251 research outputs found
Pollution in the open oceans: 2009-2013
This review of pollution in the open oceans updates a report on this topic prepared by GESAMP five years previously (Reports and Studies No. 79, GESAMP, 2009). The latter report, the first from GESAMP focusing specifically on the oceans beyond the 200 m depth contour, was prepared for purposes of the Assessment of Assessments, the preparatory phase of a regular process for assessing the state of the marine environment, led jointly by the United Nations Environment Programme (UNEP) and the Intergovernmental Oceanographic Commission (UNESCO-IOC)
Magneto-Optical Trap for Polar Molecules
We propose a method for laser cooling and trapping a substantial class of
polar molecules, and in particular titanium (II) oxide (TiO). This method uses
pulsed electric fields to nonadiabatically remix the ground-state magnetic
sublevels of the molecule, allowing us to build a magneto-optical trap (MOT)
based on a quasi-cycling transition. Monte-Carlo simulations of this
electrostatically remixed MOT (ER-MOT) demonstrate the feasibility of cooling
TiO to a temperature of 10 and trapping it with a
radiation-pumping-limited lifetime on the order of 80 ms.Comment: 4 pages, 4 figures, 1 table v2: updated to final published text and
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Sensitivity model study of regional mercury dispersion in the atmosphere
Atmospheric deposition is the most important pathway by which Hg reaches marine ecosystems, where it can be methylated and enter the base of food chain. The deposition, transport and chemical interactions of atmospheric Hg have been simulated over Europe for the year 2013 in the framework of the Global Mercury Observation System (GMOS) project, performing 14 different model sensitivity tests using two high-resolution three-dimensional chemical transport models (CTMs), varying the anthropogenic emission datasets, atmospheric Br input fields, Hg oxidation schemes and modelling domain boundary condition input. Sensitivity simulation results were compared with observations from 28 monitoring sites in Europe to assess model performance and particularly to analyse the influence of anthropogenic emission speciation and the Hg0(g) atmospheric oxidation mechanism. The contribution of anthropogenic Hg emissions, their speciation and vertical distribution are crucial to the simulated concentration and deposition fields, as is also the choice of Hg0(g) oxidation pathway. The areas most sensitive to changes in Hg emission speciation and the emission vertical distribution are those near major sources, but also the Aegean and the Black seas, the English Channel, the Skagerrak Strait and the northern German coast. Considerable influence was found also evident over the Mediterranean, the North Sea and Baltic Sea and some influence is seen over continental Europe, while this difference is least over the north-western part of the modelling domain, which includes the Norwegian Sea and Iceland. The Br oxidation pathway produces more HgII(g) in the lower model levels, but overall wet deposition is lower in comparison to the simulations which employ an O3 ∕ OH oxidation mechanism. The necessity to perform continuous measurements of speciated Hg and to investigate the local impacts of Hg emissions and deposition, as well as interactions dependent on land use and vegetation, forests, peat bogs, etc., is highlighted in this study
Dust in the Photospheric Environment: Unified Cloudy Models of M, L, and T Dwarfs
We address the problem of how dust forms and how it could be sustained in the
static photospheres of cool dwarfs for a long time. In the cool and dense gas,
dust forms easily at the condensation temperature, T_cond, and the dust can be
in detailed balance with the ambient gas so long as it remains smaller than the
critical radius, r_cr. However, dust will grow larger and segregate from the
gas when it will be larger than r_cr somewhere at the lower temperature, which
we refer to as the critical temperature, T_cr. Then, the large dust grains will
precipitate below the photosphere and only the small dust grains in the region
of T_cr < T < T_cond can be sustained in the photosphere. Thus a dust cloud is
formed. Incorporating the dust cloud, non-grey model photo- spheres in
radiative-convective equilibrium are extended to T_eff as low as 800K. Observed
colors and spectra of cool dwarfs can consistently be accounted for by a single
grid of our cloudy models. This fact in turn can be regarded as supporting
evidence for our basic assumption on the cloud formation.Comment: 50 pages with 14 postscript figures, to be published in Astrophys.
