1,608 research outputs found
Classification of aerosol properties derived from AERONET direct sun data
International audienceAerosol spectral measurements by sunphotometers can be characterized by three independent pieces of information: 1) the optical thickness (AOT), a measure of the column aerosol concentration, 2) the optical thickness average spectral dependence, given by the Angstrom exponent (?), and 3) the spectral curvature of ? (??). We propose a simple graphical method to visually convert (?, ??) to the contribution of fine aerosol to the AOT and the size of the fine aerosols. This information can be used to track mixtures of pollution aerosol with dust, to distinguish aerosol growth from cloud contamination and to observe aerosol humidification. The graphical method is applied to the analysis of yearly records at 8 sites in 3 continents, characterized by different levels of pollution, biomass burning and mineral dust concentrations. Results depict the dominance of fine mode aerosols in driving the AOT at polluted sites. In stable meteorological conditions, we see an increase in the size of the fine aerosol as the pollution stagnates and increases in optical thickness. Coexistence of coarse and fine particles is evidenced at the polluted sites downwind of arid regions
Remote sensing of soot carbon – Part 2: Understanding the absorption Ångström exponent
Recently, some authors have suggested that the absorption Ångström
exponent (AAE) can be used to deduce the component aerosol
absorption optical depths (AAODs) of carbonaceous aerosols in the
AERONET database. This AAE approach presumes that AAE ≪ 1
for soot carbon, which contrasts the traditional small particle limit
of AAE = 1 for soot carbon. Thus, we provide an overview of the
AERONET retrieval, and we investigate how the microphysics of
carbonaceous aerosols can be interpreted in the AERONET AAE
product. We find that AAE ≪ 1 in the AERONET database requires
large coarse mode fractions and/or imaginary refractive indices that
increase with wavelength. Neither of these characteristics are
consistent with the current definition of soot carbon, so we explore
other possibilities for the cause of AAE ≪ 1. AAE
is related to particle size, and coarse mode particles have
a smaller AAE than fine mode particles for a given aerosol mixture
of species. We also note that the mineral goethite has an imaginary
refractive index that increases with wavelength, is very common in
dust regions, and can easily contribute to AAE ≪ 1. We find that
AAE ≪ 1 can not be caused by soot carbon, unless soot carbon has
an imaginary refractive index that increases with wavelength
throughout the visible and near-infrared spectrums. Finally, AAE is
not a robust parameter for separating carbonaceous absorption from
dust aerosol absorption in the AERONET database
Observation-Based Study on Aerosol Optical Depth and Particle Size in Partly Cloudy Regions
This study seeks to help better understand aerosol-cloud interactions by examining statistical relationships between aerosol properties and nearby low-altitude cloudiness using satellite data. The analysis of a global dataset of MODIS (Moderate Resolution Imaging Spectroradiometer) observations reveals that the positive correlation between cloudiness and aerosol optical depth (AOD) reported in earlier studies is strong throughout the globe and during both winter and summer. Typically, AOD is 30-50 higher on cloudier-than-average days than on less cloudy days. A combination of satellite observations and MERRA-2 global reanalysis data reveals that the correlation between cloud cover and AOD is strong for all aerosol types considered: sulfate, dust, carbon, and sea salt. The observations also indicate that in the presence of nearby clouds, aerosol size distributions tend to shift toward smaller particles over large regions of the Earth. This is consistent with a greater cloud-related increase in the AOD of fine mode than of coarse mode particles. The greater increase in fine mode AOD implies that the cloudiness-AOD correlation does not come predominantly from cloud detection uncertainties. Additionally, the results show that aerosol particle size increases near clouds even in regions where it decreases with increasing cloudiness. This suggests that the decrease with cloudiness comes mainly from changes in large-scale environment, rather than from clouds increasing the number or the size of fine mode aerosols. Finally, combining different aerosol retrieval algorithms demonstrated that quality assessment flags based on local variability can help identifying when the observed aerosol populations are affected by surrounding clouds
A holistic approach to carbon-enhanced metal-poor stars
By considering the various CEMP subclasses separately, we try to derive, from
the specific signatures imprinted on the abundances, parameters (such as
metallicity, mass, temperature, and neutron source) characterizing AGB
nucleosynthesis from the specific signatures imprinted on the abundances, and
separate them from the impact of thermohaline mixing, first dredge-up, and
dilution associated with the mass transfer from the companion.To put CEMP stars
in a broad context, we collect abundances for about 180 stars of various
metallicities, luminosity classes, and abundance patterns, from our own sample
and from literature. First, we show that there are CEMP stars which share the
properties of CEMP-s stars and CEMP-no stars (which we call CEMP-low-s stars).
