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The Brewer ozone spectrophotometer (the Brewer) is one of the World Meteorological Organization (WMO) Global Atmosphere Watch (GAW)'s standard ozone-monitoring instruments since the 1980s. The entire global Brewer ozone-monitoring network is operated and maintained via a hierarchical calibration chain, which started from world reference instruments that are independently calibrated via the primary calibration method (PCM) at a premium site (National Oceanic and Atmospheric Administration's (NOAA) Mauna Loa Observatory, Hawaii). These world reference instruments have been maintained by Environment and Climate Change Canada (ECCC) in Toronto for the last 4 decades. Their calibration is transferred to the travelling standard instrument and then to network (field) Brewer instruments at their monitoring sites (all via the calibration transfer method; CTM). Thus, the measurement accuracy for the entire global network is dependent on the calibration of world reference instruments. In 2003, to coordinate regional calibration needs, the Regional Brewer Calibration Center for Europe (RBCC-E) was formed in Izaña, Spain. From that point, RBCC-E began calibrating regional references also via PCM instead of CTM. The equivalency and consistency of world and regional references are then assured during international calibration campaigns. In practice, these two calibration methods have different physical requirements, e.g., the PCM requires a stable ozone field in the short term (i.e., half-day), while the CTM would benefit from larger changes in slant ozone conditions for the calibration periods. This difference dictates that the PCM can only be implemented on Brewer instruments at certain sites and even in certain months of the year. This work is the first effort to use long-term observation records from 11 Brewer instruments at four sites to reveal the challenges in performing the PCM. By utilizing a new calibration simulation model and reanalysis ozone data, this work also quantifies uncertainties in the PCM due to short-term ozone variability. The results are validated by real-world observations and used to provide scientific advice on where and when the PCM can be performed and how many days of observations are needed to achieve the calibration goal (i.e., ensure the calibration uncertainty is within a determined criterion, i.e., ≤5 R6 units; R6 is a measurement-derived double ratio in the actual Brewer processing algorithm). This work also suggests that even if the PCM cannot be used to deliver final calibration results for mid- or high-latitude sites, the statistics of the long-term PCM fitting results can still provide key information for field Brewer instruments as stability indicators (which would provide performance monitoring and data quality assurance).

Brewer spectrophotometers are one of the most widely used instruments for measuring the Total Ozone Column (TOC) in the world, which is obtained by measuring solar radiance at a set of UV sensible wavelengths. To date, the value of the uncertainty in these measures has not been obtained quantitatively. With this work, we have carried out an exhaustive study of the uncertainties that have affected the measure of TOC with data obtained during the first ATMOZ field campaign carried out between 12-25 September 2016 at the Izaña Atmospheric Observatory, Canary Islands, Spain at 2373 m.a.s.l., organized by the Spanish Meteorological Agency (AEMET) and the World Radiation Center (PMOD/WRC). For this, we have differentiated between three uncertainty components: related to the measure (dead time, filters, etc), model components (cross sections, etc) and atmospheric components (effective temperatures and heights, etc). The total uncertainty has been obtained through the propagation of errors of the different parameters, and the cross-correlations between the model and atmospheric components, using two different data sets. With the standard algorithm we have obtained the expected 2σ-uncertainty, around 2.4% for the three RBCC-E Triad Brewer double-monochromator spectrophotometers studied (Br157, Br183 and Br185) at noon and using the Extra-Terrestrial Constant (ETC) Langley calibration. On the other hand, for these same spectrophotometers, and using an updated algorithm the 2σ-uncertainty are reduced to values around 1.3 % in the TOC measurement. In first approximation, ignoring the cross-correlations the ozone absorption coefficient covers the most of total ozone uncertainty in both algorithms, followed by the ozone optical mass, the ETC and the measurement uncertainties.

In this study, we performed a diagnostic and evolutive analysis of the bioclimatology of the Canary Islands, an Atlantic archipelago where the climate itself is a main feature promoting tourism. Among all the tourist-climate indices described in the literature, we evaluated the most widely used, which is the Tourism Climate Index (TCI) proposed by Mieczkowski (1985). Monthly mean TCI time series were calculated using meteorological data from the Spanish State Meteorological Agency database and the European Climate Assessment and Dataset. Our results show TCI values greater than 50 during almost every month in the period 1950–2018, with mean values over the entire time series between 70 and 80. According to the TCI classification scheme, these values correspond to a very good thermal comfort along all of the period. Our results also point to spring as the season with the best TCI, with maximum values around 80 for this index in April—excellent according to the TCI classification. However, we did not find a correlation between inbound arrivals and the TCI index, which might point to a lack of information available to tourists. This opens an opportunity for policymakers and tour operators to better publicize the best seasons for holidays in the islands. 2019/UEM35 3.251 JCR (2020) Q2, 60/125 Environmental Studies 0.612 SJR (2020) Q2, 72/905 Energy Engineering and Power Technology No data IDR 2019 UEC

Editado por Alberto Redondas y Stoyka Netcheva El Centro Regional de Calibración Brewer para Europa (RBCC-E) en colaboración con el Área de Instrumentación e Investigación Atmosférica del Instituto Nacional de Técnica Aeroespacial (INTA), con el apoyo del programa de Vigilancia de la Atmósfera Global (VAG) de la Organización Meteorológica Mundial (OMM) y la acción COST EUBREWNET ES1207 organizaron la 12.ª campaña europea de intercomparación. En esta campaña, el RBCC-E transfirió al resto de instrumentos participantes su propia calibración absoluta de aparatos de medición de ozono obtenida mediante el método Langley en el Observatorio de Izaña. En la intercomparación participaron un total de 20 espectrofotómetros Brewer de nueve países. Estas campañas de intercomparación, que cuentan con una gran cantidad de instrumentos participantes, ofrecen una visión general del estado actual de las mediciones de ozono realizadas por la red Brewer europea. Los instrumentos se compararon con la referencia del RBCC-E, el Brewer#185, para la medición de ozono y con el patrón de referencia europeo de la unidad QASUME del Centro Radiométrico Mundial (CRM) para la medición de radiación UV.

