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  • Articles  (22)
  • modeling
  • Springer  (22)
  • National Academy of Sciences
  • 1995-1999  (20)
  • 1975-1979  (2)
  • Geosciences  (15)
  • Natural Sciences in General  (4)
  • Medicine  (3)
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  • Articles  (22)
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  • Springer  (22)
  • National Academy of Sciences
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  • 1
    Electronic Resource
    Electronic Resource
    Monitoring and modeling of coastal waters in support of environmental preservation (1996)
    Springer
    Journal of marine science and technology 1 (1996), S. 75-84 
    Details
    ISSN: 1437-8213
    Keywords: monitoring ; modeling ; environmental preservation ; navigation ; nazardous spills ; tidal prediction
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Technology
    Notes: Abstract The improved monitoring and modeling capability resulting from recent technological advances in oceanographic sensors, computer processing power, and telecommunications can play a major role in environmental preservation. In particular, this capability can help improve: safe navigation and thus the prevention of maritime accidents that lead to hazardous spills; the effective cleanup of hazardous spills when they do occur; the real-time assessment of water quality problems; the assessment of long-term trends and variability due to both anthropogenic and climate change effects; and the understanding of key physical, chemical, and ecological processes.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02391163
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  • 2
    Electronic Resource
    Electronic Resource
    Electromagnetic Subsurface Imaging. A Case for an Adaptive Born Approximation (1997)
    Springer
    Surveys in geophysics 18 (1997), S. 477-510 
    Details
    ISSN: 1573-0956
    Keywords: electromagnetic ; modeling ; inversion ; imaging ; transient electromagnetic method
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract The process of interpretation of electromagnetic data has many facets of which fast approximate interpretation techniques is an intriguing one. A new variant of the Born approximation – the Adaptive Born approximation – is presented and exemplified through 1D and 2D imaging techniques for transient electromagnetic data. The Adaptive Born approximation is generally applicable in approximate inversion schemes for inductive electromagnetic data as a one-pass imaging algorithm. Though it is as simple to use as the ordinary Born approximation, it offers a more accurate inverse mapping. In the first part of this paper an attempt will be made to give an overview of fundamental concepts in electromagnetic subsurface imaging relevant for approximate inverse mappings and to outline major trends in present day modeling and inversion of electromagnetic data. This is of course an impossible task – certainly for this author – and much important work will not be mentioned in the limited space of the following. My apologies to the people who are not mentioned and whose research is not given credit here though it should have been. Naturally, my choice of references reflects the “schools” and circles I have been subjected to, but I hope that the list of references to developments in electromagnetic methods will point to papers of importance and thereby to other references for the interested reader.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1006593408478
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  • 3
    Electronic Resource
    Electronic Resource
    Hydrologic Response of a Wetland to Changing Moisture Conditions: Modeling Effects of Soil Heterogeneity (1998)
    Springer
    Climatic change 40 (1998), S. 211-227 
    Details
    ISSN: 1573-1480
    Keywords: wetlands ; hydrology ; modeling ; piezocone
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Prediction of the effects of external influences such as climate change on wetland systems requires the prediction of hydrologic effects. Because wetland soils are typically heterogeneous, it is particularly important to understand the extent and connectedness of hydraulically conductive soil units, since water flow may be concentrated in such units while bypassing others of lower conductivity. However, subsurface hydrologic models typically do not represent heterogeneity adequately, being limited by sparse parameterization of soil properties. Conventional techniques for mapping units of soil within wetlands are highly laborious, requiring soil coring and laboratory testing. As an alternative, we developed a portable piezocone driver and highly sensitive piezocone designed to map wetland soil units with centimeter-scale resolution in the vertical and meter-scale resolution in the horizontal dimension. This system successfully delineated several different layers of peat, sand, and limnetic sediments, and their degree of interconnectedness in an eight-meter-thick peat deposit. Monitoring of wetland response to precipitation, changes in stream stage, and overbank flooding was then used in conjunction with the piezocone data and a two-dimensional flow model to constrain the hydraulic properties of the soil units. Thus parameterized, a standard subsurface flow model was able to realistically simulate a variety of hydrologic processes relevant to climate change, including wetland-stream water exchange, the movement of wetland porewaters to the root zone of plants, and wetland desaturation under dry conditions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1005404617480