We also show that there is a strong correlation between Ba and C abundances in
the s-only CEMP stars. This strongly points at the operation of the 13C neutron
source in low-mass AGB stars. For the CEMP-rs stars (seemingly enriched with
elements from both the s- and r-processes), the correlation of the N abundances
with abundances of heavy elements from the 2nd and 3rd s-process peaks bears
instead the signature of the 22Ne neutron source. Adding the fact that CEMP-rs
stars exhibit O and Mg enhancements, we conclude that extremely hot conditions
prevailed during the thermal pulses of the contaminating AGB stars. Finally, we
argue that most CEMP-no stars (with no overabundances for the neutron-capture
elements) are likely the extremely metal-poor counterparts of CEMP
neutron-capture-rich stars. We also show that the C enhancement in CEMP-no
stars declines with metallicity at extremely low metallicity ([Fe/H]~< -3.2).
This trend is not predicted by any of the current AGB models.Comment: 27 pages, 24 figures, accepted for publication in A&
PEN as self-vetoing structural Material
Polyethylene Naphtalate (PEN) is a mechanically very favorable polymer.
Earlier it was found that thin foils made from PEN can have very high
radio-purity compared to other commercially available foils. In fact, PEN is
already in use for low background signal transmission applications (cables).
Recently it has been realized that PEN also has favorable scintillating
properties. In combination, this makes PEN a very promising candidate as a
self-vetoing structural material in low background experiments. Components
instrumented with light detectors could be built from PEN. This includes
detector holders, detector containments, signal transmission links, etc. The
current R\&D towards qualification of PEN as a self-vetoing low background
structural material is be presented.Comment: 4 pages, 7 figures, contribution to Proceedings of the sixth workshop
on Low Radioactivity Techniques 2017, 23-27 May 2017 Seoul, to be published
at AIP, editor: D. Leonar
Effect of smoke and clouds on the transmissivity of photosynthetically active radiation inside the canopy
Biomass burning activities emit high concentrations of aerosol particles to the atmosphere. Such particles can interact with solar radiation, decreasing the amount of light reaching the surface and increasing the fraction of diffuse radiation through scattering processes, and thus has implications for photosynthesis within plant canopies. This work reports results from photosynthetically active radiation (PAR) and aerosol optical depth (AOD) measurements conducted simultaneously at Reserva Biológica do Jaru (Rondonia State, Brazil) during LBA/SMOCC (Large-Scale Biosphere-Atmosphere Experiment in Amazonia/ Smoke, Aerosols, Clouds, Rainfall, and Climate) and RaCCI (Radiation, Cloud, and Climate Interactions in the Amazon during the Dry-to-Wet Transition Season) field experiments from 15 September to 15 November 2002. AOD values were retrieved from an AERONET (Aerosol Robotic Network) radiometer, MODIS (Moderate Resolution Spectroradiometer) and a portable sunphotometer from the United States Department of Agriculture – Forest Service. Significant reduction of PAR irradiance at the top of the canopy was observed due to the smoke aerosol particles layer. This radiation reduction affected turbulent fluxes of sensible and latent heats. The increase of AOD also enhanced the transmission of PAR inside the canopy. As a consequence, the availability of diffuse radiation was enhanced due to light scattering by the aerosol particles. A complex relationship was identified between light availability inside the canopy and net ecosystem exchange (NEE). The results showed that the increase of aerosol optical depth corresponded to an increase of CO<sub>2</sub> uptake by the vegetation. However, for even higher AOD values, the corresponding NEE was lower than for intermediate values. As expected, water vapor pressure deficit (VPD), retrieved at 28m height inside the canopy, can also affect photosynthesis. A decrease in NEE was observed as VPD increased. Further studies are needed to better understand these findings, which were reported for the first time for the Amazon region under smoky conditions