Editado por Alberto Redondas y Stoyka Netcheva The twelfth European Brewer Intercomparison Campaign was organized by the Regional Brewer Calibration Center for Europe (RBCC-E) in collaboration with the “Area of Instrumentation and Atmospheric Research Area” of Instituto Nacional de Tecnica Aeroespacial (INTA), with the support of the Global Atmosphere Watch (GAW) Programme of the World Meteorological Organization (WMO) and the EUBREWNET ES1207 COST Action. Alongside the campaign, the ATMOZ (Traceability of Atmospheric Total Ozone) project workshop took place at the station. In addition, this event was also used as a demonstration exercise of the calibration methodologies and error assessment developed by the project (Gröbner et al., 2018).

Based at Izaña Observatory (~2400 m a.s.l. in Tenerife), we performed 1-h resolution measurements of elemental composition of dust in the Saharan Air Layer (SAL) and studied the variability of the ratios of these elements to aluminium (elemental ratios). In a period (~1 week) of continuous dust presence (50–200 μg/m), we observed rapid variations of dust composition; some elemental ratios changed by a factor 2 in a few (5–8) hours. The lowest variability (Normalized Variability Range, %) was found for Si/Al (9%) and Fe/Al (9%), followed by the ratios of K, Ti, Mg, Mn, Ca and Sr to Al (20–80%), and the highest for S/Al, Na/Al and Cl/Al (110–160%) and a number of trace metals (Cr, Cu, Ni, Zn, Zr) and Br (>200%). This variability was induced by the alternating impacts of three of the large North African dust sources: NE Algeria (rich in evaporite minerals bearing Ca, S, Sr, K and Mg and in illite mineral), Western Sahara to Bechar region (containing Na, S and Cl rich Yermosol soils) and SW Sahara – Western Sahel (rich in illite and hematite). We traced the variability in large-scale meteorology using the so-called North African Dipole Intensity (NAFDI: strength of the subtropical Saharan high -Morocco-to the monsoon tropical low -Nigeria-). The mobilization of dust from the different sources was associated with westward propagating Harmattan pulses linked to the change of phase of NAFDI (- to +), the associated westward shifts of the Saharan Heat Low and convective monsoon inflow. We found a correlation between dust composition in the SAL and NAFDI: moderate NAFDI values (0 to +2.5) were associated with Ca, K, Na, Mg and S rich dust linked to dust sources in NE Algeria, whereas higher NAFDI values (+2.5 to +4) were linked to Fe rich dust (Ca, Na and S depleted) linked to dust sources in SW Sahara – Western Sahel. The results of this study also show that some trace elements (Br, Cr, Ni, Zn and Zr) are influenced by industrial emissions into North Africa., This study was performed within the project VARDUST-SAL (PGC2018-099166-BI00), funded by the Ministry of Science, Research and Innovation of Spain, the Research State Agency of Spain and the European Regional Development Fund (ERDF). JLD is awarded with a posdoc contract Agustín de Bethencourt, funded by the Program Fomento de Transferencia del Conocimiento, funded by the Cabildo de Tenerife. A stage of JLD at the INFN was funded by the Universidad de La Laguna

Total ozone column can be measured using Brewer sprectrophotometers which are calibrated periodically in inter- comparison campaigns with respect to a reference instrument. In 2003 the Regional Brewer Calibration Centre for Europe (RBCC-E) was established at the Izaña Atmospheric Research Centre (Canary Islands, Spain) and from 2011 it has transferred its own calibration mainly to other European Brewers using the Brewer #185 as reference instrument. The RBCC-E organizes regular inter-comparisons which are held annually alternating between Arosa (Switzerland) and El Arenosillo (Spain). This work is focused on showing the stability of the measurements of the RBCC-E Triad (Brewers #157, #183 and #185) made in the Izaña Atmospheric Observatory during the period 2005–2016. In order to study the long-term precision of the RBCC-E Triad, it must be taken into account that each Brewer performs a large number of measurements every day and, hence, it be- comes necessary to calculate a representative value of all of them. This value was calculated from methods previously used to study the long-term behaviour of the World Reference and Arosa Triads. Applying their procedures in our triad allows us to compare the three instruments. In this way, the difference between the values calculated for each Brewer and the triad mean was analyzed. In daily averages, applying the procedure used for the World Triad Reference, the RBCC-E Triad presents a relative standard deviation mean equal to 0.41 % (σ157 = 0.362 %, σ183 = 0.453 % and σ185 = 0.428 %). In opposite, using the procedure of the Arosa Triad, the RBCC-E presents a relative standard deviation around at σ = 0.5 %. In monthly averages, the method of the World Triad Reference give a relative standard deviation of 0.33 %, 0.34 % and 0.23 % for Brewers #157, #183 and #185, respectively (0.3 % in mean). Whereas, the procedure of the Arosa Triad gives a monthly values 0.3 %. In this work, two ozone datasets are analyzed: the first included all the ozone measurements available while the second only includes the simultaneous measurements of all three instruments. Furthermore, in this paper we also describe the Langley method used in the RBCC-E Triad to calculate the Extra-terrestrial constant (ETC), which is the necessary first step to ozone retrieval. Finally, the short-term, or intraday, stability is also studied to identify the effect of the solar zenith angle on the accuracy of the RBCC-E Triad.