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  • 4
    Electronic Resource
    Electronic Resource
    Hybridomas in a bioreactor cascade: modeling and determination of growth and death kinetics (1996)
    Springer
    Cytotechnology 21 (1996), S. 263-277 
    Details
    ISSN: 1573-0778
    Keywords: hybridomas ; serum-free medium ; monoclonal antibodies ; reactor series ; kinetics ; modeling
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Medicine , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Abstract Hybridomas were cultured under steady-state conditions in a series of two continuous stirred-tank reactors (CSTRs), using a serum-free medium. The substrate not completely converted in the first CSTR, was transported with the cells to the second one and very low growth rates, high death rates, and lysis of viable cells were observed in this second CSTR. These conditions are hardly accessible in a single vessel, because such experiments would be extremely time-consuming and unstable due to a low viability. In contrast to what is often observed in literature, kinetic parameters could thus be derived without the neccessity for extrapolation to lower growth rates. Good agreement with literature averages for other hybridomas was found. Furthermore, showing that the reactor series is a valuable research tool for kinetic studies under extreme conditions, the possibility to observe cell death under stable and defined steady-state conditions offers interesting opportunities to investigate apoptosis and necrosis. Additionally, a model was developed that describes hybridoma growth and monoclonal antibody production in the bioreactor cascade on the basis of glutamine metabolism. Good agreement between the model and the experiments was found.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00365349
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  • 5
    Electronic Resource
    Electronic Resource
    Mineral Favorability Mapping: A Comparison of Artificial Neural Networks, Logistic Regression, and Discriminant Analysis (1999)
    Springer
    Natural resources research 8 (1999), S. 93-109 
    Details
    ISSN: 1573-8981
    Keywords: Mineral exploration ; multivariate statistics ; modeling ; decision theory
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract A Probabilistic Neural Network (PNN) was trained to classify mineralized and nonmineralized cells using eight geological, geochemical, and geophysical variables. When applied to a second (validation) set of well-explored cells that had been excluded from the training set, the trained PNN generalized well, giving true positive percentages of 86.7 and 93.8 for the mineralized and nonmineralized cells, respectively. All artifical neural networks and statistical models were analyzed and compared by the percentages of mineralized cells and barren cells that would be retained and rejected correctly respectively, for specified cutoff probabilities for mineralization. For example, a cutoff probability for mineralization of 0.5 applied to the PNN probabilities would have retained correctly 87.66% of the mineralized cells and correctly rejected 93.25% of the barren cells of the validation set. Nonparametric discriminant analysis, based upon the Epanechnikov Kernel performed better than logistic regression or parametric discriminant analysis. Moreover, it generalized well to the validation set of well-explored cells, particularly to those cells that were mineralized. However, PNN performed better overall than nonparametric discriminant analysis in that it achieved higher percentages of correct retention and correct rejection of mineralized and barren cells, respectively. Although the generalized regression neural network (GRNN) is not designed for a binary—presence or absence of mineralization— dependent variable, it also performed well in mapping favorability by an index valued on the interval [0, 1]. However, PNN outperformed GRNN in correctly retaining mineralized cells and rejecting barren cells of the validation set.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1021886501912
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  • 6
    Electronic Resource
    Electronic Resource
    Thinking in Levels: A Dynamic Systems Approach to Making Sense of the World (1999)
    Springer
    Journal of science education and technology 8 (1999), S. 3-19 
    Details
    ISSN: 1573-1839
    Keywords: levels ; complexity ; simulation ; modeling ; science education ; mathematics education ; dynamic systems ; systems thinking
    Source: Springer Online Journal Archives 1860-2000
    Topics: Natural Sciences in General , Technology
