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Holly Peterson and Brian Lamb

Abstract

Instantaneous plume behavior is investigated via experimental and modeling results from a recent field campaign. The data consist of wind velocity and concentration measurements collected 700 m from a point source of sulfur hexafluoride during stable and neutral conditions. Fixed-point and traverse concentration data are analyzed in terms of concentration fluctuation statistics and diffusion coefficients. For the fixed-point data during this study, concentration fluctuation intensities vary between 0.7 and 4.5, intermittency factors are between 0.18 and 0.95, and peak-to-mean ratios range from 4.5 to 41.1. For traverse data, two methods are used to estimate the instantaneous diffusion coefficient σγ1, which is defined as the standard deviation of the crosswind concentration distribution of the instantaneous plume. Using the moment method, coefficients for this dataset range from 10.3 to 132.1 m. Using the peak concentrations and an assumption of a Gaussian concentration distribution in the vertical and horizontal directions, coefficients are between 6.7 and 22.4 m. The instantaneous diffusion coefficients derived from measured peak concentrations are shown to be less sensitive to plume meander than values calculated from the moment method. The values are related to simple meteorological parameters with three empirical equations, and all three equations predict diffusion coefficients within a few meters of the measured values. When used in a meandering plume model, the diffusion coefficients provide a way to estimate plume concentration fluctuation statistics from simple measurements of the wind. Overall, the model predictions are within a factor of 2 or better for the concentration mean, intensity, intermittency factor, and peak-to-mean ratio under stable through neutral conditions in the absence of large horizontal wind meander.

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Holly Peterson and Brian Lamb

Abstract

Measured wind-azimuth data are used in a simple meandering-plume model to predict observed SF6 concentration fluctuations measured downwind of a point source during a range of stability conditions. The meander component of plume diffusion is calculated as the running mean of wind-azimuth signal using travel time as the smoothing time. Instantaneous plume coefficients are derived from measured peak instantaneous concentrations in a subset of the data for distances less than 1 km. These empirical plume coefficients are shown to be a linear function of the standard deviation of the residual azimuth signal after the meander component is removed (σθI = 0.285 σ¯θR). Instantaneous concentration time series predicted with the model using the observed azimuth data are quite similar to observed time series. Sensitivity analyses indicate that the model predictions are strongly dependent upon the averaging period used to filter the azimuth signal but that the instantaneous plume width is only a weak function of the averaging period. Overall, the model predictions are within a factor of 2 or better for the concentration mean, peak-to-mean concentration ratio, plume intermittency factor, and concentration-fluctuation intensity during stable conditions when meander dominates and the instantaneous plumes are narrow.

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Richard L. Benner and Brian Lamb

Abstract

A fast response version of the continuous SF6 analyzer originally introduced in 1976 has been developed. The new continuous analyzer has a response time constant of 0.36 s, which is 4–30 times faster than previous analyzers. The very fast response of the new analyzer allows it to be used to investigate the fundamental nature of atmospheric turbulence and diffusion processes. The new design has been thoroughly tested in laboratory and field conditions over an ambient temperature range from 4° to 40°C and an elevation range of 265 to 2130 m above sea level. The reproducibility in the response of the new analyzer was approximately ±15% over a one-month period. In automobile and aircraft sampling platforms, the reproducibility in response was less than ±12% for periods as long as 7.5 h and elevation changes exceeding 1800 m. The rate of baseline change was <1 ppt min−1 in the laboratory, <10 ppt min−1 in an automobile platform, and 15 ppt min−1 in an aircraft platform. The size, weight, and power consumption of the present analyzer have been minimized to allow for convenient mobile operation of the instrument.

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Paul Dawson, David E. Stock, and Brian Lamb

Abstract

A three-dimensional, nonhydrostatic numerical code using the two-equation turbulence closure was developed to model the atmospheric transport and diffusion of pollutants over buildings and a three-dimensional hill. The standard engineering two-equation, first-order turbulence closure was modified to account for surface layer effects and the reduced production of dissipation in the region above the surface layer found in an atmospheric boundary layer.

The computations for the dispersion of a building rooftop release showed good agreement with wind tunnel measurements, except when very close to the ground. The transport and dispersion of a plume over a 300-m conical hill, Steptoe Butte, was also simulated. The computations are compared with near ground-level field measurements.

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Holly Peterson, Brian Lamb, and David Stock

Abstract

Simultaneous instantaneous concentration and wind velocity fluctuations were measured 100 to 752 m downwind of a point source release of SF6 tracer during two field studies conducted amid rolling wheat fields and at a flat desert site in eastern Washington. Data from stable, neutron, and unstable conditions are interpreted using a meandering plume model where the meander is defined to be sinusoidal and the instantaneous plume profile is Gaussian. A sensitivity analysis of the model shows that the characteristic concentration time scale is a direct function of the meander time scale and the receptor position relative to the meander centerline. For narrow instantaneous plumes relative to the meander amplitude, the predicted mean crosswind profiles of concentration, intermittency factor, concentration fluctuation intensity, and peak-to-mean ratios exhibit bimodal distributions. Conditional (nonzero) concentration fluctuation intensifies calculated from the model are scattered about 1.0;, the scatter is the result of receptor location, meander amplitude, period, and instantaneous plume width. The magnitude of the scatter from model runs covering different receptor locations is essentially equal to the magnitude of the matter from the tracer observations. The simple meandering plume model thus provides a straightforward explanation of very complex patterns observed in the tracer concentration fluctuation data in terms of wind meander, receptor location, and instantaneous plume width.

