The Experts below are selected from a list of 66 Experts worldwide ranked by ideXlab platform
Gerbrand Ceder - One of the best experts on this subject based on the ideXlab platform.
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calibrating transition metal energy levels and oxygen Bands in first principles calculations accurate prediction of redox potentials and charge transfer in lithium transition metal oxides
Physical Review B, 2015Co-Authors: Donghwa Seo, Alexander Urban, Gerbrand CederAbstract:Transition-metal (TM) oxides play an increasingly important role in technology today, including applications such as catalysis, solar energy harvesting, and energy storage. In many of these applications, the details of their electronic structure near the Fermi level are critically important for their properties. We propose a first-principles--based computational methodology for the accurate prediction of oxygen charge transfer in TM oxides and lithium TM (Li-TM) oxides. To obtain accurate electronic structures, the Heyd-Scuseria-Ernzerhof (HSE06) hybrid functional is adopted, and the amount of exact Hartree-Fock exchange (mixing parameter) is adjusted to reproduce Reference Band gaps. We show that the HSE06 functional with optimal mixing parameter yields not only improved electronic densities of states, but also better energetics (Li-intercalation voltages) for $\mathrm{LiCo}{\mathrm{O}}_{2}$ and $\mathrm{LiNi}{\mathrm{O}}_{2}$ as compared to the generalized gradient approximation (GGA), Hubbard $U$ corrected GGA ($\mathrm{GGA}+U$), and standard HSE06. We find that the optimal mixing parameters for TM oxides are system specific and correlate with the covalency (ionicity) of the TM species. The strong covalent (ionic) nature of TM-O bonding leads to lower (higher) optimal mixing parameters. We find that optimized HSE06 functionals predict stronger hybridization of the $\mathrm{Co}\phantom{\rule{0.28em}{0ex}}3d$ and $\mathrm{O}\phantom{\rule{0.28em}{0ex}}2p$ orbitals as compared to GGA, resulting in a greater contribution from oxygen states to charge compensation upon delithiation in $\mathrm{LiCo}{\mathrm{O}}_{2}$. We also find that the Band gaps of Li-TM oxides increase linearly with the mixing parameter, enabling the straightforward determination of optimal mixing parameters based on GGA ($\ensuremath{\alpha}=0.0$) and HSE06 $(\ensuremath{\alpha}=0.25)$ calculations. Our results also show that ${G}_{0}{W}_{0}@\mathrm{GGA}+U$ Band gaps of TM oxides ($M\mathrm{O},\phantom{\rule{0.28em}{0ex}}M=\mathrm{Mn},\phantom{\rule{0.28em}{0ex}}\mathrm{Co},\phantom{\rule{0.28em}{0ex}}\mathrm{Ni}$) and $\mathrm{LiCo}{\mathrm{O}}_{2}$ agree well with experimental References, suggesting that ${G}_{0}{W}_{0}$ calculations can be used as a Reference for the calibration of the mixing parameter in cases when no experimental Band gap has been reported.
A Goerner - One of the best experts on this subject based on the ideXlab platform.
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remote sensing of ecosystem light use efficiency with modis based pri
Biogeosciences, 2011Co-Authors: A Goerner, Enrico Tomelleri, Niall P Hanan, Serge Rambal, D Dragoni, Dario Papale, Markus Reichstein, Christiane SchmulliusAbstract:Abstract. Several studies sustained the possibility that a photochemical reflectance index (PRI) directly obtained from satellite data can be used as a proxy for ecosystem light use efficiency (LUE) in diagnostic models of gross primary productivity. This modelling approach would avoid the complications that are involved in using meteorological data as constraints for a fixed maximum LUE. However, no unifying model predicting LUE across climate zones and time based on MODIS PRI has been published to date. In this study, we evaluate the effectiveness with which MODIS-based PRI can be used to estimate ecosystem light use efficiency at study sites of different plant functional types and vegetation densities. Our objective is to examine if known limitations such as dependence on viewing and illumination geometry can be overcome and a single PRI-based model of LUE (i.e. based on the same Reference Band) can be applied under a wide range of conditions. Furthermore, we were interested in the effect of using different faPAR (fraction of absorbed photosynthetically active radiation) products on the in-situ LUE used as ground truth and thus on the whole evaluation exercise. We found that estimating LUE at site-level based on PRI reduces uncertainty compared to the approaches relying on a maximum LUE reduced by minimum temperature and vapour pressure deficit. Despite the advantages of using PRI to estimate LUE at site-level, we could not establish an universally applicable light use efficiency model based on MODIS PRI. Models that were optimised for a pool of data from several sites did not perform well.