    Notes: Abstract The concept of emergent "levels" (i.e., levels that arise from interactions of objects at lower levels) is fundamental to scientific theory. In this paper, we argue for an expanded role for this concept of levels in science education. We show confusion of levels (and "slippage" between levels) as the source of many of people's deep misunderstandings about patterns and phenomena in the world. These misunderstandings are evidenced not only in students' difficulties in the formal study of science but also in their misconceptions about experiences in their everyday lives. The StarLogo modeling language is designed as a medium for students to build models of multi-leveled phenomena and through these constructions explore the concept of levels. We describe several case studies of students working in StarLogo. The cases illustrate students' difficulties with the concept of levels, and how they can begin to develop richer understandings.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1009421303064
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  • 7
    Electronic Resource
    Electronic Resource
    Part I: Toward a System of Educational Engineering for Traditional Class Elements: A Case Study in an Introductory Physics Course (1997)
    Springer
    Journal of science education and technology 6 (1997), S. 173-191 
    Details
    ISSN: 1573-1839
    Keywords: Science education ; educational engineering ; modeling
    Source: Springer Online Journal Archives 1860-2000
    Topics: Natural Sciences in General , Technology
    Notes: Abstract In this paper, we present a system for formally characterizing elements of an introductory science class, measuring the performance of a class based on this characterization, and modeling the value of the class based on the measurements. This system allows the iterative improvement of any educational presentation through a model, test, iterate cycle. We propose formal practices involved in iteratively improving an educational experience be called educational engineering.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1022575606713
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  • 8
    Electronic Resource
    Electronic Resource
    Modeling Microbial Transport and Biodegradation in a Dual-porosity System (1999)
    Springer
    Transport in porous media 35 (1999), S. 49-65 
    Details
    ISSN: 1573-1634
    Keywords: modeling ; biodegradation ; microbial transport ; dual-porosity ; kinetics.
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Technology
    Notes: Abstract A mathematical model describing microbial transport and growth in a heterogeneous aquifer domain, composed of overlapping subdomains of high-permeability and low-permeability materials, is developed. Each material is conceptually visualized as a continuum which occupies the entire considered spatial aquifer domain. Based on the assumption that advection in the low-permeability domain is negligible, the mathematical model is solved by using a publically available reactive transport code. The importance of modeling microbial transport and growth in such a dual-porosity system is demonstrated through a hypothetical case study.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1006551627151
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  • 9
    Electronic Resource
    Electronic Resource
    Modeling primary substrate controlled biotransformation and transport of halogenated aliphatics in porous media (1996)
    Springer
    Transport in porous media 24 (1996), S. 203-220 
    Details
    ISSN: 1573-1634
    Keywords: biotransformation ; halogenated solvents ; anaerobic processes ; modeling ; cometabolism
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Technology
    Notes: Abstract In situ biorestoration is a groundwater remediation technique in which the indigenous aquifer bacteria are stimulated by injecting compounds to provide carbon source and energy. Stimulated bacteria may transform the target contaminants such as tetrachloroethylene (PCE) and trichloroethylene (TCE) into intermediate products. In this study, we developed a model to simulate the substrate-limited biotransformation of the halogenated solvents present in anoxic groundwater by sequential reductive dehalogenation under methanogenic conditions. The model consists of conservation of mass equations for the primary substrate, immobile indigenous biomass, organic solvents such as PCE and TCE, and their intermediate products trichloroethylene, dichloroethylene, and vinyl chloride. The utilization of primary substrate and the biotransformation of organic solvents are assumed to follow Monod kinetics. The limiting factor on bacterial growth is assumed to be the primary substrate. The microbial yield coefficient is determined from the stoichiometric equation describing the anaerobic process. The model is solved by using a finite difference technique. Results are presented for three different case studies: continuous injection of primary substrate (acetate), single-pulse injection, and double-pulse injection. The single-pulse or double-pulse injection techniques were found to be more effective than continuous injection of primary substrate. Double-pulse technique reduces the clogging of injection wells caused by excessive microbial growth around boreholes and achieves a more uniform distribution of microbial growth in the subsurface. In all cases target compounds were effectively removed. The results, however, indicate substantial levels of intermediate product accumulation. Numerical results of a simplified model which assumes an abundance of primary substrate and a constant population of biomass, compare favorably with experimental data reported in the literature.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00139845
