Hypothetical Layer Research Articles

First tier modeling of consumer dermal exposure to substances in consumer articles under REACH: A quantitative evaluation of the ECETOC TRA for consumers tool

The demonstration of safe use of chemicals in consumer products, as required under REACH, is proposed to follow a tiered process. In the first tier, simple conservative methods and assumptions should be made to quickly verify whether risks for a particular use are expected. The ECETOC TRA Consumer Exposure Tool was developed to assist in first tier risk assessments for substances in consumer products. The ECETOC TRA is not a prioritization tool, but is meant as a first screening. Therefore, the exposure assessment needs to cover all products/articles in a specific category. For the assessment of the dermal exposure for substances in articles, ECETOC TRA uses the concept of a ‘contact layer’, a hypothetical layer that limits the exposure to a substance contained in the product. For each product/article category, ECETOC TRA proposes default values for the thickness of this contact layer. As relevant experimental exposure data is currently lacking, default values are based on expert judgment alone. In this paper it is verified whether this concept meets the requirement of being a conservative exposure evaluation method. This is done by confronting the ECETOC TRA expert judgment based predictions with a mechanistic emission model, based on the well established theory of diffusion of substances in materials. Diffusion models have been applied and tested in many applications of emission modeling. Experimentally determined input data for a number of material and substance combinations are available. The estimated emissions provide information on the range of emissions that could occur in reality. First tier tools such as ECETOC TRA tool are required to cover all products/articles in a category and to provide estimates that are at least as high as is expected on the basis of current scientific knowledge.Since this was not the case, it is concluded that the ECETOC TRA does not provide a proper conservative estimation method for the dermal exposure to articles. An alternative method was proposed.

A Fluidized Bed Technique for Estimating Soil Critical Shear Stress

Soil erosion models, depending on how they are formulated, always have erodibility parameters in the erosion equations. For a process-based model like the Water Erosion Prediction Project (WEPP) model, the erodibility parameters include rill and interrill erodibility and critical shear stress (τcᵣ). These erodibility parameters are commonly determined from rainfall and runoff experiments. Recent developments in soil erosion research suggest that these experimentally determined parameters are affected by field conditions prevailing during the measurement and therefore are not inherent to the soil. The quest for fast and accurate alternatives to rainfall and runoff experiments for estimating soil erodibility parameters is an active research area. In this study, the fluidized bed technique for intrinsic soil cohesion measurement was improved and tested on four soils with various intrinsic soil properties, along with two low-cohesion materials, sand and glass beads. A strong correlation (R² = 0.82) was found between cohesion per unit length C₀ and published τcᵣ values. In addition, when the C₀ values were multiplied by a submillimeter dimension equivalent to a hypothetical layer removed at the onset of erosion, the range of stress that was yielded overlapped that of the published τcᵣ values. This method is a simple alternative to rainfall–runoff experiments, with the added advantage of being independent of extrinsic factors.

Representative Layer Theory for Diffuse Reflectance

The fractions of light absorbed by and remitted from samples consisting of different numbers of plane parallel layers can be related with the use of statistical equations. The fractions of incident light absorbed ( A), remitted ( R), and transmitted ( T) by a sample of any thickness can be related by an absorption/remission function, A( R,T): A( R,T) = [(1 - R)2 - T2]/ R = (2 - A - 2 R) A/ R = 2 A0/ R0. Being independent of sample thickness, this function is a material property in the same sense as is the linear absorption coefficient in transmission spectroscopy. The absorption and remission coefficients for the samples are obtained by extrapolating the measured absorption and remission fractions for real layers to the fraction absorbed ( A0) and remitted ( R0) by a hypothetical layer of infinitesimal thickness. A sample of particulate solids can be modeled as a series of layers, each of which is representative of the sample as a whole. In order for the layer to be representative of the properties of the individual particles of which it is comprised, it should nowhere be more than a single particle thick, and should have the same void fraction as the sample; further, the volume fraction and cross-sectional surface area fraction of each particle type in the layer should be identical to its volume fraction and surface area fraction in the sample as a whole. At lower absorption levels, the contribution of a particle of a particular type to the absorption of a sample is approximately weighted in proportion to its volume fraction, while its contribution to remission is approximately weighted in proportion to the fraction of cross-sectional surface area that the particle type makes up in the representative layer.