Ponencia presentada en: XXXV Jornadas Científicas de la AME y el XIX Encuentro Hispano Luso de Meteorología celebrado en León, del 5 al 7 de marzo de 2018. Los espectrofotómetros Brewers tienen como principal tarea medir la columna total de ozono (TOC). Estos instrumentos son calibrados respecto a instrumentos de referencia viajeros como el Brewer #017, operado por IOS y calibrado frente a la triada de Canadá o, más reciente, con respecto al Brewer #185 que forma parte de la Triada del Centro de Calibración Regional Brewer para Europa (RBCC-E) situado en el Observatorio Atmosférico de Izaña y perteneciente a la Agencia Estatal de Meteorología (AEMET). En este trabajo se pretende, en primer lugar, mostrar cómo es el ciclo anual del ozono estratosférico en las Islas Canarias, indicando su período de máxima y mínima concentración y, en segundo lugar, mostrar el grado de acuerdo entre las medidas de ozono llevadas a cabo por los tres instrumentos, Brewer #157, #183 y #185 que forman la triada RBCC-E. En este sentido, y teniendo en cuenta que cada Brewer realiza a lo largo del día un elevado número de observaciones, se visto adecuado calcular su valor medio. En este trabajo se han empleado dos conjuntos de datos para evaluar el grado de acuerdo entre los tres instrumentos. El primer conjunto de datos incluye todas las medidas hechas por los Brewers mientras el segundo sólo considera las medidas que de manera simultánea han sido realizadas por los tres instrumentos. Los resultados indican que este segundo conjunto de datos muestra una mejor estimación del acuerdo existente entre los tres Brewers. No obstante, ambos conjuntos indican que la desviación entre los valores medios diarios de los tres Brewers #157, #183 y #185 que forman la RBCC-E es ~0.5%.

Structural, elastic and electronic properties of zircon-type and scheelite-type EuVO4 are investigated experimentally, by in-situ X-ray diffraction using synchrotron radiation, and theoretically within the framework of the density functional theory (DFT) and using the PBE prescription of the exchange-correlation energy. This study was motivated by the fact that the previous knowledge of the equation of state (EOS) was inconclusive due to a large scatter of the experimental and theoretical data, and by the lack of information on the dependence of the electronic structure with pressure. Under the applied experimental conditions, the zircon-type structure transforms to a scheelite-type one at 7.4(2) GPa, whereas the calculations yield a lower zircon-scheelite-coexistence pressure of 4.8 GPa. The experimental part of the study shows that the bulk modulus of the zircon-type phase is 119(3) GPa, perfectly supported by the DFT-calculated value, 119.1 GPa. The bulk modulus for the scheelite-type polymorph is higher, with an experimental value of 135(7) GPa and a theoretical one of 137.4 GPa. Compared to those reported in previous experimental and DFT or semiempirical works, the present values for the zircon-type phase are comparable or slightly lower, whereas those for the scheelite-type phase are markedly lower. Discrepancies between the present results and earlier reported ones are attributed to differences in details of the experimental method such as the pressure transmitting medium and the pressure calibration method. The calculated band structure confirms that zircon-type EuVO4 is a direct-gap semiconductor, with a bandgap energy at zero pressure of 2.88 eV. Under compression, the bandgap of the zircon phase increases with a coefficient of 10.3 meV/GPa up to the transition pressure, at which point the present calculations show a small drop of the bandgap energy. Above the transition pressure, the bandgap energy of the scheelite phase becomes almost constant, with a small pressure coefficient of just 1.5 meV/GPa. (C) 2015 Elsevier B.V. All rights reserved. (Less)

The high spatial and temporal variability of aerosols make networks capable of measuring their properties in near real time of high scientific interest. In this work we present and discuss results of an aerosol optical depth algorithm to be used in the European Brewer Network, which provides data in near real time of more than 30 spectrophotometers located from Tamanrasset (Algeria) to Kangerlussuaq (Greenland). Using data from the Brewer Intercomparison Campaigns in the years 2013 and 2015, and the period in between, plus comparisons with Cimel sunphotometers and UVPFR instruments, we check the precision, stability, and uncertainty of the Brewer AOD in the ultraviolet range from 300 to 320 nm. Our results show a precision better than 0.01, an uncertainty of less than 0.05, and a stability similar to that of the ozone measurements for well-maintained instruments. We also discuss future improvements to our algorithm with respect to the input data, their processing, and the characterization of the Brewer instruments for the measurement of aerosols.

Lanthanum orthovanadate (LaVO4) is the only stable monazite-type rare-earth orthovanadate. In the present paper the equation of state of LaVO4is studied usingin situhigh-pressure powder diffraction at room temperature, andab initiocalculations within the framework of the density functional theory. The parameters of a second-order Birch–Murnaghan equation of state,i.e.those fitted to the experimental and theoretical data, are found to be in perfect agreement – in particular, the bulk moduli are almost identical, with values of 106 (1) and 105.8 (5) GPa, respectively. In agreement with recent reported experimental data, the compression is shown to be anisotropic. Its nature is comparable to that of some other monazite-type compounds. The softest compression direction is determined.

The ferroic phase transition in LaEr(MoO4)3 has been analyzed for the first time. It has been confirmed that this compound undergoes a phase transition from a tetragonal system (paraelectric-paraelastic phase), with space group P-421m [β-Gd2(MoO4)3 averaged phase] to an orthorhombic system (ferroelectric-ferroelastic phase), with space group Pba2 [β'-Gd2(MoO4)3 phase] in a reversible process. This phenomenon, together with the observed demixing at high temperature has been studied using different techniques. LaEr(MoO4)3 samples have been obtained by the conventional solid-state synthesis. The thermal dependence of the crystal structure was studied by powder X-ray and neutron diffraction, following a new refining procedure in which the symmetry modes of atomic displacements from the paraelectric-paraelastic structure were analyzed. Dielectric spectroscopy measurements have confirmed the structural results, showing a very smooth phase transition. Finally, calculations within the framework of Density Functional Theory show a behavior of the lattice parameters similar to that observed in our experiments.

We report on Raman scattering measurements in mercury digallium selenide (HgGa2Se4) up to 25 GPa. We also performed, for the low-pressure defect-chalcopyrite structure, lattice-dynamics ab initio calculations at high pressures which agree with experiments. Measurements evidence that the semiconductor HgGa2Se4 exhibits a pressure-induced phase transition above 19 GPa to a previously undetected structure. This transition is followed by a transformation to a Raman-inactive phase above 23.4 GPa. On downstroke from 25 GPa until 2.5 GPa, a broad Raman spectrum was observed, which has been attributed to a fourth phase, and whose pressure dependence was followed during a second upstroke. Candidate structures for the three phases detected under compression are proposed. Finally, we also report and discuss the decomposition of the sample by laser heating at pressures close to 19 GPa. As possible products of decomposition, we have identified at least the formation of trigonal selenium nanoclusters and cinnabar-type HgSe., This study was supported by the Spanish government MEC under Grant No. MAT2010-21270-004-01/03/04, by MALTA Consolider Ingenio 2010 project (CSD2007-00045), by Generalitat Valenciana through project GVA-ACOMP-2013-012, and by the Vicerrectorado de Investigacion y Desarrollo of the Universidad Politecnica de Valencia (UPV2011-0966 and UPV2011-0914). E.P.-G., J.L.-S., A.M., and P.R.-H. acknowledge computing time provided by Red Espanola de Super-computacion (RES) and MALTA-Cluster.

