Nouvelles synthéses hydrothermales du zircon: substitutions isomorphiques; relation morphologie-milieu de croissance

Zircon has been synthesized from a large number of mineral salts and also from one organic compound of zirconium. Its crystallization requires an acid reactive environment.The obtained crystals have been studied morphologically and the crystallographic forms obtained are linked to the presence of specific ions.The study of substitutions in the zircon network shows that hydro-zircon αZr(SiO₄)_1?x(OH)_4x will form only when fluorine is present. Other varieties of the α and β phases ZrSiO₄ have been obtained, substituting Hf, U, for Zr, and Ge for Si and an immiscible α phase ThSiO₄.Germanium enters the zircon network only at low temperature. Uranium and Hf, but not Th, can substitute for Zr in zircon.     

Photodissociation rates in the atmosphere below 100km

In this survey we consider the atmospheric photodissociation rates of the molecules, O₂, O₃, NO, NO₂, N₂O, N₂O₅, HNO₃, HO₂, H₂O, H₂O₂, CO₂, CH₄, CH₂O, SO₂, and H₂S. Data for the absorption cross sections and quantum yields of these molecules are assembled here along with other information pertinent to the determination of photodissociation rates. The most recent techniques for computing atmospheric photodissociation rates are discussed. Photodissociation rates for all of the molecules are given at several solar zenith angles for altitudes up to 100 kilometres. A knowledge of the photodissociation rates of atmospheric molecules is essential to the resolution of many important atmospheric problems. Pollution of the stratosphere by high-flying aircraft, and of the troposphere by other anthropogenic activities, can only be described in terms of complex photochemical-dynamical models in which photolytic processes have a dominant role. A great deal of scientific effort is presently being spent in determining the mechanisms which control ozone, nitric oxide, and excited molecular oxygen concentrations in the mesosphere. Photolytic processes are already known to be important to all of these species. The photodissociation rates presented here can be applied directly to atmospheric problems such as these, or the methodology and data contained in this work can be used to compute photorates as needed.     

On the growth of nitric and sulfuric acid aerosol particles under stratospheric conditions

We present a theory for the formation of frozen aerosol particles in the Antarctic stratosphere, the coldest region of the Earth's stratosphere. The theory is applied specifically to the formation of polar stratospheric clouds. We suggest that the condensed ices are composed primarily of nitric acid and water with small admixtures of other compounds such as H₂SO₄ and HCl in solid solution. Our assumed particle formation mechanism is in agreement with the magnitude and seasonal behavior of the optical extinctions observed in the winter polar stratosphere. Physical chemistry and thermodynamic considerations suggest that at temperatures between about 200 and 185 K, stratospheric particulates are composed primarily of frozen nitric acid solutions with a composition near that of the trihydrate. Available data suggest the particles are amorphous solid solutions and not in the crystalline hydrate form. At lower temperatures (i.e., below the forst point of pure water) cirrus-like ice clouds can form.     

Synthese et etude morphologique,radiocristallographique et spectrometrique I.R. de la wadeite K2 ZrSi3 O9; domaine de formation; remplacements isomorphiques; application a la geochimie du zirconium

The first synthesis of wadeite, K₂ZrSi₃O₉, was realised by the authors in 1970 using a hydrothermal technique. These new data were the basis for further research works about the formation of this mineral by way of dry and hydrothermal synthesis, making use of various potassic compounds. The conditions for the formation of wadeite were explained as a function of the chemical environment. The pressure—temperature range of formation was investigated from 100 to 900°C and 100 to 2000 bar. The morphology of the wadeite crystals that were obtained is discussed. Starting from wadeite structure, various isomorphic substitutions were determined (Zr/Hf; K/Rb, Cs; Si/Ge); their study resulted in an interesting diagram of crystallographic parameters as a function of the ionic radius of the substituted alkaline elements: cryst. param. = f(R⁺_{alk. element}); the bands are linear. The infra-red investigations enabled us to establish the spectrum of (Si₃O₉)^6? and (Ge₃O₉)^6? in wadeite structure as well as the substitutions caused by alkalis and transition metals. Finally a comparative experimental investigation of the ZrSiO₄-K₂ZrSi₃O₉-K₂ZrSi₆O₁₅ system brought to light new data regarding the geochemical behaviour of zirconium in igneous rocks.     

A physical model of Titan's aerosols

Microphysical simulations of Titan's stratospheric haze show that aerosol microphysics is linked to organized dynamical processes. The detached haze layer may be a manifestation of 1 cm sec-1 vertical velocities at altitudes above 300 km. The hemispherical asymmetry in the visible albedo may be caused by 0.05 cm sec-1 vertical velocities at altitudes of 150 to 200 km, we predict contrast reversal beyond 0.6 micrometer. Tomasko and Smith's (1982, Icarus 51, 65-95) model, in which a layer of large particles above 220 km altitude is responsible for the high forward scattering observed by Rages and Pollack (1983, Icarus 55, 50-62), is a natural outcome of the detached haze layer being produced by rising motions if aerosol mass production occurs primarily below the detached haze layer. The aerosol's electrical charge is critical for the particle size and optical depth of the haze. The geometric albedo, particularly in the ultraviolet and near infrared, requires that the particle size be near 0.15 micrometer down to altitudes below 100 km, which is consistent with polarization observations (Tomasko and Smith 1982, West and Smith 1991, Icarus 90, 330-333). Above about 400 km and below about 150 km Yung et al.'s (1984, Astrophys. J. Suppl. Ser. 55, 465-506) diffusion coefficients are too small. Dynamical processes control the haze particles below about 150 km. The relatively large eddy diffusion coefficients in the lower stratosphere result in a vertically extensive region with nonuniform mixing ratios of condensable gases, so that most hydrocarbons may condense very near the tropopause rather than tens of kilometers above it. The optical depths of hydrocarbon clouds are probably less than one, requiring that abundant gases such as ethane condense on a subset of the haze particles to create relatively large, rapidly removed particles. The wavelength dependence of the optical radius is calculated for use in analyzing observations of the geometric albedo. The lower atmosphere and surface should be visible outside of regions of methane absorption in the near infrared. Limb scans at 2.0 micrometers wavelength should be possible down to about 75 km altitude.     