Synthetic Spectra and Color-Temperature Relations of M Giants
As part of a project to model the integrated spectra and colors of elliptical
galaxies through evolutionary synthesis, we have refined our synthetic spectrum
calculations of M giants. After critically assessing three effective
temperature scales for M giants, we adopted the relation of Dyck et al. (1996)
for our models. Using empirical spectra of field M giants as a guide, we then
calculated MARCS stellar atmosphere models and SSG synthetic spectra of these
cool stars, adjusting the band absorption oscillator strengths of the TiO bands
to better reproduce the observational data. The resulting synthetic spectra are
found to be in very good agreement with the K-band spectra of stars of the
appropriate spectral type taken from Kleinmann & Hall (1986) as well. Spectral
types estimated from the strengths of the TiO bands and the depth of the
bandhead of CO near 2.3 microns quantitatively confirm that the synthetic
spectra are good representations of those of field M giants. The broad-band
colors of the models match the field relations of K and early-M giants very
well; for late-M giants, differences between the field-star and synthetic
colors are probably caused by the omission of spectral lines of VO and water in
the spectrum synthesis calculations. Here, we present four grids of K-band
bolometric corrections and colors -- Johnson U-V and B-V; Cousins V-R and V-I;
Johnson-Glass V-K, J-K and H-K; and CIT/CTIO V-K, J-K, H-K and CO -- for models
having 3000 K < Teff < 4000 K and -0.5 < log g < 1.5. These grids, which have
[Fe/H] = +0.25, 0.0, -0.5 and -1.0, extend and supplement the color-temperature
relations of hotter stars presented in a companion paper (astro-ph/9911367).Comment: To appear in the March 2000 issue of the Astronomical Journal. 60
pages including 15 embedded postscript figures (one page each) and 6 embedded
postscript tables (10 pages total
Mercury in the Black Sea:New Insights From Measurements and Numerical Modeling
Redox conditions and organic matter control marine methylmercury (MeHg) production. The Black Sea is the world's largest and deepest anoxic basin and is thus ideal to study Hg species along the extended redox gradient. Here we present new dissolved Hg and MeHg data from the 2013 GEOTRACES MEDBlack cruise (GN04_leg2) that we integrated into a numerical 1-D model, to track the fate and dynamics of Hg and MeHg. Contrary to a previous study, our new data show highest MeHg concentrations in the permanently anoxic waters. Observed MeHg/Hg percentage (range 9-57%) in the anoxic waters is comparable to other subsurface maxima in oxic open-ocean waters. With the modeling we tested for various Hg methylation and demethylation scenarios along the redox gradient. The results show that Hg methylation must occur in the anoxic waters. The model was then used to simulate the time evolution (1850-2050) of Hg species in the Black Sea. Our findings quantify (1) inputs and outputs of Hg-T (similar to 31 and similar to 28 kmol yr(-1)) and MeHgT (similar to 5 and similar to 4 kmol yr(-1)) to the basin, (2) the extent of net demethylation occurring in oxic (similar to 1 kmol yr(-1)) and suboxic water (similar to 6 kmol yr(-1)), (3) and the net Hg methylation in the anoxic waters of the Black Sea (similar to 11 kmol yr(-1)). The model was also used to estimate the amount of anthropogenic Hg (85-93%) in the Black Sea
Current and future levels of mercury atmospheric pollution on a global scale
An assessment of current and future emissions, air
concentrations, and atmospheric deposition of mercury worldwide is presented
on the basis of results obtained during the performance of the EU GMOS
(Global Mercury Observation System) project. Emission estimates for mercury
were prepared with the main goal of applying them in models to assess
current (2013) and future (2035) air concentrations and atmospheric
deposition of this contaminant. The combustion of fossil fuels (mainly coal) for energy and heat
production in power plants and in industrial and residential boilers, as well as artisanal and small-scale gold mining, is one of the
major anthropogenic sources of Hg emissions to the atmosphere at present.
These sources account for about 37 and 25 % of the total anthropogenic
Hg emissions globally, estimated to be about 2000 t. Emissions in
Asian countries, particularly in China and India, dominate the total
emissions of Hg. The current estimates of mercury emissions from natural
processes (primary mercury emissions and re-emissions), including mercury
depletion events, were estimated to be 5207 t year<sup>−1</sup>, which
represents nearly 70 % of the global mercury emission budget. Oceans are the most
important sources (36 %), followed by biomass burning (9 %). A comparison
of the 2035 anthropogenic emissions estimated for three different scenarios with
current anthropogenic emissions indicates a reduction of these emissions in
2035 up to 85 % for the best-case scenario.