Every summer huge amounts of desert dust particles are exported from the hyperarid subtropical Sahara to the North Atlantic the so-called Saharan Air Layer (SAL), a dry, warm and dust-laden corridor that expands from the North African coast (1-5 km.a.s.l.) to the Americas above the marine boundary layer. Because of the potential impact of the dust deposited on the ocean on marine biogeochemistry and climate, we studied the Fe solubility (in seawater) of atmospheric aerosols samples directly collected in the SAL off the North African coast, i.e. the fresh aerosols recently exported from the Sahara in the SAL. The aerosol sampling was performed at ~2400 m.a.s.l. in Izaña observatory in Tenerife island. In the total aerosols, we found low Fe concentrations and high fractional Fe solubility (FFS ~2%) in the North Atlantic free troposphere airflows and high Fe concentrations and low FFS (~0.7%) within the SAL; the resulting FFS versus total dust (or total Fe) plot shows a hyperbolic trend attributed to the conservative mixing of 'fine combustion aerosols' and 'lithogenic mineral dust'. We then focused on the soluble Fe in the SAL. Our results indicate that ~70% of soluble Fe is associated with the dissolution of submicron dust particles, probably involving Fe-bearing clays. We found a FFS of submicron dust (~6%) higher than that typically observed in submicron particles of soil dust samples (, This study is part of the project AEROATLAN – 2015-66229 -, funded by the Ministry of Economy and Competitiveness of Spain . M.I. García holds a research grant – TESIS20120054 – co-funded by the Canarian Agency for Research, Innovation and Information Society and the European Social Fund . Chemical analysis, of the long-term chemical composition of aerosols GAW program at Izaña, are performed at the IDÆA-CSIC funded by AEMET . The authors are grateful to A. Stohl for providing FLEXTRA model and to J.J. Bustos (AEMET) for computing back trajectories. Daily, 1x1 degree, MODIS-Aqua data (level 3, collection 5.1) was downloaded from the Giovanni online data system, developed and maintained by the NASA GES DISC. We also thank the brotherhood of fishermen of La Punta del Hidalgo (Cofradía de Pescadores de La Punta del Hidalgo) for collecting offshore seawater and both the Research Support General Service (SEGAI) and the research group of Chemical Analysis Applied to the Industry, Environment and Agroalimentary Products of the University of La Laguna for the laboratory facilities. Appendix A

The cationic ordering in the octahedral and tetrahedral sheets of Al-rich synthetic phlogopites with composition K(Mg3− x Al x )[Al1+ x Si3− x O10](OH)2 (0.0 < x < 1.0) has been investigated using a combined approach of Monte Carlo simulations based on the “ J formalism” (Bosenick et al. 2001; Warren et al. 2001) and 1H, 29Si MAS and {1H} → 29Si CPMAS/HETCOR solid-state NMR spectroscopic experiments. Our results are compatible with the well-established Loewenstein’s rule of Al-Al avoidance in the tetrahedral sheets, but the Si/IVAl and Mg/VIAl distributions show a tendency to segregation of the excess Al atoms, and we observe a preference for VIAl and IVAl to occupy directly neighbored octahedral and tetrahedral sites. As a result the structure is separated into clusters of original phlogopite composition {K(Mg3)[AlSi3O10](OH)2} and clusters of “eastonite” composition {K(Mg2Al)[Al2Si2O10](OH)2} that encompass a whole T-O-T layer package, although Al is solved in the phlogopite structure homogeneously on a macroscopic level.

DFT calculations have been carried out for Cu4Bi5S10 and Bi2S3 to provide an analysis of the relation between electronic structure, lone electron pairs and the local geometry. The effect of pressure is considered in Bi2S3 and the results are compared to published experimental data. Bi 3+ in Cu4Bi5S10 is found at both symmetrically and asymmetrically coordinated sites, whereas the coordination environments of Bi in Bi2S3 are asymmetric at room conditions and get more regular with increasing pressure. The charge density maps of the asymmetric sites show the lone pairs as lobes of non-shared charge. These lobes are related to an effective Bi s-Bi p hybridization resulting from coupling to S p orbitals, supporting the modern view of the origin of the stereochemically active lone pair. No effective Bi s-p hybridization is seen for the symmetric site in Cu4Bi5S10, whereas Bi s-p hybridization coexists with a much reduced lone pair in Bi2S3 at high pressure.

We use ab initio theoretical methods to study the relative stability under pressure of the zircon, scheelite and fergusonite structures in the LuVO4 and ScVO4 orthovanadates. In LuVO4, our study confirms the zircon→scheelite→fergusonite sequence of stable phases proposed in previous experimental works. In ScVO4, we find a zircon-to-scheelite phase transition in agreement with experiments. At higher pressures, our calculations show that the scheelite phase becomes unstable with respect to the fergusonite phase, although no second phase transition has been observed in experiments.

We present the results of a theoretical study on the structural stability of InAs under high pressure, with a focus on the pressure regime past the first transition from the low-pressure zincblende phase to the rock-salt (NaCl-type) phase. We have examined several candidates proposed for low-enthalpy phases at higher pressures, among them the super-Cmcm structure observed in InSb at moderately high pressure and the B10 structure, which is a distortion of the CsCI structure. Our results reproduce the established sequence of transitions and shed further light on the higher pressure regime of this material.