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tracking seasonal drought effects on ecosystem light use efficiency with satellite based pri in a mediterranean forest
Remote Sensing of Environment, 2009Co-Authors: A Goerner, Markus Reichstein, Serge RambalAbstract:article i nfo Gross primary productivity (GPP) changes occur at different time-scales and due to various mechanisms such as variations in leaf area, chlorophyll content, rubisco activity, and stomatal conductance. Diagnostic estimates of primary productivity are obviously error prone when these changes are not accounted for. Additional complications arise when factors inuencing a biome-specific maximum light use efficiency (LUE) must be estimatedoveralargearea.Inthesecasesadirectestimation ofecosystemLUEcouldreduceuncertaintyofGPP estimates. Here, we analyse whether a MODIS-based photochemical reectance index (PRI) is a useful proxy for the light use efficiency of a Mediterranean Quercus ilex forest.As the originally proposed Reference Band for PRI is not available on MODIS, we tested the Reference Bands 1 (620-670 nm), 4 (545-565 nm),12 (546-556 nm), 13 (662-672 nm), and 14 (673-683 nm) using different atmospheric correction algorithms. We repeated the analysis with different temporal resolutions of LUE (half-hourly to daily). The strongest correlation between LUE andPRIwas found when consideringonlya narrow range of viewingangles at atime (especially0-10° and 30-40°). We found that the MODIS-based PRI was able to track ecosystem LUE even during severe summer time water limitation. For this Mediterranean-type ecosystem we could show that a GPP estimation based on PRI is a huge improvement comparedtothe MODIS GPP algorithm. In this study, MODIS spectral Band 1 turned out to be the most suitable Reference Band for PRI, followed by the narrow red Bands 13 and 14. As to date no universallyapplicableReferenceBandwasidentifiedinMODIS-basedPRIstudies,weadvocatethoroughtesting for the optimal Band combination in future studies.
Serge Rambal - One of the best experts on this subject based on the ideXlab platform.
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remote sensing of ecosystem light use efficiency with modis based pri
Biogeosciences, 2011Co-Authors: A Goerner, Enrico Tomelleri, Niall P Hanan, Serge Rambal, D Dragoni, Dario Papale, Markus Reichstein, Christiane SchmulliusAbstract:Abstract. Several studies sustained the possibility that a photochemical reflectance index (PRI) directly obtained from satellite data can be used as a proxy for ecosystem light use efficiency (LUE) in diagnostic models of gross primary productivity. This modelling approach would avoid the complications that are involved in using meteorological data as constraints for a fixed maximum LUE. However, no unifying model predicting LUE across climate zones and time based on MODIS PRI has been published to date. In this study, we evaluate the effectiveness with which MODIS-based PRI can be used to estimate ecosystem light use efficiency at study sites of different plant functional types and vegetation densities. Our objective is to examine if known limitations such as dependence on viewing and illumination geometry can be overcome and a single PRI-based model of LUE (i.e. based on the same Reference Band) can be applied under a wide range of conditions. Furthermore, we were interested in the effect of using different faPAR (fraction of absorbed photosynthetically active radiation) products on the in-situ LUE used as ground truth and thus on the whole evaluation exercise. We found that estimating LUE at site-level based on PRI reduces uncertainty compared to the approaches relying on a maximum LUE reduced by minimum temperature and vapour pressure deficit. Despite the advantages of using PRI to estimate LUE at site-level, we could not establish an universally applicable light use efficiency model based on MODIS PRI. Models that were optimised for a pool of data from several sites did not perform well.
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tracking seasonal drought effects on ecosystem light use efficiency with satellite based pri in a mediterranean forest
Remote Sensing of Environment, 2009Co-Authors: A Goerner, Markus Reichstein, Serge RambalAbstract:article i nfo Gross primary productivity (GPP) changes occur at different time-scales and due to various mechanisms such as variations in leaf area, chlorophyll content, rubisco activity, and stomatal conductance. Diagnostic estimates of primary productivity are obviously error prone when these changes are not accounted for. Additional complications arise when factors inuencing a biome-specific maximum light use efficiency (LUE) must be estimatedoveralargearea.Inthesecasesadirectestimation ofecosystemLUEcouldreduceuncertaintyofGPP estimates. Here, we analyse whether a MODIS-based photochemical reectance index (PRI) is a useful proxy for the light use efficiency of a Mediterranean Quercus ilex forest.As the originally proposed Reference Band for PRI is not available on MODIS, we tested the Reference Bands 1 (620-670 nm), 4 (545-565 nm),12 (546-556 nm), 13 (662-672 nm), and 14 (673-683 nm) using different atmospheric correction algorithms. We repeated the analysis with different temporal resolutions of LUE (half-hourly to daily). The strongest correlation between LUE andPRIwas found when consideringonlya narrow range of viewingangles at atime (especially0-10° and 30-40°). We found that the MODIS-based PRI was able to track ecosystem LUE even during severe summer time water limitation. For this Mediterranean-type ecosystem we could show that a GPP estimation based on PRI is a huge improvement comparedtothe MODIS GPP algorithm. In this study, MODIS spectral Band 1 turned out to be the most suitable Reference Band for PRI, followed by the narrow red Bands 13 and 14. As to date no universallyapplicableReferenceBandwasidentifiedinMODIS-basedPRIstudies,weadvocatethoroughtesting for the optimal Band combination in future studies.
