1,012 research outputs found
Modeling chemistry in and above snow at Summit, Greenland – Part 1: Model description and results
Sun-lit snow is increasingly recognized as a chemical reactor that plays an active role in uptake, transformation, and release of atmospheric trace gases. Snow is known to influence boundary layer air on a local scale, and given the large global surface coverage of snow may also be significant on regional and global scales. We present a new detailed one-dimensional snow chemistry module that has been coupled to the 1-D atmospheric boundary layer model MISTRA. The new 1-D snow module, which is dynamically coupled to the overlaying atmospheric model, includes heat transport in the snowpack, molecular diffusion, and wind pumping of gases in the interstitial air. The model includes gas phase chemical reactions both in the interstitial air and the atmosphere. Heterogeneous and multiphase chemistry on atmospheric aerosol is considered explicitly. The chemical interaction of interstitial air with snow grains is simulated assuming chemistry in a liquid-like layer (LLL) on the grain surface. The coupled model, referred to as MISTRA-SNOW, was used to investigate snow as the source of nitrogen oxides (NOx) and gas phase reactive bromine in the atmospheric boundary layer in the remote snow covered Arctic (over the Greenland ice sheet) as well as to investigate the link between halogen cycling and ozone depletion that has been observed in interstitial air. The model is validated using data taken 10 June–13 June, 2008 as part of the Greenland Summit Halogen-HOx experiment (GSHOX). The model predicts that reactions involving bromide and nitrate impurities in the surface snow can sustain atmospheric NO and BrO mixing ratios measured at Summit, Greenland during this period
Formation of dispersive hybrid bands at an organic-metal interface
An electronic band with quasi-one dimensional dispersion is found at the
interface between a monolayer of a charge-transfer complex (TTF-TCNQ) and a
Au(111) surface. Combined local spectroscopy and numerical calculations show
that the band results from a complex mixing of metal and molecular states. The
molecular layer folds the underlying metal states and mixes with them
selectively, through the TTF component, giving rise to anisotropic hybrid
bands. Our results suggest that, by tuning the components of such molecular
layers, the dimensionality and dispersion of organic-metal interface states can
be engineered
Air–snowpack exchange of bromine, ozone and mercury in the springtime Arctic simulated by the 1-D model PHANTAS – Part 1: In-snow bromine activation and its impact on ozone
To provide a theoretical framework towards a better understanding of ozone
depletion events (ODEs) and atmospheric mercury depletion events (AMDEs) in
the polar boundary layer, we have developed a one-dimensional model that
simulates multiphase chemistry and transport of trace constituents from
porous snowpack and through the atmospheric boundary layer (ABL) as a unified
system. This paper constitutes Part 1 of the study, describing a general
configuration of the model and the results of simulations related to reactive
bromine release from the snowpack and ODEs during the Arctic spring. A common
set of aqueous-phase reactions describes chemistry both within the
liquid-like layer (LLL) on the grain surface of the snowpack and within
deliquesced "haze" aerosols mainly composed of sulfate in the atmosphere.
Gas-phase reactions are also represented by the same mechanism in the
atmosphere and in the snowpack interstitial air (SIA). Consequently, the
model attains the capacity of simulating interactions between chemistry and
mass transfer that become particularly intricate near the interface between
the atmosphere and the snowpack. In the SIA, reactive uptake on LLL-coated
snow grains and vertical mass transfer act simultaneously on gaseous HOBr, a
fraction of which enters from the atmosphere while another fraction is formed
via gas-phase chemistry in the SIA itself. A "bromine explosion", by which
HOBr formed in the ambient air is deposited and then converted
heterogeneously to Br<sub>2</sub>, is found to be a dominant process of reactive
bromine formation in the top 1 mm layer of the snowpack. Deeper in the
snowpack, HOBr formed within the SIA leads to an in-snow bromine explosion,
but a significant fraction of Br<sub>2</sub> is also produced via aqueous
radical chemistry in the LLL on the surface of the snow grains. These top-
and deeper-layer productions of Br<sub>2</sub> both contribute to the release of
Br<sub>2</sub> to the atmosphere, but the deeper-layer production is found to be
more important for the net outflux of reactive bromine. Although ozone is
removed via bromine chemistry, it is also among the key species that control
both the conventional and in-snow bromine explosions. On the other hand,