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  • 10
    Electronic Resource
    Electronic Resource
    A statistical approach to modeling transport of pollutants in ground water (1978)
    Springer
    Mathematical geology 10 (1978), S. 657-672 
    Details
    ISSN: 1573-8868
    Keywords: ground water ; pollutant transport ; modeling
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Mathematics
    Notes: Abstract The transport of pollutants in the subsurface can be affected by random geologic events. Prediction of such transport therefore requires the solution of a partial differential equation whose coefficients are random processes. A method of finding the expected (mean) values of solutions of such equations is derived. This method is used to assess the impact of fault movement and formation of breccia pipes on risk from radioactive waste disposal. Preliminary results indicate that these events, considered probabilistically, do not make a large contribution to risk.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01031897
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  • 11
    Electronic Resource
    Electronic Resource
    Inversion of dynamical indicators in quantitative basin analysis models. III. Multiwell information and two-dimensional case histories (1995)
    Springer
    Mathematical geology 27 (1995), S. 41-68 
    Details
    ISSN: 1573-8868
    Keywords: inversion methods ; basin analysis ; modeling ; Navarin Basin (Alaska) ; tomographic parameters ; GEOPETII
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Mathematics
    Notes: Abstract A dynamical tomography method, which inverts dynamical indicators to evaluate the parameters controlling geological processes as well as those in intrinsic equations of state, was introduced into a 2D fluid flow/compaction model termed GEOPETII (developed at the University of South Carolina), with the assumption of invariance to spatial location of parameters in equations of state, but allowing geologic process parameters to change with well location. Synthetic tests, including sensitivity analysis, are given to illustrate the operation of the system. The nonlinear inverse two-dimensional tomography method, together with a systematic linear search procedure, provides a useful approach to determine and constrain the parameters entering quantitative models of dynamical sedimentary evolution. Applying the method to an interpreted section from a seismic line in the Navarin Basin. Bering Sea. Alaska, the predictions of present-day formation thicknesses, porosity, and fluid pressure with depth are improved at four controlling well locations (Amoco Mishu No. 1, Exxon Redwood 1, Exxon Redwood 2, and Amoco Danielle), relative to previous results which used only a forward model. In this way the geohistory and structural development of the basin can be defined better, which helps in the reconstruction of thermal history, and so of hydrocarbon generation, migration, and accumulation histories in relation to structural and stratigraphic development.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02083567
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  • 12
    Electronic Resource
    Electronic Resource
    The geological implications of steady-state mechanisms in catastrophe theory (1976)
    Springer
    Mathematical geology 8 (1976), S. 657-662 
    Details
    ISSN: 1573-8868
    Keywords: mathematics ; topology ; modeling ; catastrophe theory
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Mathematics
    Notes: Abstract A brief introduction to catastrophe theory is presented, within the context of geological application, and a fundamental problem with a critical axiom of the theory noted. Implications for the application of this theory to modeling geological processes are noted and a solution to the problem proposed. The new approach is examined with reference to a model for sediment transport on the continental slope.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01031096
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  • 13
    Electronic Resource
    Electronic Resource
    Rhizospheric Processes Influencing the Biogeochemistry of Forest Ecosystems (1998)
    Springer
    Biogeochemistry 42 (1998), S. 107-120 
    Details
    ISSN: 1573-515X
    Keywords: acidity ; ecosystem perturbation ; fine roots ; forest soils ; modeling ; mycorrhizae ; nutrient acquisition ; rhizosphere ; soil sampling ; weathering
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Geosciences
    Notes: Abstract In the rhizosphere, biotic and abiotic processes interact to create a zone distinct from the bulk soil that may strongly influence the biogeochemistry of forest ecosystems. This paper presents a conceptual model based upon three operationally defined soil-root compartments (bulk soil, rhizosphere and soil-root interface) to assess nutrient availability in the mineral soil-root system. The model is supported by chemical and mineralogical analyses from bulk and rhizosphere soils collected from a Norway spruce forest. The rhizosphere was more intensively weathered and had accumulated more acidity, base cations and phosphorus than the bulk soil. The quantity and quality of organic matter regulate the reciprocal relationships between soil and roots with their associated biota. However, the biogeochemical role of organic matter in the rhizosphere still remains as an area in which more future research is needed. The mechanisms that may regulate nutrient availability in the rhizosphere are also discussed and related to nutrient cycling and adaptation of forests growing under nutrient poor or perturbed conditions. We suggest that the rhizosphere is not an ephemeral environment in the soil, but persists over time and is resilient against perturbation as evinced by consistent differences between rhizosphere and bulk chemistry and mineralogy over wide range of field treatments.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1005967203053