Recent experimental measurements and ab initio calculations in scheelite CaWO 4 , SrWO 4 , BaWO 4 , PbWO 4 , EuWO 4 and YLiF4 crystals reveal the existence of complex high-pressure phase diagrams, which present striking differences but also relevant similarities. In this work we show that the high-pressure structural sequence in the studied scheelites can be understood on the basis of the positions of the different ABX4 compounds in Fukunaga and Yamaoka's diagram and in Bastide's diagram. Our study can help to understand the phase diagrams and high-pressure phase transitions occurring in ABX4 compounds with scheelite, wolframite, fergusonite, zircon, or pseudoscheelite structures.

The structural properties of CaWO{sub 4}, SrWO{sub 4}, BaWO{sub 4}, PbWO{sub 4}, and EuWO{sub 4} scintillating crystals under pressure have been studied by X-ray powder diffraction, X-ray absorption near-edge structure measurements, Raman spectroscopy, and ab initio density functional theory calculations. The results obtained from these studies will be reviewed here and their differences and similitudes discussed.

We have studied the relationship between the long-term interannual variability in large-scale meteorology in western North Africa – the largest and most active dust source worldwide – and Saharan dust export in summer, when enhanced dust mobilization in the hyper-arid Sahara results in maximum dust impacts throughout the North Atlantic. We address this issue by analyzing 28 years (1987–2014) of summer averaged dust concentrations at the high-altitude Izaña observatory (~ 2400 m a.s.l.) on Tenerife, and satellite and meteorological reanalysis data. The summer meteorological scenario in North Africa (aloft 850 hPa) is characterized by a high over the the subtropical Sahara and a low over the tropics linked to the monsoon. We measured the variability of this high–low dipole-like pattern in terms of the North African dipole intensity (NAFDI): the difference of geopotential height anomalies averaged over the subtropics (30–32° N, Morocco) and the tropics (10–13° N, Bamako region) close to the Atlantic coast (at 5–8° W). We focused on the 700 hPa standard level due to dust export off the coast of North Africa tending to occur between 1 and 5 km a.s.l. Variability in the NAFDI is associated with displacements of the North African anticyclone over the Sahara and this has implications for wind and dust export. The correlations we found between the 1987–2014 summer mean of NAFDI with dust at Izaña, satellite dust observations and meteorological re-analysis data indicate that increases in the NAFDI (i) result in higher wind speeds at the north of the Inter-Tropical Convergence Zone that are associated with enhanced dust export over the subtropical North Atlantic, (ii) influence the long-term variability of the size distribution of exported dust particles (increasing the load of coarse dust) and (iii) are associated with enhanced rains in the tropical and northern shifts of the tropical rain band that may affect the southern Sahel. Interannual variability in NAFDI is also connected to spatial distribution of dust over the North Atlantic; high NAFDI summers are associated with major dust export (linked to winds) in the subtropics and minor dust loads in the tropics (linked to higher rainfall), and vice versa. The evolution of the summer NAFDI values since 1950 to the present day shows connections to climatic variability (through the Sahelian drought, ENSO (El Niño–Southern Oscillation) and winds) that have implications for dust export paths. Efforts to anticipate how dust export may evolve in future decades will require a better understanding of how the large-scale meteorological systems represented by the NAFD will evolve. The Izaña GAW program is funded by AEMET and by the Minister of Economy and Competitiveness of Spain (POLLINDUST, CGL2011-26259) J. M. Prospero’s research is supported by NSF grant AGS-0962256 5.114 JCR (2015) Q1, 6/84 Meteorology and atmospheric sciences UEC

In this paper, we combine a theoretical study of the structural phases of CaWO 4 and SrWO 4 under high pressure along with the results of angle-dispersive X-ray diffraction (ADXRD) and X-ray absorption near-edge structure (XANES) measurements of both tungstates up to approximately 20 GPa. The theoretical study was performed within the ab initio framework of the density functional theory (DFT) using a plane-wave basis set and the pseudopotential scheme, with the generalized gradient approximation (GGA) for the exchange and correlation contribution to the energy. Under normal conditions, CaWO 4 and SrWO 4 crystallize in the scheelite structure. Our results show that in a hydrostatic environment, both compounds undergo a scheelite-to-fergusonite phase transition with increasing pressure. We present a comparison of the evolution of the structural parameters, equation of state, and of the features of the transition, finding an overall excellent agreement between the experimental and theoretical results.

Recent experimental works on high-pressure X-ray diffraction have shown that YLiF 4 undergoes a reversible phase transition around 10 GPa from the ambient pressure scheelite-type to a fergusonite-type structure. Different theoretical works have proposed either the M-fergusonite or M′-fergusonite as this second stable structure. Also, small changes in photoluminescence and Raman signals have been observed around 6 GPa, and have not been completely explained yet. In this work we present a first-principles Density Functional Theory structural study of YLiF 4 . We found that the anomalies at 6 GPa can be related to the structural changes observed in a scheelite to M′-fergusonite transition. We also have studied briefly different possibilities for an experimentally observed but not completly characterised third stable structure above 17 GPa.

The structural phase transformations under pressure of ZnS are studied within the density functional formalism, using the local density approximation and pseudopotential scheme. We show results in agreement with the well-established zinc-blende and NaCl phases present at low pressures, and also confirm a recently proposed Cmcm structure found experimentally by Desgreniers et al. [Phys. Rev. B 61, 8726 (2000)] to be more stable than NaCl at pressures higher than 65 GPa.

We acknowledge the financial support of the Spanish Ministerio de Economia y Competitividad under Grants MAT2010-21270-C04-02/03/04, CTQ2009-14596-C02-01, CSD2007-00045 and the Comunidad de Madrid and European Social Fund S2009/PPQ-1551-4161893. Access to the MALTA Cluster Computer (Universidad de Oviedo), the Atlante Super-computer (Instituto Tecnologico de Canarias, Red Espanola de Supercomputacion), and the MALTA Xcalibur Diffractometer (Universidad Complutense de Madrid) is gratefully acknowledged. C. G. A. wishes to thank the Agencia Canaria de Investigacion, Innovacion y Sociedad de la Informacion, and the European Social Fund of the Gobierno de Canarias for a fellowship. J.A.S. acknowledges financial support through the Juan de la Cierva fellowship program.

