934 research outputs found
Continuous SO2 flux measurements for Vulcano Island, Italy
The La Fossa cone of Vulcano Island (Aeolian Archipelago, Italy) is a
closed conduit volcano. Today, Vulcano Island is characterized by
sulfataric activity, with a large fumarolic field that is mainly located in the
summit area. A scanning differential optical absorption spectroscopy
instrument designed by the Optical Sensing Group of Chalmers University
of Technology in Göteborg, Sweden, was installed in the framework of the
European project "Network for Observation of Volcanic and Atmospheric
Change", in March 2008. This study presents the first dataset of SO2
plume fluxes recorded for a closed volcanic system. Between 2008 and
2010, the SO2 fluxes recorded showed average values of 12 t.d—1 during the
normal sulfataric activity of Vulcano Island, with one exceptional event
of strong degassing that occurred between September and December, 2009,
when the SO2 emissions reached up to 100 t.d—1
Multi-criteria assessment of the Representative Elementary Watershed approach on the Donga catchment (Benin) using a downward approach of model complexity
International audienceThis study is part of the AMMA – African Multidisciplinary Monsoon Analysis – project and aims at a better understanding and modelling of the Donga catchment (580 km2, Benin) behaviour. For this purpose, we applied the REW concept proposed by Reggiani et al. (1998, 1999), which allows the description of the main local processes at the sub-watershed scale. Such distributed hydrological models, which represent hydrological processes at various scales, should be evaluated not only on the discharge at the outlet but also on each of the represented processes and in several points of the catchment. This kind of multi-criteria evaluation is of importance in order to assess the global behaviour of the models. We applied such multi-criteria strategy to the Donga catchment (586 km2), in Benin. The work is supported by a strategy of observation, undertaken since 1998 consisting in a network of 20 rain gauges, an automatic meteorological station, 6 discharge stations and 18 wells. The first goal of this study is to assess the model ability to reproduce the discharge at the outlet, the water table dynamics in several points of the catchment and the vadose zone dynamics at the sub-catchment scale. We tested two spatial discretisations of increasing resolution. To test the internal structure of the model, we looked at its ability to represent also the discharge at intermediary stations. After adjustment of soil parameters, the model is shown to accurately represent discharge down to a drainage area of 100 km2, whereas poorer simulation is achieved on smaller catchments. We introduced the spatial variability of rainfall by distributing the daily rainfall over the REW and obtained a very low sensitivity of the model response to this variability. Our results suggest that processes in the unsaturated zone should first be improved, in order to better simulate soil water dynamics and represent perched water tables which were not included in this first modelling study
X-Ray photoelectron spectroscopy and mass spectrometry studies of X-ray-processed solid CO2
Solid CO2 films have been grown on a stainless steel substrate and processed by X-ray bombardment for up to 6 hr.. The reactions induced were monitored using X-ray photoelectron spectroscopy (XPS) and mass spectrometry. The XPS results are twofold: direct X-ray photolysis of the CO2 ice produced CO and an unidentified O product, possibly atomic O; secondary effects resulting from surface reactions between CO, O, and residual H from the vacuum environment produced H2CO, CH3OH, and a water ice cap on the CO2 film. The rate of production of CO from direct X-ray photolysis of CO2 is measured to be 5.4 Ă— 102 molecule photon-1, corresponding to a formation cross section of 4.7 Ă— 10-20 cm2. The growth rate for the water cap is calculated to be 2.6 Ă— 10-4 monolayers s-1 for a partial pressure of H equal to 2 Ă— 10-10 Torr. The appearance of gas-phase products from the film showed a time lag which indicates that the diffusion of the product species in the bulk CO2 is affected by some time-dependent process, possibly the creation of defects in the film. A model for the observed time dependence of the dissociation products in the gas phase yields diffusion coefficients in the CO2 of 5 Ă— 10-12 and 1 Ă— 10-12 cm2 s-1, for O and CO, respectively