电子束CT三维重建技术与颅颌面外科

目的 探讨电子束CT三维重建技术在颅颌面外科的适应症和应用价值。方法 采用美国Imatron公司的电子束CT(electron beam CT,以下简称EBCT)C-150,对76例严重颅颌面病人实行薄层CT容积扫描。将所获CT图象经数字接口传至加拿大ISG公司生产的Allegro工作站进行三维重建。结果EBCT成像技术能立体的、详尽和精确的显示机体组织三维解剖结构极其相互关系。其再现畸形或病体模型的程度可以达到近乎解剖学的精度,为准确了解和掌握病情并制定合理的手术治疗计划提供了极为重要的依据,提高了手术治疗效果。结论EBCT三维重建技术是现代颅颌面外科最主要的诊断方法之一并具有重要的临床应用价值。     

Large biases and inconsistent climate change signals in ENSEMBLES regional projections

In this paper we analyze some caveats found in the state-of-the-art ENSEMBLES regional projections dataset focusing on precipitation over Spain, and highlight the need of a task-oriented validation of the GCM-driven control runs. In particular, we compare the performance of the GCM-driven control runs (20C3M scenario) with the ERA40-driven ones (“perfect” boundary conditions) in a common period (1961–2000). Large deviations between the results indicate a large uncertainty/bias for the particular RCM-GCM combinations and, hence, a small confidence for the corresponding transient simulations due to the potential nonlinear amplification of biases. Specifically, we found large biases for some RCM-GCM combinations attributable to RCM in-house problems with the particular GCM coupling. These biases are shown to distort the corresponding climate change signal, or “delta”, in the last decades of the 21st century, considering the A1B scenario. Moreover, we analyze how to best combine the available RCMs to obtain more reliable projections.     

Towards a better understanding of the evolution of the flood risk in Mediterranean urban areas: the case of Barcelona

This contribution explores the evolution of the flood risk in the Metropolitan Area of Barcelona (MAB; Northeast Spain) from 1981 to 2015, and how it has been affected by changes in land use, population and precipitation. To complete this study, we analysed PRESSGAMA and INUNGAMA databases to look for all the information related to the floods and flash floods that have affected the chosen region. The “Consorcio de Compensación de Seguros”, a state insurance company for extraordinary risks, provided data on economic damage. The extreme precipitation trend was analysed by the Fabra Observatory and El Prat-Airport Observatory, and daily precipitation data were provided by the State Meteorological Agency of Spain (AEMET) and the Meteorological Service of Catalonia (SMC). Population data were obtained from the Statistical Institute of Catalonia (IDESCAT). Changes in land use were estimated from the land use maps for Catalonia corresponding to 1956, 1993, 2000, 2005 and 2009. Prevention measures like rainwater tanks and improvements to the drainage system were also been considered. The specific case of Barcelona is presented, a city recognised by United Nations International Strategy for Disaster Reduction as a model city for urban resilience to floods. The evolution of flood events in the MAB does not show any significant trend for this period. We argue that the evolution in floods can be explained, at least in part, by the lack of trend in extreme precipitation indices, and also by the improvements in flood prevention measures.     

Towards a better understanding of the evolution of the flood risk in Mediterranean urban areas: the case of Barcelona

Dam failure constitutes a grave threat to human life. However, there is still a lack of systematic and comprehensive research on the loss of life (L) caused by dam break in China. From the perspective of protecting human life, a new calculation method for L occurred in dam break floods is put forward. Fourteen dam failure cases in China are selected as the basic data by three-dimensional stratified sampling, balancing spatial, vertical elevation and temporal representations, as well as considering various conditions of the dam collapse. The method includes three progressive steps: Firstly, some impact factors of loss of life (IFL) are selected by literature survey, i.e., severity of dam break flood (S _F), population at risk (P _R), understanding of dam break (U _B), warning time (T _W) and evacuation condition (E _C). And the other IFL of weather during dam break (W _B), dam break mode (M _B), water storage (S _W), building vulnerability (V _B), dam break time (T _B) and average distance from affected area to dam (D _D) are also taken into account to get a more comprehensive consideration. According to disaster system and disaster risk, these eleven IFL are divided into four categories. Through the improved entropy method, eight key IFL are further selected out of the eleven. Secondly, four L modules are built based on four categories, which are L-causing factor module (M ₁), L-prone environment module (M ₂), affected body module (M ₃) and rescue condition module (M ₄). Eventually, by using two methods of multivariate nonlinear regression and leave-one-out cross-validation in combination with coupled four modules, the calculation method for L is established. Compared with the results of Graham method and D&M method, the result of the proposed one is much closer to the actual value and performs better in fitting effect and regional applicability. In the application, L calculation and consequence assessment are carried out in the example of Hengjiang reservoir that has already broken down. At the same time, L calculation and risk prediction are used in the analysis of Yunshan reservoir, which is under planning. The proposed method can not only be applied to estimate L and its rate (f _L ) under various types of dam break conditions in China, but also provide a reliable consequence assessment and prediction approach to reduce the risk of L.