Dysprosium orthovanadate, DyVO4, belongs to a family of zircon-type orthovanadates showing a phase transition to scheelite-type structures at moderate pressures below 10 GPa. In the present study, the equations of state (EOSs) for both these phases were determined for the first time using high-pressure x-ray diffraction experiments and ab initio calculations based on the density functional theory. Structural parameters for scheelite-type DyVO4 were calculated from x-ray powder diffraction data as well. The high-pressure experiments were performed under pseudo-hydrostatic conditions at pressures up to 8.44 GPa and 5.5 GPa for the stable zircon-type and metastable (quenched) scheelite-type samples, respectively. Assuming as a compression model the Birch–Murnaghan EOS, we obtained the EOS parameters for both phases. The experimental bulk moduli (K0) for zircon-type and scheelite-type DyVO4 are 118(4) GPa and 153(6) GPa, respectively. Theoretical equations of state were determined by ab initio calculations using the PBE exchange–correlation energy functional of Perdew, Burke, and Ernzerhof. These calculations provide K0 values of 126.1 GPa and 142.9 GPa for zircon-type and scheelite-type DyVO4, respectively. The reliability of the present experimental and theoretical results is supported by (i) the consistency between the values yielded by the two methods (the discrepancy in K0 is as low as about 7% for each of the studied polymorphs) and (ii) their similarity to results obtained under similar compression conditions (hydrostatic or pseudo-hydrostatic) for other rare-earth orthovanadates, such as YVO4 and TbVO4.

We investigate the electronic structure of CuScO 2 thin films grown on sapphire and mica substrates by pulsed laser deposition. X-ray diffraction and microanalysis confirm that the films have the expected delafossite crystal structure and stoichiometric proportions. The electronic structure is investigated by means of X-ray and ultraviolet photoelectron spectroscopy. Electronic states in the range 0–1350 eV are identified, making reference to theoretical density-of-states calculations up to 80 eV. Photoelectron spectra near the Fermi energy confirm the p-character of the films. Optical absorption spectroscopy shows that the films are transparent up to 3.7 eV and exhibit an intense excitonic peak, with a direct gap energy of 4.24 ± 0.05 eV at room temperature. Ab initio band structure calculations confirm the direct character of CuScO 2 and allow for an assignment of the direct gap to an electronic transition at the L point of the rhombohedral Brillouin zone.

We report on high-pressure x-ray diffraction measurements up to 17.2 GPa in mercury digallium selenide (HgGa2Se4). The equation of state and the axial compressibilities for the low-pressure tetragonal phase have been determined and compared to related compounds. HgGa2Se4 exhibits a phase transition on upstroke toward a disordered rock-salt structure beyond 17 GPa, while on downstroke it undergoes a phase transition below 2.1 GPa to a phase that could be assigned to a metastable zinc-blende structure with a total cation-vacancy disorder. Thermal annealing at low- and high-pressure shows that kinetics plays an important role on pressure-driven transitions., This study was supported by the Spanish government MEC under grants nos: MAT2010-21270-C04-01/03/04 and CTQ2009-14596-C02-01, by the Comunidad de Madrid and European Social Fund (S2009/PPQ-1551 4161893), by MALTA Consolider Ingenio 2010 project (CSD2007-00045), and by the Vicerrectorado de Investigacion y Desarrollo of the Universidad Politecnica de Valencia (UPV2011-0914 PAID-05-11 and UPV2011-0966 PAID-06-11). E.P.-G., J.L.-S., A.M., and P.R.-H. acknowledge computing time provided by Red Espanola de Supercomputacion (RES) and MALTA-Cluster.

In this work, we focus on the study of the structural and elastic properties of mercury digallium selenide (HgGa2Se4) which belongs to the family of AB 2 X 4 ordered-vacancy compounds with tetragonal defect chalcopyrite structure. We have carried out high-pressure x-ray diffraction measurements up to 13.2¿GPa. Our measurements have been complemented and compared with total-energy ab initio calculations. The equation of state and the axial compressibilities for the low-pressure phase of HgGa2Se4 have been experimentally and theoretically determined and compared to other related ordered-vacancy compounds. The theoretical cation-anion and vacancy-anion distances in HgGa2Se4 have been determined. The internal distance compressibility in HgGa2Se4 has been compared with those that occur in binary HgSe and ¿¿GaSe compounds. It has been found that the Hg-Se and Ga-Se bonds behave in a similar way in the three compounds. It has also been found that bulk compressibility of the compounds decreases following the sequence ¿¿-GaSe¿>¿HgGa2Se4¿>¿HgSe.¿ Finally, we have studied the pressure dependence of the theoretical elastic constants and elastic moduli of HgGa2Se4. Our calculations report that the low-pressure phase of HgGa2Se4 becomes mechanically unstable above 13.3 GPa.. © 2013 American Institute of Physics

The electronic band-structure and band-gap dependence on the $d$ character of ${A}^{2+}$ cation in $A$WO${}_{4}$ wolframite-type oxides is investigated for different compounds ($A$ $=$ Mg, Zn, Cd, and Mn) by means of optical-absorption spectroscopy and first-principles density-functional calculations. High pressure is used to tune their properties up to 10 GPa by changing the bonding distances establishing electronic to structural correlations. The effect of unfilled $d$ levels is found to produce changes in the nature of the band gap as well as its pressure dependence without structural changes. Thus, whereas Mg, Zn, and Cd, with empty or filled $d$ electron shells, give rise to direct and wide band gaps, Mn, with a half-filled $d$ shell, is found to have an indirect band gap that is more than 1.6 eV smaller than for the other wolframites. In addition, the band gaps of MgWO${}_{4}$, ZnWO${}_{4}$, and CdWO${}_{4}$ blue-shift linearly with pressure, with a pressure coefficient of approximately 13 eV/GPa. However, the band gap of multiferroic MnWO${}_{4}$ red-shifts at \ensuremath{-}22 meV/GPa. Finally, in MnWO${}_{4}$, absorption bands are observed at lower energy than the band gap and followed with pressure based on the Tanabe-Sugano diagram. This study allows us to estimate the crystal-field variation with pressure for the MnO${}_{6}$ complexes and how it affects their band-gap closure.