Early in-flight detection of SO<sub>2</sub> via Differential Optical Absorption Spectroscopy: a feasible aviation safety measure to prevent potential encounters with volcanic plumes
Volcanic ash constitutes a risk to aviation, mainly due to its ability to cause jet engines to fail. Other risks include the possibility of abrasion of windshields and potentially serious damage to avionic systems. These hazards have been widely recognized since the early 1980s, when volcanic ash provoked several incidents of engine failure in commercial aircraft. In addition to volcanic ash, volcanic gases also pose a threat. Prolonged and/or cumulative exposure to sulphur dioxide (SO<sub>2</sub>) or sulphuric acid (H<sub>2</sub>SO<sub>4</sub>) aerosols potentially affects e.g. windows, air frame and may cause permanent damage to engines. SO<sub>2</sub> receives most attention among the gas species commonly found in volcanic plumes because its presence above the lower troposphere is a clear proxy for a volcanic cloud and indicates that fine ash could also be present. <br><br> Up to now, remote sensing of SO<sub>2</sub> via Differential Optical Absorption Spectroscopy (DOAS) in the ultraviolet spectral region has been used to measure volcanic clouds from ground based, airborne and satellite platforms. Attention has been given to volcanic emission strength, chemistry inside volcanic clouds and measurement procedures were adapted accordingly. Here we present a set of experimental and model results, highlighting the feasibility of DOAS to be used as an airborne early detection system of SO<sub>2</sub> in two spatial dimensions. In order to prove our new concept, simultaneous airborne and ground-based measurements of the plume of PopocatĂ©petl volcano, Mexico, were conducted in April 2010. The plume extended at an altitude around 5250 m above sea level and was approached and traversed at the same altitude with several forward looking DOAS systems aboard an airplane. These DOAS systems measured SO<sub>2</sub> in the flight direction and at ±40 mrad (2.3°) angles relative to it in both, horizontal and vertical directions. The approaches started at up to 25 km distance to the plume and SO<sub>2</sub> was measured at all times well above the detection limit. In combination with radiative transfer studies, this study indicates that an extended volcanic cloud with a concentration of 10<sup>12</sup> molecules cm<sup>−3</sup> at typical flight levels of 10 km can be detected unambiguously at distances of up to 80 km away. This range provides enough time (approx. 5 min) for pilots to take action to avoid entering a volcanic cloud in the flight path, suggesting that this technique can be used as an effective aid to prevent dangerous aircraft encounters with potentially ash rich volcanic clouds
Hydrological functioning of western African inland valleys explored with a critical zone model
Inland valleys are seasonally waterlogged headwater wetlands, widespread
across western Africa. Their role in the hydrological cycle in the humid,
hard-rock-dominated Sudanian savanna is not yet well understood. Thus, while
in the region recurrent floods are a major issue, and hydropower has been
recognized as an important development pathway, the scientific community
lacks precise knowledge of streamflow (Q) generation processes and how they
could be affected by the presence of inland valleys. Furthermore, inland
valleys carry an important agronomic potential, and with the strong
demographic rates of the region, they are highly subject to undergoing land
cover changes. We address both the questions of the hydrological functioning
of inland valleys in the Sudanian savanna of western Africa and the impact of
land cover changes on these systems through deterministic sensitivity
experiments using a physically based critical zone model (ParFlow-CLM)
applied to a virtual generic catchment which comprises an inland valley.
Model forcings are based on 20Â years of data from the AMMA-CATCH observation
service and parameters are evaluated against multiple field data (Q,
evapotranspiration – ET –, soil moisture, water table levels, and water
storage) acquired on a pilot elementary catchment. The hydrological model
applied to the conceptual lithological/pedological model proposed in this
study reproduces the main behaviours observed, which allowed those virtual
experiments to be conducted. We found that yearly water budgets were highly
sensitive to the vegetation distribution: average yearly ET for a
tree-covered catchment (944 mm) exceeds that of herbaceous cover (791 mm).
