation results showed that the PMD model satisfied the criteria for most of the statistical performance measures considered. This study concludes that the PMD model can be used effectively in predicting particulate matter concentrations with dry deposition in unstable conditions (B and C) at different wind speed categories for near-field downwind distances of an area source.

Large-Eddy Simulation of Flow and Pollutant Transports in and Above Two-Dimensional Idealized Street Canyons

A large-eddy simulation (LES) model, using the one-equation subgrid-scale (SGS) parametrization, was developed to study the flow and pollutant transport in and above urban street canyons. Three identical two-dimensional (2D) street canyons of unity aspect ratio, each consisting of a ground-level area source of constant pollutant concentration, are evenly aligned in a cross-flow in the streamwise direction x. Theflow falls into the skimming flow regime. A larger computational domain is adopted to accurately resolve the turbulence above roof level and its influence on the flow characteristics in the street canyons. The LES calculated statistics of wind and pollutant transports agree well with other field, laboratory and modelling results available in the literature. The maximum wind velocity standard devi- ations σi in the streamwise (σu), spanwise (σv) and vertical (σw) directions are located near the roof-level windward corners. Moreover, a second σw peak is found at z ≈ 1.5h (h is the building height) over the street canyons. Normalizing σi by the local friction velocity u∗ ,i t is found that σu/u∗ ≈ 1.8, σv/u∗ ≈ 1. 3a ndσw/u∗ ≈ 1.25 exhibiting rather uniform values in the urban roughness sublayer. Quadrant analysis of the vertical momentum flux u �� w �� shows that, while the inward and outward interactions are small, the sweeps and ejections dominate themomentumtransportoverthestreetcanyons.Inthe x direction,thetwo-pointcorrelations of velocity Rv,x and Rw,x drop to zero at a separation larger than h but Ru,x (= 0.2) persists even at a separation of half the domain size. Partitioning the convective transfer coefficient � T of pollutant into its removal and re-entry components, an increasing pollutant re-entrain- ment from 26. 3t o 43.3% in the x direction is revealed, suggesting the impact of background pollutant on the air quality in street canyons.

Sensitivity of Two Dispersion Models (AERMOD and ISCST3) to Input Parameters for a Rural Ground-Level Area Source

As of December 2006, the American Meteorological Society/U.S. Environmental Protection Agency (EPA) Regulatory Model with Plume Rise Model Enhancements (AERMOD-PRIME; hereafter AERMOD) replaced the Industrial Source Complex Short Term Version 3 (ISCST3) as the EPA-preferred regulatory model. The change from ISCST3 to AERMOD will affect Prevention of Significant Deterioration (PSD) increment consumption as well as permit compliance in states where regulatory agencies limit property line concentrations using modeling analysis. Because of differences in model formulation and the treatment of terrain features, one cannot predict a priori whether ISCST3 or AERMOD will predict higher or lower pollutant concentrations downwind of a source. The objectives of this paper were to determine the sensitivity of AERMOD to various inputs and compare the highest downwind concentrations from a ground-level area source (GLAS) predicted by AERMOD to those predicted by ISCST3. Concentrations predicted using ISCST3 were sensitive to changes in wind speed, temperature, solar radiation (as it affects stability class), and mixing heights below 160 m. Surface roughness also affected downwind concentrations predicted by ISCST3. AERMOD was sensitive to changes in albedo, surface roughness, wind speed, temperature, and cloud cover. Bowen ratio did not affect the results from AERMOD. These results demonstrate AERMOD’s sensitivity to small changes in wind speed and surface roughness. When AERMOD is used to determine property line concentrations, small changes in these variables may affect the distance within which concentration limits are exceeded by several hundred meters.

Sensitivity of Two Dispersion Models (AERMOD and ISCST3) to Input Parameters for a Rural Ground-Level Area Source

Sensitivity of Two Dispersion Models (AERMOD and ISCST3) to Input Parameters for a Rural Ground-Level Area Source

Comparison of Dispersion Models for Ammonia Emissions from a Ground-Level Area Source

Dispersion models are important tools for determining and regulating pollutant emissions from many sources, including ground-level area sources such as feedyards, dairies, and agricultural field operations. This study compares the calculated emission fluxes of ammonia from a feedyard in the Texas panhandle using four dispersion models: Industrial Source Complex Short Term Version 3 (ISCST3), AERMOD-PRIME, WindTrax, and AUSTAL. ISCST3 and AERMOD are Gaussian plume models, while WindTrax and AUSTAL are backward and forward Lagrangian stochastic models, respectively. Identical measured downwind ammonia concentration data were entered into each model. The results of this study indicate that calculated emission rates and/or emission factors are model specific, and no simple conversion factor can be used to adjust emission rates and/or factors between models. Therefore, emission factors developed using one model should not be used in other models to determine downwind pollutant concentrations.

The Recovery of Ammonia and Hydrogen Sulflde from Ground-Level Area Sources Using Dynamic Isolation Flux Chambers: Bench-Scale Studies

Controlled bench-scale laboratory experiments were conducted to evaluate the recovery of ammonia (NH3) and hydrogen sulflde (H2S) from dynamic isolation flux chambers. H2S (80–4000 ppb) and NH3 (5000–40,000 ppb) samples were diffused through the flux chamber to simulate ground level area source emissions while measuring the inlet and outlet flux chamber concentrations simultaneously. Results showed that the recovery of H2S during a 30-min sampling time was almost complete for concentrations >2000 ppb. At the lowest concentration of 80 ppb, 92.55% of the H2S could be recovered during the given sampling period. NH3 emissions exhibited similar behavior between concentrations of 5000–40,000 ppb. Within the 30-min sampling period, 92.62% of the 5000-ppb NH3 sample could be recovered. Complete recovery was achieved for concentrations >40,000 ppb. Predictive equations were developed for gas adsorption. From these equations, the maximum difference between chamber inlet and outlet concentrations of NH3 or H2S was predicted to be 7.5% at the lowest concentration used for either gas. In the calculation of emission factors for NH3 and H2S, no adsorption correction factor is recommended for concentrations >37,500 ppb and 2100 ppb for NH3 and H2S, respectively. The reported differences in outlet and inlet concentration above these ranges are outside the full-scale sensitivity of the gas sensing equipment. The use of 46–90 m of Teflon tubing with the flux chambers has apparently no effect on gas adsorption, because recovery was completed almost instantaneously at the beginning of the tests.