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Julia E. Flaherty, David Stock, and Brian Lamb

Abstract

A 3D computational fluid dynamics study using Reynolds-averaged Navier–Stokes modeling was conducted and validated with field data from the Joint Urban 2003 dispersion study in Oklahoma City, Oklahoma. The modeled flow field indicated that the many short buildings in this domain had a relatively small effect on the flow field, whereas the few tall buildings considerably influenced the transport and diffusion of tracer gas through the domain. Modeled values were compared with observations along a vertical profile located about 500 m downwind of the source. The isothermal base case using the standard k–ε closure model was within 50% of the concentration measurements, and a convective case with ground and building surfaces 10°C hotter than ambient temperatures improved the modeled profile to within 30% of observations. Varying wind direction and source location had a marked effect on modeled concentrations at the vertical profile site. Ground-level concentrations were 6 times the observed values when the approach flow wind direction was changed by +15° and were nearly zero when the wind direction was changed by −15°. Similar results were obtained when the source was moved 50 m to the east and to the west, respectively. All cases underestimated wind speed and turbulent kinetic energy near the surface, although adding heat significantly improved the magnitude of the modeled turbulent kinetic energy. Model results based upon a Reynolds stress closure scheme were also compared with the vertical concentration profiles. Neither the isothermal case nor the thermal buoyancy case resulted in an improvement over the standard k–ε model.

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James P. Rydock and Brian K. Lamb

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An apparatus for the simultaneous measurement of two tracers, sulfur hexafluoride (SF6) and a perfluorocarbon compound, is introduced. The new instrument is a modification of a commercially available fast-response, continuous analyzer for single halogenated atmospheric tracer studies. A two-channel flow system was implemented consisting of an alumina cartridge in one channel and a glass beads cartridge of equal flow resistance in the second channel. The alumina passes only sulfur hexafluoride, while the glass beads pass both SF6 and the perfluorocarbon tracer. The SF6 is quantified directly from the electron capture detector (ECD) signal in the alumina channel, and the perfluorocarbon concentration is obtained from the difference of the ECD responses in the two channels. The dual-tracer analyzer is field portable for mobile operations or fixed-location monitoring, has a response time of 1.2 s, and has limits of detection of about 15 pptv for SF6 and 10 pptv for perfluoromethylcyclohexane, which was the principal perfluorocarbon tracer used in this study. The present instrument configuration, which requires periodic purging of the adsorbent trap, can obtain continuous measurements for a 10–15-min segment in every half hour of operation. Dual-tracer data from a field demonstration test are presented.

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Alex Guenther, Brian Lamb, and Ronald Petersen

Abstract

The ability of a modified Industrial Source Complex model to simulate concentration distributions resulting from high wind speeds (neutral conditions) has been evaluated by comparison to data from a wind tunnel study of a Prudhoe Bay, AK oil-gathering center characterized by short stacks and interconnecting buildings. The model tends to predict correctly the concentrations further downwind (500–1700 m) and underpredict the higher concentrations in the building wake region from 3 to 10 building heights downwind (100–350 m). The model cannot be used to predict concentrations at downwind distances less than 100 m where the maximum concentrations typically occurred. A large variation in the amount of building enhanced diffusion was observed for different wind direction. The Huber-Snyder algorithm could simulate lateral diffusion best when the combined width of all buildings of a height similar to the release height, up to a maximum of 5HB (building height), was considered the dominant length scale. Enhanced lateral dispersion was observed even when the plume reached a height above 1.2HB, estimated with Briggs plume rise equations, at a downwind distance of 2HB.

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Alex Guenther, Brian Lamb, and David Stock

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Plume downwash at a large oil-gathering facility in the Prudhoe Bay, Alaska oil-field reservation was simulated in a series of numerical experiments. The purpose of this study was to investigate the potential of the numerical model as a means of assessing the impact of pollutants emitted from buoyant sources influenced by complex aerodynamic wakes. The model is a three-dimensional, Cartesian coordinate, finite difference code that solves the nonhydrostatic, time-averaged equations for the conservation of momentum and energy. The code uses a modified form of the standard first-order, two-equation (k–ε) engineering turbulence closure model.

Wind tunnel and field investigations of dispersion at this arctic industrial complex indicate that dispersion is significantly influenced by building-generated airflow disturbances. We have used the numerical model to simulate directly the mean features of the flow field and dispersion from a buoyant source at an industrial site. The flow features varied depending on the size, number, and orientation of the buildings. A recirculation cavity was present in all model simulations and varied from 0.8 HB to 2 HB (building height). This agrees closely with results of wind tunnel studies. The model simulates a velocity defect of 0.6, a factor of 3.4 increase (relative to the approach flow) in turbulent kinetic energy (k), a factor of 5 increase in dissipation of k(ε), and a 45% increase in turbulent viscosity at a downwind distance of 2 HB from the building. At a downwind distance of 5 HB, the plume rise of the simulated thermal plume decreased by 70% compared to the no-building case while the vertical and horizontal widths of the plume increased by 45% and 30%, respectively. These results generally reproduce the plume downwash and dispersion observed in wind tunnel and field investigations.

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Joseph P. Krasnec, David E. Demaray, Brian Lamb, and Richard Benner

Abstract

The design of an improved sampler for the automatic collection of air samples during atmospheric halogenated tracer experiments is described. In this approach, each sample is collected in a small volume (20 to 150 ml) syringe using a rack and pinion drive powered by a precision stepper motor. The sampling rate is controlled via a quartz crystal oscillator and clock to yield averaging periods from 2 to 120 minutes per syringe, and as many as twelve syringe samples can be collected sequentially. Start-up of the sampler is pre-selected using a digital clock and calendar. The sampler is battery-powered, portable and designed for rugged field use. Sample return rates as high as 97% have been achieved in a number of large tracer field programs. Typical results from several of these investigations are also presented.

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