A Wind Tunnel Model for Quantifying Fluxes in the Urban Boundary Layer   总被引：9，自引：6，他引：3  

Janet F. Barlow  Stephen E. Belcher 《Boundary-Layer Meteorology》2002,104(1):131-150

Transport of pollution and heatout of streets into the boundary layer above is not currently understood and so fluxes cannot be quantified. Scalar concentration within the street is determined by the flux out of it and so quantifying fluxes for turbulent flow over a rough urban surface is essential. We have developed a naphthalene sublimation technique to measure transfer from a two-dimensional street canyon in a wind tunnel for the case of flow perpendicular to the street. The street was coated with naphthalene, which sublimes at room temperature, so that the vapour represented the scalar source. The transfer velocity w_T relates the flux out of the canyon to the concentration within it and is shown to be linearly related to windspeed above the street. The dimensionless transfer coefficient w_T/U represents the ventilation efficiency of the canyon (here, w_T is a transfer velocity,U is the wind speed at the boundary-layer top). Observed values are between 1.5 and 2.7 ×10^-3 and, for the case where H/W0 (ratio of buildingheight to street width), values are in the same range as estimates of transfer from a flat plate, giving confidence that the technique yields accurate values for street canyon scalar transfer. w_T/U varies with aspect ratio (H/W), reaching a maximum in the wake interference regime (0.3 < H/W < 0.65). However, when upstream roughness is increased, the maximum in w_T/U reduces, suggesting that street ventilation is less sensitive to H/W when the flow is in equilibrium with the urban surface. The results suggest that using naphthalene sublimation with wind-tunnel models of urban surfaces can provide a direct measure of area-averaged scalar fluxes.  相似文献   

The effect of distribution of iron-oxyhydroxide grain coatings on the transport of bacterial cells in porous media   总被引：3，自引：0，他引：3  

E. P. Knapp  Janet S. Herman  George M. Hornberger  Aaron L. Mills 《Environmental Geology》1998,33(4):243-248

 Among the demonstrated processes influencing the transport of bacteria through aquifers, the deposition of cells on mineral surfaces is one of the most important. For example, understanding the transport of introduced bacteria through aquifers is essential to designing some in situ bioremediation schemes. The impact of the presence and distribution of Fe(III)-oxyhydroxide-coated sand grains on bacterial transport through porous media was evaluated in column experiments in which bacteria (short rods; 1.2 μm length) were eluted through columns of quartz sand (0.5–0.6 mm in diameter) for several conditions of chemical heterogeneity of mineral substrate. Fe(III)-oxyhydroxide-coated sand was present as 10% of the mass, and it was arranged in three treatments: (1) homogeneously distributed, and present as a discrete layer (2) at the top and (3) at the bottom of 14-cm-long sand columns. A pulse input of 10⁸ cells ml^–1 was introduced in an artificial groundwater solution flowing at 14 cm h^–1 through the column, and eluted cells were counted. Peak breakthrough occurred at 1.0 pore volume. A large proportion of cells were retained; 14.7–15.8% of the cells were recovered after three pore volumes of solution had eluted through clean quartz sand, and only 2.1–4.0% were recovered from the Fe(III)-oxyhydroxide-coated sand mixtures. The three physical arrangements of the chemical heterogeneity resulted in essentially the same breakthrough of cells, indicating that the spatial distribution of iron coating does not affect the transport of bacteria. The results of the column transport experiments, which mimic hydrogeological conditions encountered in field problems, are consistent with our mechanistic understanding of bacterial sorption. Received: 10 April 1996 · Accepted: 17 February 1997  相似文献   

The perturbed structure of the neutral atmospheric boundary layer over irregular terrain     