Thulium phosphate (TmPO${}_{4}$) with the tetragonal zircon-type structure has been studied using angle-dispersive powder x-ray diffraction and Raman spectroscopy in a diamond anvil cell up to 55 GPa (at room temperature). The results from both experimental methods reveal a pressure-induced structural phase transition from zircon to a scheelite-type structure at 20 GPa. At the same pressure, a second phase with the monazite structure is also formed at a low concentration, which decreases and eventually vanishes with increasing pressure. Our ab initio total-energy and lattice-dynamics calculations are in good agreement with experimental findings. The calculations indicate that the main transition to the scheelite structure is related with the softening of a silent ${B}_{1u}$ mode. Upon decompression, TmPO${}_{4}$ reverses back to the original zircon structure showing significant hysteresis. The results are discussed in relation to the observed general structural systematics and phase transition sequences in zircon-type $A$PO${}_{4}$ orthophosphates under pressure.

We have performed experimental and theoretical studies of the structural stability of YAsO4 and YCrO4 at high pressures. X-ray diffraction experiments together with ab initio total-energy and lattice-dynamics calculations have allowed us to completely characterize a pressure-induced structural phase transition from the zircon to the scheelite structure in both compounds. Furthermore, total-energy calculations have been performed to check the relative stabilities of different candidate structures at different pressures and allow us to propose for YAsO4 the zircon \rightarrow scheelite \rightarrow SrUO4-type sequence of structures. In this sequence, sixfold arsenic coordination is attained for the SrUO4-type structure above 32 GPa. The whole sequence of transitions is discussed in comparison with YVO4, YPO4, YNbO4, YMoO4, and YTaO4. Also a comparative discussion of lattice-dynamics properties is presented. The band-gap for YAsO4 and YCrO4 and band structure for YAsO4 are also reported. Finally, the room-temperature equation of state of different compounds is also obtained.

HgWO 4 at ambient pressure is characterized using a combination of ab initio calculations, X-ray diffraction and Raman scattering measurements. The effect of low pressure and temperature on the structural stability is analysed. Extending our ab initio study to the range of higher pressures, a sequence of stable phases up to 30GPa is proposed. © 2011 Taylor & Francis., We thank J.M. Menendez for his help in the use of the GIBBS code. This work has been supported by the Spanish MEC under Projects MAT2007-65990-C03-01/03, MAT2010-21270-C04-03/04 and CSD-2007-00045 and by the "Vicerrectorado de Innovacion y Desarrollo de la UPV" (PAID-05-2009 through project UPV2010-0096). We gratefully acknowledge computational time provided by the "Red Espanola de Supercomputacion" at the supercomputer "Atlante". S. R. acknowledges financial support from the "Vicerrectorado de Investigacion de la UPV" through grant PAID-02-09-3085.

We report results of experimental and theoretical lattice-dynamics studies on scandium orthovanadate up to 35 GPa. Raman-active modes of the low-pressure zircon phase are measured up to 8.2 GPa, where the onset of an irreversible zircon-to-scheelite phase transition is detected. Raman-active modes in the scheelite structure are observed up to 16.5 GPa. Beyond 18.2 GPa we detected a gradual splitting of the ${E}_{g}$ modes of the scheelite phase, indicating the onset of a second phase transition. Raman symmetries, frequencies, and pressure coefficients in the three phases of ScVO${}_{4}$ are discussed in the light of ab initio lattice-dynamics calculations that support the experimental results. The results on all the three phases of ScVO${}_{4}$ are compared with those previously reported for related orthovanadates.

We have studied by means of high-pressure x-ray diffraction the structural stability of ${\text{Ni}}_{2}{\text{Mo}}_{3}\text{N}$, ${\text{Co}}_{3}{\text{Mo}}_{3}\text{N}$, and ${\text{Fe}}_{3}{\text{Mo}}_{3}\text{N}$. We also report ab initio computing modeling of the high-pressure properties of these compounds, ${\text{Pd}}_{2}{\text{Mo}}_{3}\text{N}$ and ${\text{Pt}}_{2}{\text{Mo}}_{3}\text{N}$. We have found that the nitrides remain stable in the ambient-pressure cubic structure at least up to 50 GPa and determined their equation of state. All of them have a bulk modulus larger than 300 GPa. Single-crystal elastic constants have been calculated in order to quantify the stiffness of the investigated nitrides. We found that they should have a Vickers hardness similar to that of cubic spinel nitrides such as $\ensuremath{\gamma}{\text{-Si}}_{3}{\text{N}}_{4}$.

We have synthesized monoclinic mercury tungstate $({\text{HgWO}}_{4})$ and characterized its structural and vibrational properties at room conditions. Additionally, we report the structural and lattice dynamical behavior of ${\text{HgWO}}_{4}$ under high pressure studied by means of x-ray diffraction and Raman-scattering measurements up to 16 GPa and 25 GPa, respectively. The pressure dependence of the structural parameters and Raman-active first-order phonons of monoclinic $C2/c$ ${\text{HgWO}}_{4}$ are discussed in the light of our theoretical first-principles total-energy and lattice dynamics calculations. Our measurements show that the monoclinic phase of ${\text{HgWO}}_{4}$ is stable up to the maximum pressure reached in these experiments. Theoretically predicted possible high-pressure phases and the theoretical pressure dependence of infrared-active modes are also reported.

We have performed a theoretical and experimental study of the structural stability of terbium phosphate at high pressures. Theoretical ab initio total-energy and lattice-dynamics calculations together with x-ray diffraction experiments have allowed us to completely characterize a phase transition at {approx}9.8 GPa from the zircon to the monazite structure. Furthermore, total-energy calculations have been performed to check the relative stability of 17 candidate structures at different pressures and allow us to propose the zircon {yields} monazite {yields} scheelite {yields} SrUO{sub 4}-type sequence of stable structures with increasing pressure. In this sequence, sixfold P coordination is attained for the SrUO{sub 4}-type structure above 64 GPa. The whole sequence of transitions is discussed in association with the high-pressure structural behavior of oxides isomorphic to TbPO{sub 4}.