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  • 14
    Electronic Resource
    Electronic Resource
    The use of system identification techniques in the analysis of oculomotor burst neuron spike train dynamics (1996)
    Springer
    Journal of computational neuroscience 3 (1996), S. 347-368 
    Details
    ISSN: 1573-6873
    Keywords: oculomotor ; burst neurons ; system identification ; saccade ; modeling
    Source: Springer Online Journal Archives 1860-2000
    Topics: Computer Science , Medicine , Physics
    Notes: Abstract The objective of system identification methods is to construct a mathematical model of a dynamical system in order to describe adequately the input-output relationship observed in that system. Over the past several decades, mathematical models have been employed frequently in the oculomotor field, and their use has contributed greatly to our understanding of how information flows through the implicated brain regions. However, the existing analyses of oculomotor neural discharges have not taken advantage of the power of optimization algorithms that have been developed for system identification purposes. In this article, we employ these techniques to specifically investigate the “burst generator” in the brainstem that drives saccadic eye movements. The discharge characteristics of a specific class of neurons, inhibitory burst neurons (IBNs) that project monosynaptically to ocular motoneurons, are examined. The discharges of IBNs are analyzed using different linear and nonlinear equations that express a neuron's firing frequency and history (i.e., the derivative of frequency), in terms of quantities that describe a saccade trajectory, such as eye position, velocity, and acceleration. The variance accounted for by each equation can be compared to choose the optimal model. The methods we present allow optimization across multiple saccade trajectories simultaneously. We are able to investigate objectively how well a specific equation predicts a neuron's discharge pattern as well as whether increasing the complexity of a model is justifiable. In addition, we demonstrate that these techniques can be used both to provide an objective estimate of a neuron's dynamic latency and to test whether a neuron's initial firing rate (expressed as an initial condition) is a function of a quantity describing a saccade trajectory (such as initial eye position).
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00161093
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  • 15
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    Some Considerations on Modeling Heat and Mass Transfer in Porous Media (1997)
    Springer
    Transport in porous media 28 (1997), S. 233-251 
    Details
    ISSN: 1573-1634
    Keywords: capillary pressure ; conservation equations ; constitutive equations ; liquid water ; modeling ; thermodynamic equilibrium
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Technology
    Notes: Abstract In this paper some considerations are presented about the equations needed to set up a model of the process of heat and mass transfer in porous media. A clear classification is made of the various types of equations used and of their physical meaning. Special attention is paid to the thermodynamic equilibrium equations and to their derivation since they are too often taken for granted. The importance of the various transport mechanisms (of mass and energy) is analyzed and the consequences that can arise when some term is neglected are indicated.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1006525729566
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  • 16
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    Single Phase Flow in Partially Fissured Media (1997)
    Springer
    Transport in porous media 28 (1997), S. 285-306 
    Details
    ISSN: 1573-1634
    Keywords: fissured media ; homogenization ; dual porosity ; modeling ; microstructure ; porous media
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Technology
    Notes: Abstract Totally fissured media in which the individual cells are isolated by the fissure system are effectively described by double porosity models with microstructure. Such models contain the geometry of the individual cells in the medium and the flux across their interface with the fissure system which surrounds them. We extend these results to a dual-permeability model which accounts for the secondary flux arising from direct cell-to-cell diffusion within the solid matrix. Homogenization techniques are used to construct a new macroscopic model for the flow of a single phase compressible fluid through a partially fissured medium from an exact but highly singular microscopic model, and it is shown that this macroscopic model is mathematically well posed. Preliminary numerical experiments illustrate differences in the behaviour of solutions to the partially fissured from that of the totally fissured case.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1006562120265