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Two global chemical transport models (GLEMOS and ECHMERIT) have been used
for the evaluation of future mercury pollution levels considering future
emission scenarios. Projections of future changes in mercury deposition on a
global scale simulated by these models for three anthropogenic emissions
scenarios of 2035 indicate a decrease in up to 50 % deposition in the
Northern Hemisphere and up to 35 % in Southern Hemisphere for the best-case scenario.
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The EU GMOS project has proved to be a very important research instrument
for supporting the scientific justification for the Minamata
Convention and monitoring of the implementation of targets of this
convention, as well as the EU Mercury Strategy. This project provided the
state of the art with regard to the development of the latest emission
inventories for mercury, future emission scenarios, dispersion modelling of
atmospheric mercury on a global and regional scale, and source–receptor
techniques for mercury emission apportionment on a global scale
Chemical cycling and deposition of atmospheric mercury in Polar Regions: review of recent measurements and comparison with models
Mercury (Hg) is a worldwide contaminant that can cause adverse health effects to wildlife and humans. While atmospheric modeling traces the link from emissions to deposition of Hg onto environmental surfaces, large uncertainties arise from our incomplete understanding of atmospheric processes (oxidation pathways, deposition, and re-emission). Atmospheric Hg reactivity is exacerbated in high latitudes and there is still much to be learned from polar regions in terms of atmospheric processes. This paper provides a synthesis of the atmospheric Hg monitoring data available in recent years (2011–2015) in the Arctic and in Antarctica along with a comparison of these observations with numerical simulations using four cutting-edge global models. The cycle of atmospheric Hg in the Arctic and in Antarctica presents both similarities and differences. Coastal sites in the two regions are both influenced by springtime atmospheric Hg depletion events and by summertime snowpack re-emission and oceanic evasion of Hg. The cycle of atmospheric Hg differs between the two regions primarily because of their different geography. While Arctic sites are significantly influenced by northern hemispheric Hg emissions especially in winter, coastal Antarctic sites are significantly influenced by the reactivity observed on the East Antarctic ice sheet due to katabatic winds. Based on the comparison of multi-model simulations with observations, this paper discusses whether the processes that affect atmospheric Hg seasonality and interannual variability are appropriately represented in the models and identifies research gaps in our understanding of the atmospheric Hg cycling in high latitudes
Nucleologenesis in the Caenorhabditis elegans Embryo
In the Caenorhabditis elegans nematode, the oocyte nucleolus disappears prior to fertilization. We have now investigated the re-formation of the nucleolus in the early embryo of this model organism by immunostaining for fibrillarin and DAO-5, a putative NOLC1/Nopp140 homolog involved in ribosome assembly. We find that labeled nucleoli first appear in somatic cells at around the 8-cell stage, at a time when transcription of the embryonic genome begins. Quantitative analysis of radial positioning showed the nucleolus to be localized at the nuclear periphery in a majority of early embryonic nuclei. At the ultrastructural level, the embryonic nucleolus appears to be composed of a relatively homogenous core surrounded by a crescent-shaped granular structure. Prior to embryonic genome activation, fibrillarin and DAO-5 staining is seen in numerous small nucleoplasmic foci. This staining pattern persists in the germline up to the ∼100-cell stage, until the P4 germ cell divides to give rise to the Z2/Z3 primordial germ cells and embryonic transcription is activated in this lineage. In the ncl-1 mutant, which is characterized by increased transcription of rDNA, DAO-5-labeled nucleoli are already present at the 2-cell stage. Our results suggest a link between the activation of transcription and the initial formation of nucleoli in the C. elegans embryo
Neural coding in a single sensory neuron controlling opposite seeking behaviours in Caenorhabditis elegans
Unveiling the neural codes for intricate behaviours is a major challenge in neuroscience. The neural circuit for the temperature-seeking behaviour of Caenorhabditis elegans is an ideal system to dissect how neurons encode sensory information for the execution of behavioural output. Here we show that the temperature-sensing neuron AFD transmits both stimulatory and inhibitory neural signals to a single interneuron AIY. In this circuit, a calcium concentration threshold in AFD acts as a switch for opposing neural signals that direct the opposite behaviours. Remote control of AFD activity, using a light-driven ion pump and channel, reveals that diverse reduction levels of AFD activity can generate warm- or cold-seeking behaviour. Calcium imaging shows that AFD uses either stimulatory or inhibitory neuronal signalling onto AIY, depending on the calcium concentration threshold in AFD. Thus, dual neural regulation in opposite directions is directly coupled to behavioural inversion in the simple neural circuit
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