aqueous-phase radical chemistry initiated by photolytic OH formation in the
LLL is also a significant contributor to the in-snow source of Br<sub>2</sub>
and can operate without ozone, whereas the delivery of Br<sub>2</sub> to the
atmosphere becomes much smaller after ozone is depleted. Catalytic ozone loss
via bromine radical chemistry occurs more rapidly in the SIA than in the
ambient air, giving rise to apparent dry deposition velocities for ozone from
the air to the snow on the order of 10<sup>−3</sup> cm s<sup>−1</sup> during
daytime. Overall, however, the depletion of ozone in the system is caused
predominantly by ozone loss in the ambient air. Increasing depth of the
turbulent ABL under windy conditions will delay the buildup of reactive
bromine and the resultant loss of ozone, while leading to the higher column
amount of BrO in the atmosphere. During the Arctic spring, if moderately
saline and acidic snowpack is as prevalent as assumed in our model runs on
sea ice, the shallow, stable ABL under calm weather conditions may undergo
persistent ODEs without substantial contributions from blowing/drifting snow
and wind-pumping mechanisms, whereas the column densities of BrO in the ABL
will likely remain too low in the course of such events to be detected
unambiguously by satellite nadir measurements
GEM-AQ, an on-line global multiscale chemical weather system: model description and evaluation of gas phase chemistry processes
International audienceTropospheric chemistry and air quality processes were implemented on-line in the Global Environmental Multiscale model. The integrated model, GEM-AQ, has been developed as a platform to investigate chemical weather at scales from global to urban. The model was exercised for five years (2001?2005) to evaluate its ability to simulate seasonal variations and regional distributions of trace gases such as ozone, nitrogen dioxide and carbon monoxide on the global scale. The model results presented are compared with observations from satellites, aircraft measurement campaigns and balloon sondes
Long-range nonlocal flow of vortices in narrow superconducting channels
We report a new nonlocal effect in vortex matter, where an electric current
confined to a small region of a long and sufficiently narrow superconducting
wire causes vortex flow at distances hundreds of inter-vortex separations away.
The observed remote traffic of vortices is attributed to a very efficient
transfer of a local strain through the one-dimensional vortex lattice, even in
the presence of disorder. We also observe mesoscopic fluctuations in the
nonlocal vortex flow, which arise due to "traffic jams" when vortex
arrangements do not match a local geometry of a superconducting channel.Comment: a slightly longer version of a tentatively accepted PR
Luttinger parameters and momentum distribution function for the half-filled spinless fermion Holstein model: A DMRG approach
We reexamine the nature of the metallic phase of the one-dimensional
half-filled Holstein model of spinless fermions. To this end we determine the
Tomonaga-Luttinger-liquid correlation parameter by large-scale
density-matrix renormalisation-group (DMRG) calculations, exploiting (i) the
leading-order scaling relations between the ground-state energy and the
single-particle excitation gap and (ii) the static charge structure factor in
the long-wavelength limit. While both approaches give almost identical results
for intermediate-to-large phonon frequencies, we find contrasting behaviour in
the adiabatic regime: (i) (attractive) versus (ii)
(repulsive). The latter result for the correlation exponent is corroborated by
data obtained for the momentum distribution function , which puts the
existence of an attractive metallic state in the spinless fermion Holstein
model into question. We conclude that the scaling relation must be modified in
the presence of electron-phonon interactions with noticeable retardation.Comment: 6 pages, 5 figures, revised versio
The effect of an in-plane magnetic field on the interlayer transport of quasiparticles in layered superconductors
We consider the quasiparticle c-axis conductivity in highly anisotropic
layered compounds in the presence of the magnetic field parallel to the layers.
We show that at low temperatures the quasiparticle interlayer conductivity
depends strongly on the orientation of the in-plane magnetic field if the
excitation gap has nodes on the Fermi surface. Thus measurements of the
angle-dependent c-axis (out-of-plane) magnetoresistance, as a function of the
orientation of the magnetic field in the layers, provide information on the
momentum dependence of the superconducting gap (or pseudogap) on the Fermi
surface. Clean and highly anisotropic layered superconductors seem to be the
best candidates for probing the existence and location of the nodes on the
Fermi surface.Comment: 4 pages RevTeX, including 2 PostScript figures, to appear in Phys.