Christiane Schmullius - One of the best experts on this subject based on the ideXlab platform.
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remote sensing of ecosystem light use efficiency with modis based pri
Biogeosciences, 2011Co-Authors: A Goerner, Enrico Tomelleri, Niall P Hanan, Serge Rambal, D Dragoni, Dario Papale, Markus Reichstein, Christiane SchmulliusAbstract:Abstract. Several studies sustained the possibility that a photochemical reflectance index (PRI) directly obtained from satellite data can be used as a proxy for ecosystem light use efficiency (LUE) in diagnostic models of gross primary productivity. This modelling approach would avoid the complications that are involved in using meteorological data as constraints for a fixed maximum LUE. However, no unifying model predicting LUE across climate zones and time based on MODIS PRI has been published to date. In this study, we evaluate the effectiveness with which MODIS-based PRI can be used to estimate ecosystem light use efficiency at study sites of different plant functional types and vegetation densities. Our objective is to examine if known limitations such as dependence on viewing and illumination geometry can be overcome and a single PRI-based model of LUE (i.e. based on the same Reference Band) can be applied under a wide range of conditions. Furthermore, we were interested in the effect of using different faPAR (fraction of absorbed photosynthetically active radiation) products on the in-situ LUE used as ground truth and thus on the whole evaluation exercise. We found that estimating LUE at site-level based on PRI reduces uncertainty compared to the approaches relying on a maximum LUE reduced by minimum temperature and vapour pressure deficit. Despite the advantages of using PRI to estimate LUE at site-level, we could not establish an universally applicable light use efficiency model based on MODIS PRI. Models that were optimised for a pool of data from several sites did not perform well.
Pablo J Zarcotejada - One of the best experts on this subject based on the ideXlab platform.
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monitoring water stress and fruit quality in an orange orchard under regulated deficit irrigation using narrow Band structural and physiological remote sensing indices
Isprs Journal of Photogrammetry and Remote Sensing, 2012Co-Authors: Stavros Stagakis, Victoria Gonzalezdugo, Patricio Cid, M L Guillencliment, Pablo J ZarcotejadaAbstract:Abstract This paper deals with the monitoring of water status and the assessment of the effect of stress on citrus fruit quality using structural and physiological remote sensing indices. Four flights were conducted over a citrus orchard in 2009 using an unmanned aerial vehicle (UAV) carrying a multispectral camera with six narrow spectral Bands in the visible and near infrared. Physiological indices such as the Photochemical Reflectance Index (PRI 570 ), a new structurally robust PRI formulation that uses the 515 nm as the Reference Band (PRI 515 ), and a chlorophyll ratio (R 700 /R 670 ) were compared against the Normalized Difference Vegetation Index (NDVI), Renormalized Difference Vegetation Index (RDVI) and Modified Triangular Vegetation Index (MTVI) canopy structural indices for their performance in tracking water status and the effects of sustained water stress on fruit quality at harvest. The irrigation setup in the commercial orchard was compared against a treatment scheduled to satisfy full requirements (based on estimated crop evapotranspiration) using two regulated deficit irrigation (RDI) strategies. The water status of the trees throughout the experiment was monitored with frequent field measurements of stem water potential ( Ψ x ), while titratable acidity (TA) and total soluble solids (TSS) were measured at harvest on selected trees from each irrigation treatment. The high spatial resolution of the multispectral imagery (30 cm pixel size) enabled identification of pure tree crown components, extracting the tree reflectance from shaded, sunlit and aggregated pixels. The physiological and structural indices were then calculated from each tree at the following levels: (i) pure sunlit tree crown, (ii) entire crown, aggregating the within-crown shadows, and (iii) simulating a lower resolution pixel, including tree crown, sunlit and shaded soil pixels. The resulting analysis demonstrated that both PRI formulations were able to track water status, except when water stress altered canopy structure. In such cases, PRI 570 was more affected than PRI 515 by the structural changes caused by sustained water stress