ET differences between the two covers vary between 12 % and 24 % of
the precipitation of the year for the wettest and driest years, respectively.
Consequently, the tree-covered catchment produces a yearly Q amount of
28 % lower on average as compared to a herbaceous-covered catchment,
ranging from 20 % for the wettest year to 47 % for a dry year. Trees
also buffer interannual variability in ET by 26 % (with respect to
herbaceous). On the other hand, pedological features (presence – or absence
– of the low-permeability layer commonly found below inland valleys,
upstream and lateral contributive areas) had limited impact on yearly water
budgets but marked consequences for intraseasonal hydrological processes
(sustained/non-sustained baseflow in the dry season, catchment water storage
redistribution). Therefore, subsurface features and vegetation cover of
inland valleys have potentially significant impacts on downstream
water-dependent ecosystems and water uses as hydropower generation, and
should focus our attention.</p
Modelling constraints on the emission inventory and on vertical dispersion for CO and SO<sub>2</sub> in the Mexico City Metropolitan Area using Solar FTIR and zenith sky UV spectroscopy
International audienceEmissions of air pollutants in and around urban areas lead to negative health impacts on the population. To estimate these impacts, it is important to know the sources and transport mechanisms of the pollutants accurately. Mexico City has a large urban fleet in a topographically constrained basin leading to high levels of carbon monoxide (CO). Large point sources of sulfur dioxide (SO2) surrounding the basin lead to episodes with high concentrations. An Eulerian grid model (CAMx) and a particle trajectory model (FLEXPART) are used to evaluate the estimates of CO and SO2 in the current emission inventory using mesoscale meteorological simulations from MM5. Vertical column measurements of CO are used to constrain the total amount of emitted CO in the model and to identify the most appropriate vertical dispersion scheme. Zenith sky UV spectroscopy is used to estimate the emissions of SO2 from a large power plant and the Popocatépetl volcano. Results suggest that the models are able to identify correctly large point sources and that both the power plant and the volcano impact the MCMA. Modelled concentrations of CO based on the current emission inventory match observations suggesting that the current total emissions estimate is correct. Possible adjustments to the spatial and temporal distribution can be inferred from model results. Accurate source and dispersion modelling provides feedback for development of the emission inventory, verification of transport processes in air quality models and guidance for policy decisions
The Chandra Source Catalog
The Chandra Source Catalog (CSC) is a general purpose virtual X-ray
astrophysics facility that provides access to a carefully selected set of
generally useful quantities for individual X-ray sources, and is designed to
satisfy the needs of a broad-based group of scientists, including those who may
be less familiar with astronomical data analysis in the X-ray regime. The first
release of the CSC includes information about 94,676 distinct X-ray sources
detected in a subset of public ACIS imaging observations from roughly the first
eight years of the Chandra mission. This release of the catalog includes point
and compact sources with observed spatial extents <~ 30''. The catalog (1)
provides access to the best estimates of the X-ray source properties for
detected sources, with good scientific fidelity, and directly supports
scientific analysis using the individual source data; (2) facilitates analysis
of a wide range of statistical properties for classes of X-ray sources; and (3)
provides efficient access to calibrated observational data and ancillary data
products for individual X-ray sources, so that users can perform detailed
further analysis using existing tools. The catalog includes real X-ray sources
detected with flux estimates that are at least 3 times their estimated 1 sigma
uncertainties in at least one energy band, while maintaining the number of
spurious sources at a level of <~ 1 false source per field for a 100 ks
observation. For each detected source, the CSC provides commonly tabulated
quantities, including source position, extent, multi-band fluxes, hardness
ratios, and variability statistics, derived from the observations in which the
source is detected. In addition to these traditional catalog elements, for each
X-ray source the CSC includes an extensive set of file-based data products that
can be manipulated interactively.Comment: To appear in The Astrophysical Journal Supplement Series, 53 pages,
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