Assimilation of SPOT/VEGETATION NDVI data into a sahelian vegetation dynamics model

This paper presents a method to monitor the dynamics of herbaceous vegetation in the Sahel. The approach is based on the assimilation of Normalized Difference Vegetation Index (NDVI) data acquired by the VEGETATION instrument on board SPOT 4/5 into a simple sahelian vegetation dynamics model. The study region is located in the Gourma region of Mali. The vegetation dynamics model is coupled with a radiative transfer model (the SAIL model). First, it is checked that the coupled models allow for a realistic simulation of the seasonal and interannual variability of NDVI over three sampling sites from 1999 to 2004. The data assimilation scheme relies on a parameter identification technique based on an Evolution Strategies algorithm. The simulated above-ground herbage mass resulting from NDVI assimilation is then compared to ground measurements performed over 13 study sites during the period 1999-2004. The assimilation scheme performs well with 404 kg DM/ha of average error (n = 126 points) and a correlation coefficient of r = 0.80 (to be compared to the 463 kg DM/ha and r = 0.60 of the model performance without data assimilation). Finally, the sensitivity of the herbage mass model estimates to the quality of the meteorological forcing (rainfall and net radiation) is analyzed thanks to a stochastic approach.     

Mapping of Sahelian vegetation parameters from ERS scatterometer data with an evolution strategies algorithm

The West African Sahel rainfall regime is known for its spatio-temporal variability at different scales which has a strong impact on vegetation development. This study presents results of the combined use of a simple water balance model, a radiative transfer model and ERS scatterometer data to produce map of vegetation biomass and thus vegetation cover at a spatial resolution of 25 km. The backscattering coefficient measured by spaceborne wind scatterometers over Sahel shows a marked seasonality linked to the drastic changes of both soil and vegetation dielectric properties associated to the alternating dry and wet seasons. For lack of a direct observation, METEOSAT rainfall estimates are used to calculate temporal series of soil moisture with the help of a water balance model. This a priori information is used as input of the radiative transfer model that simulates the interaction between the radar wave and the surface components (soil and vegetation). Then, an inversion algorithm is applied to retrieve vegetation aerial mass from the ERS scatterometer data. Because of the nonlinear feature of the inverse problem to be solved, the inversion is performed using a global stochastic nonlinear inversion method. A good agreement is obtained between the inverse solutions and independent field measurements with mean and standard deviation of −54 and 130 kg of dry matter by hectare (kg DM/ha), respectively. The algorithm is then applied to a 350,000 km² area including the Malian Gourma and Seno region and a Sahelian part of Burkina Faso during two contrasted seasons (1999 and 2000). At the considered resolution, the obtained herbaceous mass maps show a global qualitative consistency (r²=0.71) with NDVI images acquired by the VEGETATION instrument.     

On the influence of canopy structure on the radar backscattering of mangrove forests

This paper is the third of a series which aims to evaluate the effects of canopy structure on the polarimetric radar response of mangrove forests. It complements the experimental and theoretical study of closed canopies presented in the previous papers by analysing two different mangrove stands of equal biomass but which greatly differ in their structure. For the three considered frequencies (C-, L- and P-band), experimental observations show that the back-scattering from the open declining stand is higher than that of the closed forest. The corresponding enhancement factor increases with wavelength and shows maximum values for the HH polarization. The identification of the scattering mechanisms occurring between the incident radar wave and the forest components was performed with the assistance of a polarimetric scattering model based on a radiative transfer approach. For the co-polarizations, results of the simulation study confirm that the backscatter enhancement is mainly due to an increase of either the surface scattering or the interaction component. For the cross-polarization HV at L- and P-bands, the increase of the volume component, originating from a stronger interaction with bigger branches, is found to be responsible for the observed enhancement. These findings confirm the large effect of the canopy structure on the forest backscatter and give rise to two important applications. First, the mapping of open declining mangrove stands appears feasible by using either the backscattering coefficient values, especially at P-HH and P-HV, or the HH-VV phase difference at P-band. Second, the use of the σ °-biomass statistical relationships must be restricted to homogeneous closed canopies.     

Monitoring seasonal changes of a mixed temperate forest using ERSSAR observations

Temporal variations of environmental research satellite (ERS)-1/2 backscattering coefficients acquired over a mixed deciduous forest are analyzed with an aim toward relating the observed radiometric variations to changes either in the vegetation seasonal cycle or in the structural parameters. Overall, the results are somewhat pessimistic. Temporal σ⁰ plots show chaotic variations, which are difficult to relate to the seasonal changes of forest parameters and particularly to the foliage dynamics. Furthermore, no distinction between stand types or between deciduous species is found to be possible, and nearly identical temporal plots are observed for all the stands, suggesting that the radar signatures are partly under the influence of nonforest parameters. Besides, the effect of meteorological events are difficult to evaluate. Discrimination between deciduous stands and conifers is nevertheless possible, since the radiometric difference between the two species is about 1 dB. With an overall sensitivity to standing biomass of about 0.1 dB/50 tons per hectare, ERS SARs can be considered as almost insensitive to biomass variations. For the young stands, the C-band response is found to be dominated by stand structure, whereas the backscattering coefficient saturates for biomass values higher than 50 and 80 t DM ha^-1 for deciduous and conifers, respectively     

Development and operation of alternative oxygen electrode materials for hydrogen production by high temperature steam electrolysis

