13,342 research outputs found
Decadal trends in aerosol chemical composition at Barrow, Alaska: 1976–2008
Aerosol measurements at Barrow, Alaska during the past 30 years have identified the long range transport of pollution associated with Arctic Haze as well as ocean-derived aerosols of more local origin. Here, we focus on measurements of aerosol chemical composition to assess (1) trends in Arctic Haze aerosol and implications for source regions, (2) the interaction between pollution-derived and ocean-derived aerosols and the resulting impacts on the chemistry of the Arctic boundary layer, and (3) the response of aerosols to a changing climate. Aerosol chemical composition measured at Barrow, AK during the Arctic haze season is compared for the years 1976–1977 and 1997–2008. Based on these two data sets, concentrations of non-sea salt (nss) sulfate (SO<sub>4</sub><sup>=</sup>) and non-crustal (nc) vanadium (V) have decreased by about 60% over this 30 year period. Consistency in the ratios of nss SO<sub>4</sub><sup>=</sup>/ncV and nc manganese (Mn)/ncV between the two data sets indicates that, although emissions have decreased in the source regions, the source regions have remained the same over this time period. The measurements from 1997–2008 indicate that, during the haze season, the nss SO<sub>4</sub><sup>=</sup> aerosol at Barrow is becoming less neutralized by ammonium (NH<sub>4</sub><sup>+</sup>) yielding an increasing sea salt aerosol chloride (Cl<sup>&minus;</sup>) deficit. The expected consequence is an increase in the release of Cl atoms to the atmosphere and a change in the lifetime of volatile organic compounds (VOCs) including methane. In addition, summertime concentrations of biogenically-derived methanesulfonate (MSA<sup>&minus;</sup>) and nss SO<sub>4</sub><sup>=</sup> are increasing at a rate of 12 and 8% per year, respectively. Further research is required to assess the environmental factors behind the increasing concentrations of biogenic aerosol
Probing a non-biaxial behavior of infinitely thin hard platelets
We give a criterion to test a non-biaxial behavior of infinitely thin hard
platelets of symmetry based upon the components of three order
parameter tensors. We investigated the nematic behavior of monodisperse
infinitely thin rectangular hard platelet systems by using the criterion.
Starting with a square platelet system, and we compared it with rectangular
platelet systems of various aspect ratios. For each system, we performed
equilibration runs by using isobaric Monte Carlo simulations. Each system did
not show a biaxial nematic behavior but a uniaxial nematic one, despite of the
shape anisotropy of those platelets. The relationship between effective
diameters by simulations and theoretical effective diameters of the above
systems was also determined.Comment: Submitted to JPS
Development of planar pixel modules for the ATLAS high luminosity LHC tracker upgrade
The high-luminosity LHC will present significant challenges for tracking systems. ATLAS is preparing to upgrade the entire tracking system, which will include a significantly larger pixel detector. This paper reports on the development of large area planar detectors for the outer pixel layers and the pixel endcaps. Large area sensors have been fabricated and mounted onto 4 FE-I4 readout ASICs, the so-called quad-modules, and their performance evaluated in the laboratory and testbeam. Results from characterisation of sensors prior to assembly, experience with module assembly, including bump-bonding and results from laboratory and testbeam studies are presented
Computing the Casimir energy using the point-matching method
We use a point-matching approach to numerically compute the Casimir
interaction energy for a two perfect-conductor waveguide of arbitrary section.