R. Michael Jones  W. H. Hooke 《Boundary-Layer Meteorology》1986,36(4):395-416

The differential equations for first-order (linear) response of the planetary boundary layer are formulated for flow over periodic terrain, for variations in both surface roughness and terrain elevation. A simple second-order closure model of the turbulence is used, and Coriolis forces are neglected. Flow over a periodic terrain produces corresponding periodic structure in all meteorological fields above the surface. The periodic structure consists of two components. The first is very nearly evanescent with height. It corresponds to the motion that would be observed were the atmosphere inviscid. The second component, introduced by turbulent viscosity, exhibits a phase variation with height in addition to a decay in amplitude. W.K.B. [Wentzel-Kramers-Brillouin] approximations for the two components are developed, and the coupling between the components is discussed. The formulation for calculating solutions by numerical integration is developed, including specification of appropriate boundary conditions. Calculations are presented in a companion paper.Wave Propagation Laboratory.Environmental Science Group.  相似文献   

The geochemical environment     

Janet S. Herman 《Engineering Geology》1989,26(4):301-317

Much of the assessment of high-level radioactive waste repositories will be accomplished through the application of geochemical models. Such models will be used to understand geochemical processes in the repository and the surrounding environment. The purpose of this paper is to review the nature of the currently available geochemical models. The kinds of calculations that a number of these models perform include aqueous speciation, mineral solubility, and mass transfer reactions. Presently, there are limitations on the success of calculation of adsorption reactions and rate-limited reactions. Ongoing development of geochemical models requires that more work be done to compile thermodynamic and kinetic data, incorporate additional geochemical processes and parameters into existing codes, and assess the accuracy of all simulations through validation studies and sensitivity analyses.  相似文献   

Spitzer observations of M83 and the hot star, H ii region connection     

Robert H. Rubin  Janet P. Simpson  Sean W. J. Colgan  Reginald J. Dufour  Katherine L. Ray  Edwin F. Erickson  Michael R. Haas  Adalbert W. A. Pauldrach  Robert I. Citron 《Monthly notices of the Royal Astronomical Society》2007,377(4):1407-1418

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Spiral waves and the secondary star in the nova-like variable V3885 Sgr     

Louise E. Hartley  James R. Murray  Janet E. Drew  Knox S. Long 《Monthly notices of the Royal Astronomical Society》2005,363(1):285-292

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Women and seed management: A study of two villages in Bangladesh     

Emily Oakley  Janet Henshall Momsen 《Singapore journal of tropical geography》2007,28(1):90-106

Seed management has until recently been neglected in agrobiodiversity research. Gender dimensions in seed selection, processing, storage and exchange embody critical task areas in the human management of diverse crop species and varieties. This paper focuses on women's roles in seed management in field and homegarden crops in Bangladesh and reports on the findings of a study in two villages in Tangail District. Seventy-five women were surveyed with a questionnaire and subsets interviewed in-depth; focus groups conducted for women and men provided a means of cross-checking responses. Three distinct areas of agrobiodiversity were addressed: the gendered divisions of labour among agricultural tasks; the pervasiveness of seed saving among respondents; and seed management practices and techniques. The results indicate that while men have greater authority over field activities, homegarden production is the exclusive domain of women. Seed management is seen as an extension of women's domestic duties: women are responsible for all seed processing, storage and exchange for field as well as homegarden crops. The vast majority of seeds sown are saved on-farm by women, revealing important cultural, economic and environmental implications for agrobiodiversity conservation and local differences.  相似文献   

Isotopic and Elemental Imaging of Geological Materials by Laser Ablation Inductively Coupled Plasma-Mass Spectrometry   总被引：4，自引：0，他引：4  

Jon D. Woodhead  John Hellstrom  Janet M. Hergt  Alan Greig  Roland Maas 《Geostandards and Geoanalytical Research》2007,31(4):331-343

Laser ablation ICP-MS represents a promising new development for the acquisition of elemental and isotopic images from a variety of different materials. Compared to existing methods for imaging, it offers relatively rapid throughput, very wide dynamic range, a relatively clean mass spectrum, utility at a variety of scales (from μm to cm) and multi-element/isotopic capability. Although developing rapidly in the biological sciences, the method has not yet seen widespread application to geological materials. This contribution documents some preliminary experiments aimed at understanding the fundamental aspects of elemental and isotopic image acquisition using laser ablation ICP-MS. In particular, we note that ablation cell designs must be optimised to promote rapid system response, in contrast to the signal-smoothing that is often preferred for simple spot analyses. Furthermore, experimental parameters must be carefully evaluated on a system-by-system basis to avoid the blurring effects of re-sampling phenomena. With careful attention to these details, and development of appropriate data processing software, laser ablation ICP-MS imaging has much to offer workers in the Earth and environmental sciences.  相似文献   