Millimeter wave surface resistance of RBa2Cu3O(7-delta) (R=Y,Eu,Dy,Sm,Er) superconductors
The measurements are reported of the millimeter wave surface resistance R(sub s) at 58.6 GHz of bulk samples of RBa2Cu3O(7-delta) (R = Y,Eu,Dy,Sm,Er) and of YBa2Cu3O(7-delta) superconducting films, in the temperature range from 20 to 300 K. The bulk samples were prepared by cold pressing the powders of RBa2Cu3O(7-delta) into one in. disks. The powders were prepared by several sinterings in one atmosphere of oxygen at 925 C, with grindings between sinterings, to obtain the superconducting phase. The thin films were deposited on SrTiO3 and LaGaO3 substrates by pulsed laser ablation. Each sample was measured by replacing the end wall of a gold-plated Te sub 013 circular mode copper cavity with the sample and determining the cavity quality factor . From the difference in the Q-factor of the cavity, with and without the sample, the R(sub s) of the sample was determined
CH in stellar atmospheres: an extensive linelist
The advent of high-resolution spectrographs and detailed stellar atmosphere
modelling has strengthened the need for accurate molecular data.
Carbon-enhanced metal-poor (CEMP) stars spectra are interesting objects with
which to study transitions from the CH molecule. We combine programs for
spectral analysis of molecules and stellar-radiative transfer codes to build an
extensive CH linelist, including predissociation broadening as well as newly
identified levels. We show examples of strong predissociation CH lines in CEMP
stars, and we stress the important role played by the CH features in the
Bond-Neff feature depressing the spectra of barium stars by as much as 0.2
magnitudes in the 3000 -- 5500 \AA\ range. Because of the extreme
thermodynamic conditions prevailing in stellar atmospheres (compared to the
laboratory), molecular transitions with high energy levels can be observed.
Stellar spectra can thus be used to constrain and improve molecular data.Comment: 33pages, 15 figures, accepted in A&A external data available at
http://www.astro.ulb.ac.be/~spectrotools
A VLT-UVES spectrscopic analysis of C-rich Fe-poor stars
Large surveys of very metal-poor stars have revealed in recent years that a
large fraction of these objects were carbon-rich, analogous to the more
metal-rich CH-stars. The abundance peculiarities of CH-stars are commonly
explained by mass-transfer from a more evolved companion. In an effort to
better understand the origin and importance for Galactic evolution of Fe-poor,
C-rich stars, we present abundances determined from high-resolution and high
signal-to-noise spectra obtained with the UVES instrument attached to the
ESO/VLT. Our analysis of carbon-enhanced objects includes both CH stars and
more metal-poor objects, and we explore the link between the two classes. We
also present preliminary results of our ongoing radial velocity monitoring.Comment: 3 pages, 4 figures, presented at Cool Stars 13, Hamburg, 200
On the map: Nature and Science editorials
Bibliometric mapping of scientific articles based on keywords and technical terms in abstracts is now frequently used to chart scientific fields. In contrast, no significant mapping has been applied to the full texts of non-specialist documents. Editorials in Nature and Science are such non-specialist documents, reflecting the views of the two most read scientific journals on science, technology and policy issues. We use the VOSviewer mapping software to chart the topics of these editorials. A term map and a document map are constructed and clusters are distinguished in both of them. The validity of the document clustering is verified by a manual analysis of a sample of the editorials. This analysis confirms the homogeneity of the clusters obtained by mapping and augments the latter with further detail. As a result, the analysis provides reliable information on the distribution of the editorials over topics, and on differences between the journals. The most striking difference is that Nature devotes more attention to internal science policy issues and Science more to the political influence of scientists
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