5,452 research outputs found
STRATAQ: A three-dimensional Chemical Transport Model of the stratosphere
International audienceA three-dimensional (3-D) Chemical Transport Model (CTM) of the stratosphere has been developed and used for a test study of the evolution of chemical species in the arctic lower stratosphere during winter 1996/97. This particular winter has been chosen for testing the model's capabilities for its remarkable dynamical situation (very cold and strong polar vortex) along with the availability of sparse chlorine, HNO3 and O3 data, showing also very low O3 values in late March/April. Due to those unusual features, the winter 1996/97 can be considered an excellent example of the impact of both dynamics and heterogeneous reactions on the chemistry of the stratosphere. Model integration has been performed from January to March 1997 and the resulting long-lived and short-lived tracer fields compared with available measurements. The model includes a detailed gas phase chemical scheme and a parameterization of the heterogeneous reactions occurring on liquid aerosol and polar stratospheric cloud (PSC) surfaces. The transport is calculated using a semi-lagrangian flux scheme, forced by meteorological analyses. In such form, the STRATAQ CTM model is suitable for short-term integrations to study transport and chemical evolution related to "real" meteorological situations. Model simulation during the chosen winter shows intense PSC formation, with noticeable local HNO3 capture by PSCs, and the activation of vortex air leading to chlorine production and subsequent O3 destruction. The resulting model fields show generally good agreement with satellite data (MLS and TOMS), although the available observations, due to their limited number and time/space sparse nature, are not enough to effectively constraint the model. In particular, the model seems to perform well in reproducing the rapid processing of air inside the polar vortex on PSC converting reservoir species in active chlorine. In addition, it satisfactorily reproduces the morphology of the continuous O3 decline as shown by the satellite during the investigated period, with a tendency, however, to underestimate the total column values inside the polar vortex during late winter. As possible causes of this model/observation difference we suggest an incorrect estimation of the vertical transport and of the tropospheric contribution
Assimilation of stratospheric ozone in the chemical transport model STRATAQ
International audienceWe describe a sequential assimilation approach useful for assimilating tracer measurements into a three-dimensional chemical transport model (CTM) of the stratosphere. The numerical code, developed largely according to Kha00, uses parameterizations and simplifications allowing assimilation of sparse observations and the simultaneous evaluation of analysis errors, with reasonable computational requirements. Assimilation parameters are set by using ?2 and OmF (Observation minus Forecast) statistics. The CTM used here is a high resolution three-dimensional model. It includes a detailed chemical package and is driven by UKMO (United Kingdom Meteorological Office) analyses. We illustrate the method using assimilation of Upper Atmosphere Research Satellite/Microwave Limb Sounder (UARS/MLS) ozone observations for three weeks during the 1996 antarctic spring. The comparison of results from the simulations with TOMS (Total Ozone Mapping Spectrometer) measurements shows improved total ozone fields due to assimilation of MLS observations. Moreover, the assimilation gives indications on a possible model weakness in reproducing polar ozone values during springtime
An Introduction to the Covariant Quantization of Superstrings
We give an introduction to a new approach to the covariant quantization of
superstrings. After a brief review of the classical Green--Schwarz superstring
and Berkovits' approach to its quantization based on pure spinors, we discuss
our covariant formulation without pure spinor constraints. We discuss the
relation between the concept of grading, which we introduced to define vertex
operators, and homological perturbation theory, and we compare our work with
recent work by others. In the appendices, we include some background material
for the Green-Schwarz and Berkovits formulations, in order that this
presentation be self contained.Comment: LaTex, 23 pp. Contribution to the Proceedings of the Workshop in
String Theory, Leuven 2002, some references added and a comment on ref. [16
Propaedeutic study for the delivery of nucleic acid-based molecules from PLGA microparticles and stearic acid nanoparticles
We studied the mechanism governing the delivery of nucleic acid-based drugs (NABD) from microparticles and nanoparticles in zero shear conditions, a situation occurring in applications such as in situ delivery to organ parenchyma. The delivery of a NABD molecule from poly(DL-lactide-co-glycolide) (PLGA) microparticles and stearic acid (SA) nanoparticles was studied using an experimental apparatus comprising a donor chamber separated from the receiver chamber by a synthetic membrane. A possible toxic effect on cell biology, as evaluated by studying cell proliferation, was also conducted for just PLGA microparticles. A mathematical model based on the hypothesis that NABD release from particles is due to particle erosion was used to interpret experimental release data. Despite zero shear conditions imposed in the donor chamber, particle erosion was the leading mechanism for NABD release from both PLGA microparticles and SA nanoparticles. PLGA microparticle erosion speed is one order of magnitude higher than that of competing to SA nanoparticles. Finally, no deleterious effects of PLGA microparticles on cell proliferation were detected. Thus, the data here reported can help optimize the delivery systems aimed at release of NABD from micro- and nanoparticles
Slavnov-Taylor Parameterization for the Quantum Restoration of BRST Symmetries in Anomaly-Free Gauge Theories
It is shown that the problem of the recursive restoration of the
Slavnov-Taylor (ST) identities at the quantum level for anomaly-free gauge
theories is equivalent to the problem of parameterizing the local approximation
to the quantum effective action in terms of ST functionals, associated with the
cohomology classes of the classical linearized ST operator. The ST functionals