Theory for water management in membranes for polymer electrolyte fuel cells

Water management was discussed theoretically for membranes in polymer electrolyte fuel cells in which the polymer electrolyte membranes were contaminated with foreign impurity cations. Water transport in a contaminated two-cation system membrane was considered by assuming an ‘infected zone’ of finite thickness, a hypothetical layer of mixed cations, H+ and the contaminant ions Qn+, stretching from the catalyst layer ∣ membrane interface into the membrane. Solving the flux equation of water with some boundary conditions, the water concentration profile across the membrane was derived in analytical form. Some characteristic variables such as water content, the net water flux and the membrane resistance overvoltage were calculated systematically as functions of several relevant parameters in fuel cell operations. It was discovered theoretically that both the current density and the membrane thickness are vital parameters in the management of water in fuel cell membranes, and this tendency becomes larger when the membrane is contaminated by impurity ions, especially when such ions are localized at the anode ∣ membrane interface. It is noted from the point of view of water management that special caution should be directed in order to protect the membrane from contamination.

Blood flow resistance during hemodilution: effect of plasma composition.

To investigate the causes of wide variations in reported effects of hemodilution on flow resistance of vascular beds. (a) In a meta-analysis of 28 prior studies, resistance values at hematocrits of zero (R0) and 0.45 (R0.45) were derived. Study design characteristics (presence of vasodilatory reserve or leukocytes, species, tissue, hemodiluent) were tested by ANOVA for their relation to the ratio R0/R0.45. (b) Experiments were performed to determine flow resistance during hemodilution in the rat mesentery with (n = 8) and without (n = 11) pretreatment with heparinase, which modifies the endothelial glycocalyx. (c) A mathematical flow simulation for mesenteric microvascular networks was used to predict resistance effects of hemodilution and of a hypothetical layer on the endothelial surface. (a) In prior studies using native plasma for hemodilution R0 averaged 50 +/- 8% of R0.45, while in studies using artificial solutions R0 averaged 32 +/- 12% of R0.45. The larger reduction of flow resistance upon dilution with artificial media is independent of viscosity and oncotic pressure. Other design characteristics did not show strong significant effects. (b) Present experiments showed large reductions of flow resistance with saline hemodilution which were nearly halved after heparinase pretreatment. (c) Resistance effects of hemodilution with plasma or after heparinase treatment agree with model predictions based on tube flow rheology of blood. The larger resistance effects of dilution with artificial media can be explained by the removal of an endothelial surface layer of approximately 1.5 microns thickness. The results imply that changes of plasma composition, due to use of artificial infusion media, influence peripheral resistance and tissue perfusion. They are consistent with the hypothesis that interactions between endothelial glycocalyx structures and plasma components lead to formation of a thick layer at the endothelial surface which increases flow resistance.

A model for the Kolbe reaction of acetate in a parallel-plate reactor

A mathematical model is developed for the study of the Kolbe oxidative dimerization of acetate to ethane and carbon dioxide in a parallel-plate reactor operating at a fixed cell potential, with hydrogen evolution being the cathode reaction. The volume of gas evolved into the interelectrode gap is tracked by constructing a hypothetical gas layer which increases in thickness with the streamwise direction in a manner determined by solution to the model equations; concurrently, the liquid flows in a hypothetical layer which decreases in thickness. The three-component gas phase is assumed to be ideal, and the liquid phase is an aqueous mixture of five species: acetate, proton, sodium and hydroxyl ions and acetic acid. The model predicts the concentration profiles and the streamwise variation of the gas-void fraction, reaction current density and liquid and gas velocities. Gas evolution causes a decreasing current density in the streamwise direction and an increasing gas and liquid velocity. The concentrations of acetic acid and proton decrease in the streamwise direction, while hydroxyl concentration increases; the decrease in acetate concentration, however, is not significant until the local base-to-acid ratio is near unity because of the buffering effect from undissociated acetic acid. The average current density increases with inlet solution velocity and cell potential and asymptotically approaches the secondary current limit. There exists an optimal interelectrode separation where the cell resistance is minimum. The average current density exhibits a shallow maximum with the baseto-acid ratio of the feed, but decreases precipitously when the ratio is near unity due to the rapid decrease in the proton concentration.

Effective Permittivity and Permeability of a Fibers Grating

ABSTRACT A diffraction problem for a grating consisting of thin parallel cylinders (fibers) is considered in the low frequency approximation. Relative permittivity and permeability are supposed to be constant in each fiber. An electric field vector of an incident plane wave is parallel to the fibers. Results of this approximation are compared with the field scattered by a homogeneous hypothetical layer. Thus, its relative effective permittivity (εe ), permeability (µe), refractive index (ηe), and impedance (z e) are determined. It has been shown that such an equivalent material is sometimes characterized by negative values of either Im(εe ) or Im(µe) (with the time dependence e−iωt), and may be interpreted as a medium with an internal generation.