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  • 17
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    Part II: A Computationally Based Modeling System for Class Elements Using Formal Observer-Based Experimental Connections (1997)
    Springer
    Journal of science education and technology 6 (1997), S. 193-211 
    Details
    ISSN: 1573-1839
    Keywords: Science education ; educational engineering ; modeling
    Source: Springer Online Journal Archives 1860-2000
    Topics: Natural Sciences in General , Technology
    Notes: Abstract This article extends and refines the modeling system presented previously (Stewart, 1997). The initial system was sufficient for the optimization of delivery of education at a departmental level. This system is greatly more powerful, precise, and scientific, and fulfills the role of a modeling system for the research and development of educational practices. The model is applied to two widely diverse educational processes, Student Actions and Do Homework Problem, establishing the formalism and demonstrating its usefulness. The use of a rigorous computational syntax imposes completeness criteria on the modeling itself and uniformity. Experimental definition of the formation process of the patterns allows anyone to introduce new features of a model. This and the uniformity allows the models to become the property of the education community, not merely a single researcher, in the same way that mathematical models allow scientists to utilize and build upon previous research.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1022527723551
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  • 18
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    Optimally Engineering Traditional Introductory Physics Classes (1997)
    Springer
    Journal of science education and technology 6 (1997), S. 297-314 
    Details
    ISSN: 1573-1839
    Keywords: Science education ; educational engineering ; modeling
    Source: Springer Online Journal Archives 1860-2000
    Topics: Natural Sciences in General , Technology
    Notes: Abstract This paper presents a measurement of the time and resources committed to traditional student actions such as reading and working homework. The perception of the educational value of each basic action for both students and faculty is captured. From this information, basic educational efficiencies are computed for a traditional mechanics course and a non-traditional hands-on Electricity and Magnetism course. The calculations show an allocation of resources in the traditional course which uses the most student time in the least educationally valuable activity. The computed efficiencies also show overseen student note-taking as potentially a very valuable general tool. The techniques presented allow any institution to carry out quantitative educational engineering of their course offerings at the highest level.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1022502211969
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  • 19
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    An Investigation of CIO Photchemistry in the Chemically Perturbed Arctic Vortex (1999)
    Springer
    Journal of atmospheric chemistry 32 (1999), S. 61-81 
    Details
    ISSN: 1573-0662
    Keywords: chlorine monoxide ; photochemistry ; modeling ; partitioning
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Geosciences
    Notes: Abstract A new lightweight in situ instrument designed to measure ClO was flown on a balloon launched into the arctic vortex at dawn on February 3, 1995 at Kiruna, Sweden during the Second European Stratospheric Arctic and Mid-latitude Experiment (SESAME), together with instruments to measure ozone and long-lived tracers. Observations on ascent and descent at different solar zenith angles are compared to results from Lagrangian and box model calculations that assume the airmasses at similar potential temperatures had comparable photochemical histories. Between 20 and 22 km, in a region where ClO was significantly enhanced, a model constrained by currently recommended rate parameters significantly underestimates the abundances of ClO that were observed on ascent at high solar zenith angles, whereas the agreement is much better if a smaller ClO-Cl2O2 equilibrium constant, one inferred from previous ER-2 aircraft observations of ClO in the Arctic during nighttime, is assumed. On ascent, ClO is additionally enhanced in a narrow region between 20 and 21 km. We believe the most plausible explanation for this feature is rapid photolysis of OClO produced by the slow bimolecular reaction ClO + ClO over the 48 hours prior to the observations when the airmass was warmed to 225 K by adiabatic compression while in polar darkness. These results suggest that under special circumstances, OClO can be produced by a reaction other than one involving BrO, and, hence, OClO is not necessarily a universal proxy for BrO abundances in the perturbed polar vortex.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1006136712267
