Holly Peterson

Interpretation of Measured Tracer Concentration Fluctuations Using a Sinusoidal Meandering Plume Model

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...

An Investigation of Instantaneous Diffusion and Concentration Fluctuations

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 vertic...

Instantaneous Spread of Plumes in the Surface Layer

Abstract Data are presented from two recent tracer campaigns regarding relative diffusion of surface-level plumes. One study consists of tests performed amid flat, rural terrain near Galen, Montana, while other experiments were conducted above a poplar forest with uniform canopy density and height near Boardman, Oregon. In both cases, sulfur hexafluoride was released near the surface at a constant rate, and fast-response analyzers were used to measure plume concentrations along crosswind traverses and at fixed locations within 1 km of the source. This paper characterizes horizontal plume spread on near-instantaneous time frames during 29 tests, and the field data are used to test seven empirical and theoretical approaches for estimating relative diffusion coefficients using on-site wind data. Five of the equations utilize simple turbulence statistics to predict plume spread as a function of downwind distance; one method utilizes stability-based power-law formulas; and the last technique invokes second-ord...

A Simple Model to Predict Scalar Dispersion within a Successively Thinned Loblolly Pine Canopy

Abstract Bark beetles kill millions of acres of trees in the United States annually by using chemical signaling to attack host trees en masse. As an attempt to control infestations, forest managers use synthetic semiochemical sources to attract beetles to traps and/or repel beetles from high-value resources such as trees and stands. The purpose of this study was to develop a simple numerical technique that may be used by forest managers as a guide in the placement of synthetic semiochemicals. The authors used a one-dimensional, one-equation turbulence model (k–lm) to drive a three-dimensional transport and dispersion model. Predictions were compared with observations from a unique tracer gas experiment conducted in a successively thinned loblolly pine canopy. Predictions of wind speed and turbulent kinetic energy compared well with observations. Scalar concentration was predicted well and trends of maximum observed concentration versus leaf area index were captured within 30 m of the release location. A h...

Comparison of Results from a Meandering-Plume Model with Measured Atmospheric Tracer Concentration Fluctuations

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...

Pheromone Movement in Four Stand Thinning Scenarios: High Frequency Plume Observations

An atmospheric tracer experiment using SF6 was designed to assess changes in the dispersive environment in the trunk space of a southern pine forest through four thinning regimes. The experimental plot was thinned from dense boles and thick understory (>140 ft2 (13 m2) basal area) in four stages with the final basal area being 70 ft2 (6.5 m2 ). Observations indicate that thinned stands are less susceptible to bark beetle attack and one possible reason could be that the plumes of pheromone the insects use for signaling cannot achieve the intended purpose in the thinned stand. The pheromone mechanism could be altered through direct dilution of the gaseous plume, through a reduction in spatial coherence of a plume making it more difficult to follow back to a source or through loss of the plume vertically from the stand environment through lofting due to surface heating and loss of overhead containment. This work is part of a larger program to improve the success of forest insect management strategies using pheromone. Data analysis is on-going but the high frequency tracer plumes examined in this study show that the plumes are more dilute and much less coherent spatially in the thinned stands. The plumes are filamentous in nature in all cases but wander (whip) across a larger volume of space in the thinned stands.

Mass-Balance of Pheromone Surrogate Plumes in the Canopy Trunk Space

A large, multi-year study has been completed to evaluate the dispersion of insect pheromone in forest canopy trunk spaces. The study was undertaken to guide operational pest managers in the placement of pheromone sources in forest canopies. A very dense network of tracer sampling devices was deployed and over 200 average (half-hourly) plumes consisting of over 13000 chemical samples are available for analysis. This spatially dense near-field array provides an important opportunity to conduct a mass balance exercise. The subset of tests analyzed here indicates that mass recovery in this situation is possible within a factor of two and often close to 100%, though even in this dense sampler field there is substantial uncertainty and a tendency to over recover (>100%).

Diffusion of Insect Pheromones in a Forest Canopy: Co-Located Tracer/Electroantennogram Experiments

Sulfur hexafluoride (SF6) is a tracer gas that has been widely used to simulate behavior of air pollutants in the atmosphere. In this study, however, tracer experiments were conducted to study diffusion of insect pheromone through a forest canopy. Experiments were performed during morning and afternoon hours in August of 2003 at a forested field site near Pellston, Michigan. An SF6 source was collocated with a gypsy moth pheromone source, and concentration data for both gases were collected downwind with near-instantaneous equipment at a source-to-receptor distance of 5 m. In addition, a sonic anemometer was used to measure turbulent changes in horizontal and vertical winds throughout the study. The results illustrate the use of SF6 as a surrogate for insect pheromone, and to evaluate the performance of a new, portable, electroantennogram (EAG) device.

Near-Field Trunk Space Dispersion

A near-field tracer study was conducted to study the movement and dispersion of gas in the lower forest canopy in an attempt to improve guidance for forest managers deploying anti-aggregation pheromone sources to protect high value forest stands. Data are shown from three forest canopies and include over 13000 chemical tracer samples compiled into half hour dispersion fields around a point source. A high frequency sampler was also deployed to ascertain the structure of the gas plumes at 1 Hz. The plumes showed strong Gaussian tendencies in many cases and very high peak-to-mean ratios. Average maximum ÷/Q values were relatively consistent over the canopies studied though high variance in the maximum ÷/Q values was observed.