An Alternative Sea Level Pressure Reduction and a Statistical Comparison of Geostrophic Wind Estimates with Observed Surface Winds

Abstract A method for station or grid point reduction of surface pressure to sea level or some other level is presented that shows improvement over the standard reduction method in the western United States. This method (MAPS SLP-Mesoscale Analysis and Prediction System sea level pressure) uses the 700 hPa temperature to estimate an “effective” surface temperature from which the temperature of the hypothetical layer beneath the ground is estimated. The use of this “effective” temperature instead of the observed surface temperature is responsible for the improved reduction since it varies more smoothly over space and time and is more representative of the temperature variation found above the boundary layer. The MAPS SLP reduction was compared with the standard reduction and altimeter setting reduction in statistical comparisons of geostrophic wind estimates with observed winds and in a case study. A 21-month comparison between geostrophic and observed winds was made over different geographical regions, ti...

Insect Migration, Flight Periodicity and the Boundary Layer

In an earlier attempt to find a reference framework to classify the kinds of migratory flight adopted by different kinds of insects, I used the term 'boundary layer' to describe a hypothetical layer of air near the ground within which it was supposed that insects were able to control their movements relative to the ground because their flight speed exceeded wind speed; outside the boundary layer they would, perforce, move downwind (Taylor 1958). Evidence for such a layer was found by analysing existing data on the profile of insect density near the ground, although the sampling techniques then in use were questionable (Taylor 1960). The boundary layer concept has apparently proved useful (see Southwood 1966; Johnson 1969); it has helped interpretation of experimental work, e.g. on insect density in wind shadows (Lewis 1967) and has also provided a basis for classifying migratory flight of aphids (Taylor & Palmer 1972). Diurnal flight periodicity and body size were suggested as major determinants of the migratory potential of insects (Taylor 1958). For example, it was suggested that migration tends to be minimal in the small Nematocera that are behaviourally adapted to fly within a shallow boundary layer at night when atmospheric lift is minimal and, as a consequence, are restricted to travelling the short distances their own powers of flight can sustain. In contrast, the small day-flyers are typified by those aphids that have developed a migratory response in which initial flight upwards towards the light, at times when maximum upward air transport is available, frees them from the limitations of the boundary layer and enables them to enlarge their migratory ambit by using the wind as a vehicle. Large day-flyers, like butterflies, could migrate directionally within their boundary layers, whilst large nocturnal migrants, like moths, might migrate downwind (Lewis & Taylor 1965). Until now, however, despite its common acceptance, the boundary layer has remained more conceptual than experimental; it is a term of convenience with few of its implications investigated. I have, therefore, measured accurately a series of twenty-four vertical density profiles for different insect taxa and examined them in relation to periodicity of flight and body size, to investigate the more obvious deductions from the hypothesis. No other accurately measured low level density x height profiles in exposed situations exist, except those for Oscinella frit L. (Calnaido, French & Taylor 1965), Noctuidae and Geometridae (Taylor & French 1974), and some three-point profiles for Lepidoptera (Taylor & Carter 1961). Service (1971a, b, 1973) has produced profiles, for three groups of Nematocera, inside woodland.

The “Kennelly-Heaviside” Layer

EVERY radio engineer is familiar with what is called the “Heaviside Layer” and its supposed functions. I find, however, that the hypothesis of an electrically conducting stratum in the upper air was clearly enunciated in an article by Prof. A. E, Kennelly, of Harvard University, published in the Electrical World and Engineer of New York on March 15, 1902. The official date of Heaviside's disclosure of his hypothesis is December 19, 1902. About the same date H. Poincare, A. Blondel, and C. E. Guillaume made similar hypotheses. If names are to be attached to this hypothetical layer it should be called, in equity, the “Kennelly-Heaviside” layer, a name which is beginning to be used in America.

The Isothermal Layer of the Atmosphere

MR. DINES makes a happy choice of terms when he writes of the isothermal column (NATURE, January 21, p. 341). Each of the unrejected traces is interpreted to show a more or less isothermal column, and it is by mentally piecing together these columns into a sort of honeycomb that the miscalled isothermal layer is brought into existence. It must not be forgotten that this hypothetical layer has a very uneven floor, and that each cell in the honeycomb has its own particular temperature. This is a complex structure. I certainly think it more feasible to ascribe the sudden and sustained minimum in the temperature curve, which is the gist of this discussion, to some idiosyncrasy or limit to which all the instruments, foreign as well as native, are subject night and day, and on the down as well as the up journey. In your issue of January 21 I referred to the falling density of the air current, upon which current the whole experiment depends. If the trace shows a uniform temperature during the upper 9 kilometres of an ascent, there is no escape from the conclusion that the temperature of the air has steadily fallen to compensate for its tenuity, and if we assume an adequate compression of the hydrogen before the rubber gives way, there is a further compensation required for loss of speed.