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  • 20
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    Iodine Chemistry and its Role in Halogen Activation and Ozone Loss in the Marine Boundary Layer: A Model Study (1999)
    Springer
    Journal of atmospheric chemistry 32 (1999), S. 375-395 
    Details
    ISSN: 1573-0662
    Keywords: aerosol ; iodine chemistry ; halogen chemistry ; marine boundary layer ; modeling ; ozone loss ; sea salt
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Geosciences
    Notes: Abstract A detailed set of reactions treating the gas and aqueous phase chemistry of the most important iodine species in the marine boundary layer (MBL) has been added to a box model which describes Br and Cl chemistry in the MBL. While Br and Cl originate from seasalt, the I compounds are largely derived photochemically from several biogenic alkyl iodides, in particular CH2I2, CH2ClI, C2H5I, C3H7I, or CH3I which are released from the sea. Their photodissociation produces some inorganic iodine gases which can rapidly react in the gas and aqueous phase with other halogen compounds. Scavenging of the iodine species HI, HOI, INO2, and IONO2 by aerosol particles is not a permanent sink as assumed in previous modeling studies. Aqueous-phase chemical reactions can produce the compounds IBr, ICl, and I2, which will be released back into the gas phase due to their low solubility. Our study, although highly theoretical, suggests that almost all particulate iodine is in the chemical form of IO-3. Other aqueous-phase species are only temporary reservoirs and can be re-activated to yield gas phase iodine. Assuming release rates of the organic iodine compounds which yield atmospheric concentrations similar to some measurements, we calculate significant concentrations of reactive halogen gases. The addition of iodine chemistry to our reaction scheme has the effect of accelerating photochemical Br and Cl release from the seasalt. This causes an enhancement in ozone destruction rates in the MBL over that arising from the well established reactions O(1D) + H2O → 2OH, HO2 + O3 → OH + 2O2, and OH + O3 → HO2 + O2. The given reaction scheme accounts for the formation of particulate iodine which is preferably accumulated in the smaller sulfate aerosol particles.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1006179901037
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  • 21
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    Electronic Resource
    Modeling the temperature response of nitrification (1996)
    Springer
    Biogeochemistry 35 (1996), S. 433-445 
    Details
    ISSN: 1573-515X
    Keywords: ammonium oxidation ; growth rate ; maintenance energy ; modeling ; nitrification ; soil nitrate ; temperature
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Geosciences
    Notes: Abstract To model nitrification rates in soils, it is necessary to have equations that accurately describe the effect of environmental variables on nitrification rates. A variety of equations have been used previously to describe the effect of temperature on rates of microbial processes. It is not clear which of these best describes the influence of temperature on nitrification rates in soil. I compared five equations for describing the effects of temperature on nitrification in two soils with very different temperature optima from a California oak woodland-annual grassland. The most appropriate equation depended on the range of temperatures being evaluated. A generalized Poisson density function best described temperature effects on nitrification rates in both soils over the range of 5 to 50 °C; however, the Arrhenius equation best described temperature effects over the narrower range of soil temperatures that normally occurs in the ecosystem (5 to 28 °C). Temperature optima for nitrification in most of the soils were greater than even the highest soil temperatures recorded at the sites. A model accounting for increased maintenance energy requirements at higher temperatures demonstrates how net energy production, rather than the gross energy production from nitrification, is maximized during adaptation by nitrifier populations to soil temperatures.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02183035
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  • 22
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    Electronic Resource
    Modeling of phlebothrombosis (1997)
    Springer
    Bulletin of experimental biology and medicine 124 (1997), S. 1151-1153 
    Details
    ISSN: 1573-8221
    Keywords: thrombosis ; modeling ; external jugular vein
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Medicine
    Notes: Abstract A new model of phlebothrombosis has been developed. Thrombotic mass in preparedin vitro by mixing dog's blood with thrombin. Retracted clot is injected transcutaneously in ligated segment of the jugular vein. The veins are examined visually and microscopically 1, 3, 5, 7, 10, and 14 days after injection. It is found that the thrombus is loosely bound to the vascular wall within 8–10 days. Endothelial desquamation starts on the 3rd–5th day.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02445688
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