of dimension <=4 correspond to the invariant counterterms, those of dimension
>4 generate the non-symmetric counterterms upon projection on the action-like
sector. At orders higher than one in the loop expansion there are additional
contributions to the non-invariant counterterms, arising from known lower order
terms. They can also be parameterized by using the ST functionals. We apply the
method to Yang-Mills theory in the Landau gauge with an explicit mass term
introduced in a BRST-invariant way via a BRST doublet. Despite being
non-unitary, this model provides a good example where the method devised in the
paper can be applied to derive the most general solution for the action-like
part of the quantum effective action, compatible with the fulfillment of the ST
identities and the other relevant symmetries of the model, to all orders in the
loop expansion. The full dependence of the solution on the normalization
conditions is given.Comment: 23 pages. Final version published in the journa
Continuous approximation of binomial lattices
A systematic analysis of a continuous version of a binomial lattice,
containing a real parameter and covering the Toda field equation as
, is carried out in the framework of group theory. The
symmetry algebra of the equation is derived. Reductions by one-dimensional and
two-dimensional subalgebras of the symmetry algebra and their corresponding
subgroups, yield notable field equations in lower dimensions whose solutions
allow to find exact solutions to the original equation. Some reduced equations
turn out to be related to potentials of physical interest, such as the
Fermi-Pasta-Ulam and the Killingbeck potentials, and others. An instanton-like
approximate solution is also obtained which reproduces the Eguchi-Hanson
instanton configuration for . Furthermore, the equation under
consideration is extended to --dimensions. A spherically symmetric form
of this equation, studied by means of the symmetry approach, provides
conformally invariant classes of field equations comprising remarkable special
cases. One of these enables us to establish a connection with the
Euclidean Yang-Mills equations, another appears in the context of Differential
Geometry in relation to the socalled Yamabe problem. All the properties of the
reduced equations are shared by the spherically symmetric generalized field
equation.Comment: 30 pages, LaTeX, no figures. Submitted to Annals of Physic
Early mandibular canine-lateral incisor transposition: case report
Purpose. The main aim of the present study is to present a case of mandibular transposition between lateral incisor and
canine in a paediatric patient.
Materials and methods. A fixed multibracket orthodontic treatment was performed by means of a modified welded arch
as to correct the transposition and obtaining a class I functional and symmetrical occlusion, also thanks to the early diagnosis
of the eruption anomaly.
Results. Our case report shows that a satisfactory treatment of mandibular transpositions is obtained when detected at
an early stage of the tooth development.
Conclusions. The main treatment options to be taken into consideration in case of a mandibular transposition are two:
correcting the transposition or aligning it leaving the dental elements in their transposed order; in both cases, the followups
show a stable condition, maintained without relapses. Several factors, such as age of the patient, occlusion, aesthetics,
patientâs collaboration, periodontal support and duration of treatment have to be considered as to prevent potential damage
to dental elements and support appliances. The choice between the two treatment approaches for mandibular lateral
incisor/canine transpositions mainly depends on the time the anomaly is detected
Knots, Braids and BPS States in M-Theory
In previous work we considered M-theory five branes wrapped on elliptic
Calabi-Yau threefold near the smooth part of the discriminant curve. In this
paper, we extend that work to compute the light states on the worldvolume of
five-branes wrapped on fibers near certain singular loci of the discriminant.
We regulate the singular behavior near these loci by deforming the discriminant
curve and expressing the singularity in terms of knots and their associated
braids. There braids allow us to compute the appropriate string junction
lattice for the singularity and,hence to determine the spectrum of light BPS
states. We find that these techniques are valid near singular points with N=2
supersymmetry.Comment: 38 page
Drug Nanocrystals: Theoretical Background of Solubility Increase and Dissolution Rate Enhancement
The peculiar higher solubility of drug nanocrystals compared to macrocrystals appeals to the pharmaceutical field. Indeed, until now, about 70 % of the potential drug candidates are discarded due to low bioavailability related with poor solubility in water. Since a modern and efficient design strategy for nanocrystal-based delivery systems requires the knowledge of the theoretical relation between nanocrystal size and solubility, the aim of this paper is to build up a physically-oriented thermodynamic model relating to nanocrystal dimensions with their melting temperature, enthalpy, solubility and dissolution rate. In particular, the developed model will be applied to vinpocetine, a poorly soluble drug used in the treatment of various types of cerebrovascular circulatory disorders
Drug Nanocrystals: Theoretical Background of Solubility Increase and Dissolution Rate Enhancement
The peculiar higher solubility of drug nanocrystals compared to macrocrystals appeals to the pharmaceutical field. Indeed, until now, about 70 % of the potential drug candidates are discarded due to low bioavailability related with poor solubility in water. Since a modern and efficient design strategy for nanocrystal-based delivery systems requires the knowledge of the theoretical relation between nanocrystal size and solubility, the aim of this paper is to build up a physically-oriented thermodynamic model relating to nanocrystal dimensions with their melting temperature, enthalpy, solubility and dissolution rate. In particular, the developed model will be applied to vinpocetine, a poorly soluble drug used in the treatment of various types of cerebrovascular circulatory disorders
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