throughout the season. Both PRI formulations were proven to serve as pre-visual water stress indicators linked to fruit quality TSS and TA parameters ( r 2 = 0.69 for PRI 515 vs TSS; r 2 = 0.58 vs TA). In contrast, the chlorophyll (R 700 /R 670 ) and structural indices (NDVI, RDVI, MTVI) showed poor relationships with fruit quality and water status levels ( r 2 = 0.04 for NDVI vs TSS; r 2 = 0.19 vs TA). The two PRI formulations showed strong relationships with the field-measured fruit quality parameters in September, the beginning of stage III, which appeared to be the period most sensitive to water stress and the most critical for assessing fruit quality in citrus. Both PRI 515 and PRI 570 showed similar performance for the two scales assessed (sunlit crown and entire crown), demonstrating that within-crown component separation is not needed in citrus tree crowns where the shaded vegetation component is small. However, the simulation conducted through spatial resampling on tree + soil aggregated pixels revealed that the physiological indices were highly affected by soil reflectance and between-tree shadows, showing that for TSS vs PRI 515 the relationship dropped from r 2 = 0.69 to r 2 = 0.38 when aggregating soil + crown components. This work confirms a previous study that demonstrated the link between PRI 570 , water stress, and fruit quality, while also making progress in assessing the new PRI formulation (PRI 515 ), the within-crown shadow effects on the physiological indices, and the need for high resolution imagery to target individual tree crowns for the purpose of evaluating the effects of water stress on fruit quality in citrus.
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assessing structural effects on pri for stress detection in conifer forests
Remote Sensing of Environment, 2011Co-Authors: Rocio Hernandezclemente, Rafael M Navarrocerrillo, Lola Suarez, Fermin Morales, Pablo J ZarcotejadaAbstract:Abstract The retrieval of indicators of vegetation stress from remote sensing imagery is an important issue for the accurate assessment of forest decline. The Photochemical Reflectance Index (PRI) has been demonstrated as a physiological index sensitive to the epoxidation state of the xanthophyll cycle pigments and to photosynthetic efficiency, serving as a proxy for short-term changes in photosynthetic activity, stress condition, and pigment absorption, but highly affected by illumination conditions, viewing geometry and canopy structure. In this study, a diurnal airborne campaign was conducted over Pinus sylvestris and Pinus nigra forest areas with the Airborne Hyperspectral Scanner (AHS) to evaluate the effects of canopy structure on PRI when used as an indicator of stress in a conifer forest. The AHS airborne sensor was flown at two times (8:00 GMT and 12:00 GMT) over forest areas under varying field-measured stress levels, acquiring 2 m spatial resolution imagery in 80 spectral Bands in the 0.43–12.5 μm spectral range. Five formulations of PRI (based on R531 as a xanthophyll-sensitive spectral Band) were calculated using different Reference wavelengths, such as PRI570 (Reference Band RREF = R570), and the PRI modifications PRIm1 (RREF = R512), PRIm2 (RREF = R600), PRIm3 (RREF = R670), and PRIm4 (RREF = R570, R670), along with other structural indices such as NDVI, SR, OSAVI, MSAVI and MTVI2. In addition, thermal Bands were used for the retrieval of the land surface temperature. A radiative transfer modeling method was conducted using the LIBERTY and INFORM models to assess the structural effects on the PRI formulations proposed, studying the sensitivity of PRIm indices to detect stress levels while minimizing the effects caused by the conifer architecture. The PRI indices were related to stomatal conductance, xanthophyll epoxidation state (EPS) and crown temperature. The modeling analysis showed that the coefficient of variation (CV) for PRI was 50%, whereas the CV for PRIm1 (Band R512 as a Reference) was only 20%. Simulation and experimental results demonstrated that PRIm1 (RREF = R512) was less sensitive than PRI (RREF = R570) to changes in Leaf Area Index (LAI) and tree densities. PRI512 was demonstrated to be sensitive to EPS at both leaf (r2 = 0.59) and canopy level (r2 = 0.40), yielding superior performance than PRI570 (r2 = 0.21) at the canopy level. In addition, PRI512 was significantly related to water stress indicators such as stomatal conductance (Gs; r2 = 0.45) and water potential (Ψ; r2 = 0.48), yielding better results than PRI570 (Gs, r2 = 0.21; Ψ, r2 = 0.21) due to the structural effects found on the PRI570 index at the canopy level.