High temperature steam electrolysis (HTSE) is one of the most promising ways for hydrogen mass production. To make this technology suitable from an economical point of view, each component of the system has to be optimized, from the balance of plant to the single solid oxide electrolysis cell. At this level, the optimization of the oxygen electrode is of particular interest since it contributes to a large extent to the cell polarization resistance. The present paper is focused on alternative oxygen electrode materials with improved performances compared to the usual ones mainly based on perovskite structure. Two nickelates, with compositions La₂NiO_4+δ and Nd₂NiO_4+δ are investigated and evaluated in HTSE operation at the button cell level. The performances of the Ln₂NiO_4+δ - containing cells (Ln = La, Nd) is improved compared to a cell containing the classical Sr-doped LaMnO₃ (LSM) perovskite oxygen electrode showing that nickelates are promising candidates for HTSE oxygen electrodes, especially for operation below 800 °C. Indeed, current densities determined at 1.3 V are 1.1 times larger for the La₂NiO_4+δ - containing cell and 1.6 times larger for the Nd₂NiO_4+δ one compared to the LSM - containing cell at 850 °C, whereas at 750 °C they are 1.8 and 4.4 times larger, respectively. Thanks to the use of a reference electrode, by coupling impedance spectroscopy and polarization measurements, the overpotential of each working electrode is deconvoluted from the complete cell voltage under HTSE operating conditions.     

Hybrid Capsules via Self‐Assembly of Thermoresponsive and Interfacially Active Bionanoparticle–Polymer Conjugates

New bionanoparticles have been prepared from horse spleen ferritin by grafting thermoresponsive poly(N‐isopropyl acrylamide) (PNIPAAm) and photo‐crosslinkable 2‐(dimethyl maleinimido)‐N‐ethyl‐acrylamide (DMIAAm) from the protein surface. The 72 addressable amino groups on the exterior of HSF were modified with N‐hydroxysuccinimide‐activated 2‐bromo‐isobutyrate to form a macro‐initiator for atom transfer radical polymerization, which was performed in water/DMF solutions at low temperature. The modification of the HSF and the presence of the polymer shell were confirmed by size exclusion chromatography (SEC), sodium dodecyl sulfate‐polyacrylamide gel‐electrophoresis, transmission electron microscopy, and scanning force microscopy. The thermoresponsive behavior of the ferritin‐PNIPAAm conjugates was investigated in solution by UV–vis spectroscopy showing a phase transition in the form of a cloud point around 32 °C. Further, dynamic light scattering revealed an increasing hydrodynamic radius around this transition, indicating aggregation of the particles at elevated temperatures which was confirmed by transmission electron microscopy. Initial experiments show that the particles are highly surface active, much more than the individual components alone, which was demonstrated by pendant‐drop interfacial tension measurements. This leads to the fact that they form stable Pickering emulsions, i.e., emulsion droplets decorated with polymer‐modified bionanoparticles which can be cross‐linked successively. This allows the formation of capsules with thermoresponsiveness for controlled release purposes, e.g., in drug delivery.     

Effect of Ga irradiation on the magneto-optic spectra of Pt/Co/Pt sandwiches

Interfacial changes in rf sputtered Pt/Co(2.6 nm)/Pt sandwiches grown onto sapphire (Al₂O₃) substrates induced by irradiation of 30 keV Ga⁺ ions at low dose (10¹⁴ ions/cm²) have been investigated by magneto-optic polar Kerr rotation (PKR) spectroscopy between 1 and 5 eV. The irradiation resulted in an increase of PKR over the whole spectral range. The measured PKR spectra were compared with those computed from the transfer matrix formalism using known polar Kerr rotation and ellipticity spectra for Co and five Co_xPt_1 − x alloys. The comparison between measured and computed PKR spectra provided an in-depth profile of Co and Pt ion distributions across the sandwich and confirmed that irradiation favors alloying in the vicinity of the two interfaces. These results are in a good agreement with the profile evaluated independently by TRIDYN simulations. Our results evidence an asymmetry in the irradiation effect due to an excess of Pt-Co alloying at the upper interface. Moreover, the observation of a negative PKR peak around 3.2 eV states definitively the presence of a chemically ordered Co_0.75Pt_0.25 alloy phase inside the irradiated film structure.     

Use of Surfactants to Enhance the Removal of PAHs from Soil

The efficiency of soil remediation is often limited by the low aqueous solubility of Polycyclic Aromatic Hydrocarbons, PAHs. Surfactants can then be used to enhance the removal of PAHs from soils. The dissolution of pure solid deposit of benzo(a)pyrene, B(a)P, has shown that cationic surfactants are the most efficient to increase the aqueous solubility of B(a)P, when compared to neutral or anionic surfactants. In this paper we compare by using soil suspension washings, the efficiency of two surfactants (i) a cationic surfactant, the benzyldimethyl dodecylammonium bromide, BDDA, and (ii) a neutral one, the t-octylphenoxypolyethoxy ethanol, triton X-100. The losses of surfactant, by adsorption on soil or precipitation, were measured together with the release of B(a)P, chosen as a model molecule, representative of all the PAHs. The efficiency of surfactants used in blend was then compared to the efficiency of surfactants alone.     

Magnetization reversal in Pt/Co(0.5 nm)/Pt nano-platelets patterned by focused ion beam lithography

Arrays of ultrathin Pt/Co(0.5 nm)/Pt nano-platelets with lateral sizes ranging from 30 nm to 1 μm have been patterned by focused ion beam (FIB) lithography under a weak Ga(+) ion fluence. From polar magneto-optical Kerr microscopy it is demonstrated that nano-platelets are ferromagnetic with perpendicular anisotropy down to a size of 50 nm. The irradiation process creates a magnetically soft ring at the nano-platelet periphery in which domain nucleation is initiated at a low field. The magnetization reversal in nano-platelets can be interpreted using a confined droplet model. All of the results prove that ultimate FIB patterning is suitable for preparing discrete magnetic recording media or small magnetic memory elements and nano-devices.