Strontium Isotope Analysis of Kimberlitic Groundmass Perovskite via LA-MC-ICP-MS   总被引：1，自引：0，他引：1  

Chad Paton  Jon D. Woodhead  Janet M. Hergt  David Phillips  Simon Shee 《Geostandards and Geoanalytical Research》2007,31(4):321-330

Strontium isotopic composition is a potentially powerful tracer in studies of kimberlitic rocks but the results from even the most carefully collected and stringently prepared bulk-rock samples are still hampered by contamination and alteration effects. Here we describe a LA-MC-ICP-MS technique which can obtain accurate, high precision Sr_i ratios from 50–150 μm kimberlitic groundmass perovskite without requiring time-consuming mineral separation procedures. Since perovskite is a robust magmatic phase with an extremely low Rb/Sr ratio, the effects of late-stage crustal contamination, post-emplacement alteration and age correction are minimised and results are more representative of primary melt compositions, while additionally preserving powerful grain-scale spatial and textural information. We demonstrate that the adopted protocol overcomes isobaric interferences from Kr⁺, Rb⁺, Er²⁺ and Yb²⁺, and that Ca dimers and Ca argides do not detectably affect the quality of ⁸⁷Sr/⁸⁶Sr ratios produced. To illustrate the utility of the technique, contrasting bulk-rock and in situ perovskite results from eleven Proterozoic kimberlites are documented.  相似文献   

Arsenic sequestration by sorption processes in high-iron sediments   总被引：3，自引：0，他引：3  

Robert A. Root  Kate M. Campbell  Janet G. Hering 《Geochimica et cosmochimica acta》2007,71(23):5782-5803

High-iron sediments in North Haiwee Reservoir (Olancha, CA), resulting from water treatment for removal of elevated dissolved arsenic in the Los Angeles Aqueduct system, were studied to examine arsenic partitioning between solid phases and porewaters undergoing shallow burial. To reduce arsenic in drinking water supplies, ferric chloride and a cationic polymer coagulant are added to the aqueduct upstream of Haiwee Reservoir, forming an iron-rich floc that scavenges arsenic from the water. Analysis by synchrotron X-ray absorption spectroscopy (XAS) showed that the aqueduct precipitate is an amorphous hydrous ferric oxide (HFO) similar to ferrihydrite, and that arsenic is associated with the floc as adsorbed and/or coprecipitated As(V). Arsenic-rich floc and sediments are deposited along the inlet channel as aqueduct waters enter the reservoir. Sediment core samples were collected in two consecutive years from the edge of the reservoir along the inlet channel using 30- or 90-cm push cores. Cores were analyzed for total and extractable arsenic and iron concentrations. Arsenic and iron speciation and mineralogy in sediments were examined at selected depths by synchrotron XAS and X-ray diffraction (XRD). Sediment-porewater measurements were made adjacent to the core sample sites using polyacrylamide gel probe samplers. Results showed that sediment As(V) is reduced to As(III) in all cores at or near the sediment-water interface (0-4 cm), and only As(III) was observed in deeper sediments. Analyses of EXAFS spectra indicated that arsenic is present in the sediments mostly as a bidentate-binuclear, inner-sphere sorption complex with local atomic geometries similar to those found in laboratory studies. Below about 10 cm depth, XAS indicated that the HFO floc had been reduced to a mixed Fe(II, III) solid with a local structure similar to that of synthetic green rust (GR) but with a slightly contracted average interatomic Fe-Fe distance in the hydroxide layer. There was no evidence from XRD for the formation of a crystalline GR phase. The release of dissolved iron (presumably Fe²⁺) and arsenic to solution, as monitored by in situ gel probes, was variable but, in general, occurred at greater depths than arsenic reduction in the sediments by spectroscopic observations and appears to be near or below the depth at which sediment GR was identified. These data point to reductive dissolution of the sorbent iron phase as the primary mechanism of release of sorbed arsenic to solution.  相似文献