Rev. Let
Structural instability associated with the tilting of CuO6 octahedra in La2-xSrxCuO4
Comprehensive inelastic neutron-scattering measurements were performed to
study the soft optical phonons in La2-xSrxCuO4 at x=0.10, 0.12 and 0.18. We
found at x=0.18 that the softening of Z-point phonon, suggesting incipient
structural transition from the low-temperature orthorhombic (LTO) to
low-temperature tetragonal (LTT) phase, breaks at Tc, which is consistent with
the previous report by Lee et al. for the optimally doped x=0.15 sample. As for
x=0.10 and 0.12, on the other hand, the softening continues even below Tc. It
is thus clarified that the breaking of soft phonon is characteristic of
La2-xSrxCuO4 in the optimally and overdoped regions. In the course of studying
the soft phonons, we discovered that a central peak remains above the LTO to
high-temperature tetragonal (HTT) phase transition at Ts1 and splits into
incommensurate components along the (1 1 0)HTT direction at higher
temperatures. This is a common feature for both x=0.12 and 0.18 and their
temperature dependences of the splitting 2d can be scaled by using a
renormalized temperature T/Ts1. In the high temperature limit, d saturates
around d ~ 0.12 r.l.u., which value is close to the splitting of incommensurate
magnetic signals. This implies that the incipient lattice modulation starts
appearing at very high temperature. Details of this modulation and its
relations with other properties are, however, not yet clarified.Comment: 7 pages, 5 eps figure
The N2K Consortium. II. A Transiting Hot Saturn Around HD 149026 With a Large Dense Core
Doppler measurements from Subaru and Keck have revealed radial velocity
variations in the V=8.15, G0IV star HD 149026 consistent with a Saturn-Mass
planet in a 2.8766 day orbit. Photometric observations at Fairborn Observatory
have detected three complete transit events with depths of 0.003 mag at the
predicted times of conjunction. HD 149026 is now the second brightest star with
a transiting extrasolar planet. The mass of the star, based on interpolation of
stellar evolutionary models, is 1.3 +/- 0.1 solar masses; together with the
Doppler amplitude, K=43.3 m s^-1, we derive a planet mass Msin(i)=0.36 Mjup,
and orbital radius of 0.042 AU. HD 149026 is chromospherically inactive and
metal-rich with spectroscopically derived [Fe/H]=+0.36, Teff=6147 K, log g=4.26
and vsin(i)=6.0 km s^-1. Based on Teff and the stellar luminosity of 2.72 Lsun,
we derive a stellar radius of 1.45 Rsun. Modeling of the three photometric
transits provides an orbital inclination of 85.3 +/- 1.0 degrees and (including
the uncertainty in the stellar radius) a planet radius of 0.725 +/- 0.05 Rjup.
Models for this planet mass and radius suggest the presence of a ~67 Mearth
core composed of elements heavier than hydrogen and helium. This substantial
planet core would be difficult to construct by gravitational instability.Comment: 25 pages, 5 figures, accepted by the Astrophysical Journa
LINE-1 Hypomethylation in Cancer Is Highly Variable and Inversely Correlated with Microsatellite Instability
BACKGROUND: Alterations in DNA methylation in cancer include global hypomethylation and gene-specific hypermethylation. It is not clear whether these two epigenetic errors are mechanistically linked or occur independently. This study was performed to determine the relationship between DNA hypomethylation, hypermethylation and microsatellite instability in cancer. METHODOLOGY/PRINCIPAL FINDINGS: We examined 61 cancer cell lines and 60 colorectal carcinomas and their adjacent tissues using LINE-1 bisulfite-PCR as a surrogate for global demethylation. Colorectal carcinomas with sporadic microsatellite instability (MSI), most of which are due to a CpG island methylation phenotype (CIMP) and associated MLH1 promoter methylation, showed in average no difference in LINE-1 methylation between normal adjacent and cancer tissues. Interestingly, some tumor samples in this group showed increase in LINE-1 methylation. In contrast, MSI-showed a significant decrease in LINE-1 methylation between normal adjacent and cancer tissues (P<0.001). Microarray analysis of repetitive element methylation confirmed this observation and showed a high degree of variability in hypomethylation between samples. Additionally, unsupervised hierarchical clustering identified a group of highly hypomethylated tumors, composed mostly of tumors without microsatellite instability. We extended LINE-1 analysis to cancer cell lines from different tissues and found that 50/61 were hypomethylated compared to peripheral blood lymphocytes and normal colon mucosa. Interestingly, these cancer cell lines also exhibited a large variation in demethylation, which was tissue-specific and thus unlikely to be resultant from a stochastic process. CONCLUSION/SIGNIFICANCE: Global hypomethylation is partially reversed in cancers with microsatellite instability and also shows high variability in cancer, which may reflect alternative progression pathways in cancer
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