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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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K. Jerry Allwine, Brian K. Lamb, and Robert Eskridge

Abstract

During January 1989, five nighttime SF6 tracer experiments were conducted in Roanoke, Virginia. The experiments were designed to help identify and understand the dispersion characteristics of a basin atmosphere during winter stagnation conditions. The basin studied was the Roanoke basin located on the eastern slopes of the Appalachian Mountains. This paper documents this tracer study and gives results from the experiment conducted on the night of 16–17 January 1989. A cold-air pool formed in the basin beginning after the evening transition period and filled to near the elevation of the lowest mountain barrier. A simple model of the ascent rate of the top of this cold-air pool is proposed. A sharp potential temperature jump was present at the top of this fully developed cold-air pool. Vertical measurements of tracer showed the initial ground-level plume to become elevated and ride over the top of the cold-air pool. Horizontal plume spread was enhanced over that expected from turbulent diffusion alone, by shear in wind-direction vertical profiles. The tracer concentrations within the cold-air pool increased slowly with time, even after the release was terminated. After sunrise, the elevated plume appeared to fumigate to the ground.

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Julia E. Flaherty, Brian Lamb, K. Jerry Allwine, and Eugene Allwine

Abstract

An atmospheric tracer dispersion study known as Joint Urban 2003 was conducted in Oklahoma City, Oklahoma, during July of 2003. As part of this field program, vertical concentration profiles were measured at approximately 1 km from the downtown ground-level tracer gas release locations. These profiles showed that the urban landscape was very effective in mixing the plume vertically. In general, the lowest concentration measured along the profile was within 50% of the highest concentration in any given 5-min measurement period. The general slope of the concentration profiles was bounded by a Gaussian distribution with Briggs’s urban equations (stability classes D and E/F) for vertical dispersion. However, measured concentration maxima occurred at levels above the surface, which would not be predicted by Gaussian formulations. Variations in tracer concentration observed in the time series between different release periods were related to changes in wind direction as opposed to changes in turbulence. This was demonstrated using data from mobile analyzers that captured the width of the plume by traveling east to west along nearby streets. These mobile-van-analyzer data were also used to compute plume widths. Plume widths increased for wind directions at larger angles to the street grid, and a simple model comprising adjusted open-country dispersion coefficients and a street channeling component, were used to describe the measured widths. This dispersion dataset is a valuable asset not only for developing advanced tools for emergency-response situations in the event of a toxic release but also for refining air-quality models.

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Jielun Sun, Steven P. Oncley, Sean P. Burns, Britton B. Stephens, Donald H. Lenschow, Teresa Campos, Russell K. Monson, David S. Schimel, William J. Sacks, Stephan F. J. De Wekker, Chun-Ta Lai, Brian Lamb, Dennis Ojima, Patrick Z. Ellsworth, Leonel S. L. Sternberg, Sharon Zhong, Craig Clements, David J. P. Moore, Dean E. Anderson, Andrew S. Watt, Jia Hu, Mark Tschudi, Steven Aulenbach, Eugene Allwine, and Teresa Coons

A significant fraction of Earth consists of mountainous terrain. However, the question of how to monitor the surface–atmosphere carbon exchange over complex terrain has not been fully explored. This article reports on studies by a team of investigators from U.S. universities and research institutes who carried out a multiscale and multidisciplinary field and modeling investigation of the CO2 exchange between ecosystems and the atmosphere and of CO2 transport over complex mountainous terrain in the Rocky Mountain region of Colorado. The goals of the field campaign, which included ground and airborne in situ and remote-sensing measurements, were to characterize unique features of the local CO2 exchange and to find effective methods to measure regional ecosystem–atmosphere CO2 exchange over complex terrain. The modeling effort included atmospheric and ecological numerical modeling and data assimilation to investigate regional CO2 transport and biological processes involved in ecosystem–atmosphere carbon exchange. In this report, we document our approaches, demonstrate some preliminary results, and discuss principal patterns and conclusions concerning ecosystem–atmosphere carbon exchange over complex terrain and its relation to past studies that have considered these processes over much simpler terrain.

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Edward G. Patton, Thomas W. Horst, Peter P. Sullivan, Donald H. Lenschow, Steven P. Oncley, William O. J. Brown, Sean P. Burns, Alex B. Guenther, Andreas Held, Thomas Karl, Shane D. Mayor, Luciana V. Rizzo, Scott M. Spuler, Jielun Sun, Andrew A. Turnipseed, Eugene J. Allwine, Steven L. Edburg, Brian K. Lamb, Roni Avissar, Ronald J. Calhoun, Jan Kleissl, William J. Massman, Kyaw Tha Paw U, and Jeffrey C. Weil

The Canopy Horizontal Array Turbulence Study (CHATS) took place in spring 2007 and is the third in the series of Horizontal Array Turbulence Study (HATS) experiments. The HATS experiments have been instrumental in testing and developing subfilterscale (SFS) models for large-eddy simulation (LES) of planetary boundary layer (PBL) turbulence. The CHATS campaign took place in a deciduous walnut orchard near Dixon, California, and was designed to examine the impacts of vegetation on SFS turbulence. Measurements were collected both prior to and following leafout to capture the impact of leaves on the turbulence, stratification, and scalar source/sink distribution. CHATS utilized crosswind arrays of fast-response instrumentation to investigate the impact of the canopy-imposed distribution of momentum extraction and scalar sources on SFS transport of momentum, energy, and three scalars. To directly test and link with PBL parameterizations of canopy-modified turbulent exchange, CHATS also included a 30-m profile tower instrumented with turbulence instrumentation, fast and slow chemical sensors, aerosol samplers, and radiation instrumentation. A highresolution scanning backscatter lidar characterized the turbulence structure above and within the canopy; a scanning Doppler lidar, mini sodar/radio acoustic sounding system (RASS), and a new helicopter-observing platform provided details of the PBL-scale flow. Ultimately, the CHATS dataset will lead to improved parameterizations of energy and scalar transport to and from vegetation, which are a critical component of global and regional land, atmosphere, and chemical models. This manuscript presents an overview of the experiment, documents the regime sampled, and highlights some preliminary key findings.

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