We present the method and describe the procedure used to obtain the numerical
results. At first, our technique is tested for geometries with known solutions,
such as concentric and eccentric cylinders. Then, we apply the point-matching
technique to compute the Casimir interaction energy for new geometries such as
concentric corrugated cylinders and cylinders inside conductors with focal
lines.Comment: 11 pages, 18 figure
External and intrinsic anchoring in nematic liquid crystals: A Monte Carlo study
We present a Monte Carlo study of external surface anchoring in nematic cells
with partially disordered solid substrates, as well as of intrinsic anchoring
at free nematic interfaces. The simulations are based on the simple hexagonal
lattice model with a spatially anisotropic intermolecular potential. We
estimate the corresponding extrapolation length by imposing an elastic
deformation in a hybrid cell-like nematic sample. Our estimates for
increase with increasing surface disorder and are essentially
temperature--independent. Experimental values of are approached only when
both the coupling of nematic molecules with the substrate and the anisotropy of
nematic--nematic interactions are weak.Comment: Revisions primarily in section I
Pulsar J0453+1559: A Double Neutron Star System with a Large Mass Asymmetry
To understand the nature of supernovae and neutron star (NS) formation, as
well as binary stellar evolution and their interactions, it is important to
probe the distribution of NS masses. Until now, all double NS (DNS) systems
have been measured to have a mass ratio close to unity (q 0.91). Here we
report the measurement of the individual masses of the 4.07-day binary pulsar
J0453+1559 from measurements of the rate of advance of periastron and Shapiro
delay: The mass of the pulsar is 1.559(5) and that of its companion
is 1.174(4) ; q = 0.75. If this companion is also a neutron star
(NS), as indicated by the orbital eccentricity of the system (e=0.11), then its
mass is the smallest precisely measured for any such object. The pulsar has a
spin period of 45.7 ms and a spin derivative of 1.8616(7) x; from these
we derive a characteristic age of ~ 4.1 x years and a magnetic field of
~ 2.9 x G,i.e, this pulsar was mildly recycled by accretion of matter
from the progenitor of the companion star. This suggests that it was formed
with (very approximately) its current mass. Thus NSs form with a wide range of
masses, which is important for understanding their formation in supernovae. It
is also important for the search for gravitational waves released during a
NS-NS merger: it is now evident that we should not assume all DNS systems are
symmetric
An electron Talbot interferometer
The Talbot effect, in which a wave imprinted with transverse periodicity
reconstructs itself at regular intervals, is a diffraction phenomenon that
occurs in many physical systems. Here we present the first observation of the
Talbot effect for electron de Broglie waves behind a nanofabricated
transmission grating. This was thought to be difficult because of Coulomb
interactions between electrons and nanostructure gratings, yet we were able to
map out the entire near-field interference pattern, the "Talbot carpet", behind
a grating. We did this using a Talbot interferometer, in which Talbot
interference fringes from one grating are moire'-filtered by a 2nd grating.
This arrangement has served for optical, X-ray, and atom interferometry, but
never before for electrons. Talbot interferometers are particularly sensitive
to distortions of the incident wavefronts, and to illustrate this we used our
Talbot interferometer to measure the wavefront curvature of a weakly focused
electron beam. Here we report how this wavefront curvature demagnified the
Talbot revivals, and we discuss applications for electron Talbot
interferometers.Comment: 5 pages, 5 figures, updated version with abstrac
Anharmonic stacking in supercoiled DNA
Multistep denaturation in a short circular DNA molecule is analyzed by a
mesoscopic Hamiltonian model which accounts for the helicoidal geometry.
Computation of melting profiles by the path integral method suggests that
stacking anharmonicity stabilizes the double helix against thermal disruption
of the hydrogen bonds. Twisting is essential in the model to capture the
importance of nonlinear effects on the thermodynamical properties. In a ladder
model with zero twist, anharmonic stacking scarcely affects the thermodynamics.
Moderately untwisted helices, with respect to the equilibrium conformation,
show an energetic advantage against the overtwisted ones. Accordingly
moderately untwisted helices better sustain local fluctuational openings and
make more unlikely the thermally driven complete strand separation.Comment: In pres
Gender, foundation degrees and the knowledge economy
This article questions the concept of ‘education for employment’, which constructs a discourse of individual and societal benefit in a knowledge‐driven economy. Recent policy emphasis in the European Union promotes the expansion of higher education and short‐cycle vocational awards such as the intermediate two‐year Foundation Degree recently introduced into England and Wales. Studies of vocational education and training (VET) and the knowledge economy have focused largely on the governance of education and on the development and drift of policy. Many VET programmes have also been considered for their classed, raced and gendered take‐up and subsequent effect on employment. This article builds on both fields of study to engage with the finer cross‐analyses of gender, social class, poverty, race and citizenship. In its analysis of policy texts the article argues that in spite of a discourse of inclusivity, an expanded higher education system has generated new inequalities, deepening social stratification. Drawing on early analyses of national quantitative data sets, it identifies emerging gendered, classed and raced patterns and considers these in relation to occupationally and hierarchically stratified labour markets, both within and without the knowledge economy
Filmic geographies: audio-visual, embodied-material
Although conventionally described as a ‘visual’ method, film-making is also increasingly used within research on embodiment. However, much remains to be said about the ability of filmic methods to enhance researchers’ capacity to think and research through the body. Drawing on my experience of making four research films, in this paper, I attempt to advance this agenda in three steps. First, I introduce anthropological work on the filming body to shed light on the technologically-mediated encounters that enfold around a camera and discuss how they might inform geographical thinking. Second, I describe the corporeally-mediated object ecologies that take shape within the filming setting and highlight how a camera might make objects ‘speak’. Finally, I discuss the affective dimension of screening research films to research participants and the contribution of such intense events to the articulation of collective matters of concerns. Through these three themes, I make the case for understanding knowledge production as located not merely in encounters with filmed audio-visual content, but also in the embodied-material encounters of bodies and objects around the filming and screening apparatus. I finally discuss the implications of these reflections for conceptualising the ‘body’ within embodied methods in social and cultural geography
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