A three-dimensional steady-state atmospheric dispersion-deposition model for emissions from a ground-level area source

Abstract An analytical solution to the steady-state three-dimensional atmospheric dispersion equation has been developed for the transport of non-buoyant emissions from a continuous ground-level area source. The model incorporates power law profiles for the variation of wind speed and vertical eddy diffusivity with height, represents the lateral eddy diffusivity as a function of wind speed and the crosswind dispersion coefficient, and includes dry deposition as a removal mechanism. The model is well suited for accurate prediction of emission concentration levels in the vicinity of an area source, as well as farther downwind, under neutral or stable atmospheric conditions. The impact of the important model parameters on contaminant dispersion is examined. The results from several simulations, compared with point and line sources of equivalent source strength, indicate that at short downwind distances, predictions of contaminant concentrations emitted from area sources may be unacceptably inaccurate unless the structure of the source is properly taken into account.

An approximate analytical solution to the diffusion equation for short-range dispersion from a continuous ground-level source

An easily-evaluated expression for the dimensionless concentration profile χ(z/z0,χ/z0, z0/L) = = cu*/kQ (or z0cu*/kQ) downwind of a continuous ground-level area (or line) source in the stable surface layer is obtained by integrating the diffusion equation using the Shwetz approximation method (c = concentration, Q = source strength, k = von Karman's constant). The analytical solution compares closely with concentration profiles obtained using a trajectory-simulation model over a useful range of heights, the important discrepancies occurring at the upper edge of the plume. The analytical solution is used to generate predictions of ground-level concentration for the Project Prairie Grass experiments; good agreement with the observations is obtained at all downwind distances (50 to 800 m).

CCl 3F (Freon -11) as an indicator of transport processes in an urban atmosphere—a case study in Melbourne

In view of the lack of surface uniformity, and the importance of relatively uniform ground level area sources of pollution in an urban environment, point source diffusion experiments are of limited usefulness in studying transport processes in an urban atmosphere. While it is not feasible to carry out controlled area source diffusion experiments, we argue here that CCl 3F is an existing pollutant which for many cities represents a good ground level area source. For cities such as Melbourne where industrial use of CCl 3F occurs, the effect of a few strong point sources superimposed on the area sources may need to be accounted for. As an example of the use of CCl 3F as an urban atmospheric tracer, vertical concentration profiles and simultaneous profiles of relevant meteorological parameters were obtained at Aspendale during a very stable June night. The profiles show a marked variation of concentration with height which is explained qualitatively by the change in wind direction with height and the variation with direction of the source strength (as determined by a source inventory) around Aspendale. The futility of estimating pollutant concentrations at height from ground level measurements is clear from our results, as is the importance of a knowledge of the non-uniformity of wind (both vertically and horizontally) due to local effects, such as cold air drainage.

Numerical experiments on the influence of CO 2 release at ground level on crop assimilation and water use

A model to simulate the effect of increased carbon dioxide levels on the gas exchange by a crop canopy was developed. The distribution of carbon dioxide in the canopy, released from a ground level area source, was calculated by a method reported elsewhere. In this submodel, the wind speed in the crop surface boundary layer, the CO 2 concentration at the upwind edge of the release area, the CO 2 release rate, and the aerodynamic canopy parameters (crop height, zero-plane displacement and roughness length) were the inputs. The radiant energy distribution inside the canopy was computed by a modified Duncan-Stewart submodel, using leaf architecture and optics as inputs. Using the outputs of the submodels and the air temperature, dewpoint and effective soil water potential as inputs, a leaf gas and energy exchange submodel calculated CO 2 and water vapor fluxes for each leaf layer, each characterized by physiological parameters that were assumed constant with height. The gas exchange of two sorghum canopies was calculated by the proposed model for a crop grown in central Texas. The efficiency of CO 2 enrichment (the increase in the CO 2 assimilation rate divided by the CO 2 release rate) was significant only at a high irradiance level and a low wind speed. An efficiency of 13% was predicted for a sparse and rough canopy at a global irradiance of about 1,000 W m −2 and at a wind speed of 0.5 m sec −1. It was 16% for a dense and smooth canopy. The CO 2 concentration halfway into the denser canopy was 3.6 g m −3 (1,997 ppm) higher than the ambient level, at a wind speed of 0.5 m sec −1 at 2 m height, for a release rate of 0.01 g m −1 sec −1 (360 kg ha −1 h −1). When CO 2 was continuously released in a sparse and rough canopy during a calm (average wind speed of 1.2 m sec −1) and clear day at a rate of 36 g m −2 h −1, the CO 2 concentration at the crop height was maintained at around 390 ppm. Daytime CO 2 assimilation was increased from 64 g m −1 without release to 84 g m −2 with release. The efficiency in this case was 4%, and would have been 5% for a dense and smooth canopy. The daily water use efficiency (crop assimilation divided by the crop water use) was enhanced from 0.7% to 1.1%. The results indicate that the proposed model is useful to evaluate the possibilities of CO 2 enrichment in field crops in terms of crop CO 2 assimilation and water use under steady state conditions.