In this work we report high-pressure (HP) and high-temperature (HT) ex situ and in situ experiments in ${\text{BaWO}}_{4}$. Starting from powder samples of ${\text{BaWO}}_{4}$, scheelite structure $(I{4}_{1}/a)$, we reached conditions of 2.5--5.5 GPa and 400--1100 K using a Paris-Edinburgh press. The quenched samples were characterized by x-ray diffraction and Raman measurements at ambient conditions. Depending upon the final $P\text{\ensuremath{-}}T$ conditions we found either the scheelite or the monoclinic ${\text{BaWO}}_{4}$-II $(P{2}_{1}/n)$ structure. We also performed HP-HT in situ Raman measurements in a single crystal of ${\text{BaWO}}_{4}$ using a resistive-heated diamond-anvil cell. The transition from scheelite to the ${\text{BaWO}}_{4}$-II phase was observed at 5 GPa for $T=621\text{ }\text{K}$. Ab initio lattice-dynamics calculations have been performed in order to characterize the vibrations of the ${\text{BaWO}}_{4}$-II phase. Finally we carried out in situ powder angle-dispersive x-ray diffraction synchrotron measurements on ${\text{BaWO}}_{4}$ compound following different $P\text{\ensuremath{-}}T$ paths, extending its measured phase diagram in the 2--6 GPa and 300--2000 K range.

Tetragonal scheelite-type CaWO4 nanocrystals recently prepared by a hydrothermal method show an enhancement of its structural symmetry with the decrease in nanocrystal size. The analysis of the volume dependence of the structural parameters in CaWO4 nanocrystals with the help of ab initio total-energy calculations shows that the enhancement of the symmetry in the scheelite-type nanocrystals is a consequence of the negative pressure exerted on the nanocrystals; i.e., the nanocrystals are under tension. Besides, the behavior of the structural parameters in CaWO4 nanocrystals for sizes below 10 nm suggests an onset of a scheelite-to-zircon phase transformation in good agreement with the predictions from our ab initio calculations. CaWO4 nanocrystals exhibit a reconstructive-type mechanism for the scheelite-to-zircon phase transition that seems to follow the tetragonal path that links both structures. This result is in contrast with the mechanism recently proposed for this transition in bulk ZrSiO4 where the transition goes through an intermediate monoclinic phase. Daniel.Errandonea@uv.es

The compression process in the α-phase of europium trimolybdate was revised employing several experimental techniques. X-ray diffraction (using synchrotron and laboratory radiation sources), Raman scattering and photoluminescence experiments were performed up to a maximum pressure of 21 GPa. In addition, the crystal structure and Raman mode frequencies have been studied by means of first-principles density functional based methods. Results suggest that the compression process of α-Eu2(MoO4)3 can be described by three stages. Below 8 GPa, the α-phase suffers an isotropic contraction of the crystal structure. Between 8 and 12 GPa, the compound undergoes an anisotropic compression due to distortion and rotation of the MoO4 tetrahedra. At pressures above 12 GPa, the amorphization process starts without any previous occurrence of a crystalline-crystalline phase transition in the whole range of pressure. This behavior clearly differs from the process of compression and amorphization in trimolybdates with β′-phase and tritungstates with α-phase., We thank Diamond Light Source for access to beamline I15 (EE1746) that contributed to the results presented here. Part of the diffraction measurements were performed at the 'Servicio Integrado de Difraccion de Rayos X (SIDIX)' of University of La Laguna. This work has been supported by Ministerio de Economia y Competitividad of Spain (MINECO) for the research projects through the National Program of Materials (MAT2010-21270-C04-01/02/03/04, MAT2013-46649-C41/2/3/4-P and MAT2013-43319-P), the Consolider-Ingenio 2010 MALTA (CSD2007-00045), the project of Generalitat Valenciana (GVA-ACOMP/2014/243) and by the European Union FEDER funds. C Guzman-Afonso wishes to thank ACIISI and FSE for a fellowship. J A Sans thanks the FPI and 'Juan de la Cierva' programs for fellowships.

Room-temperature Raman scattering has been measured in lead tungstate up to 17 GPa. We report the pressure dependence of all the Raman modes of the tetragonal scheelite phase PbWO4-I or stolzite, space group I41 /a, which is stable at ambient conditions. Upon compression the Raman spectrum undergoes significant changes around 6.2 GPa due to the onset of a partial structural phase transition to the monoclinic PbWO4-III phase space group P21 /n. Further changes in the spectrum occur at 7.9 GPa, related to a scheelite-to-fergusonite transition. This transition is observed due to the sluggishness and kinetic hindrance of the I → III transition. Consequently, we found the coexistence of the scheelite, PbWO4-III, and fergusonite phases from 7.9 to 9 GPa, and of the last two phases up to 14.6 GPa. We have performed ab initio latticedynamics calculations, which have greatly helped us in assigning the Raman modes of the three phases and discussing their pressure dependence. The Raman modes of the free WO4 molecule are discussed.

We report the results of both angle-dispersive x-ray diffraction and x-ray absorption near-edge structure studies in $\mathrm{Ba}\mathrm{W}{\mathrm{O}}_{4}$ and $\mathrm{Pb}\mathrm{W}{\mathrm{O}}_{4}$ at pressures of up to $56\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ and $24\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, respectively. $\mathrm{Ba}\mathrm{W}{\mathrm{O}}_{4}$ is found to undergo a pressure-driven phase transition at $7.1\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ from the tetragonal scheelite structure (which is stable under normal conditions) to the monoclinic fergusonite structure whereas the same transition takes place in $\mathrm{Pb}\mathrm{W}{\mathrm{O}}_{4}$ at $9\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. We observe a second transition to another monoclinic structure which we identify as that of the isostructural phases $\mathrm{Ba}\mathrm{W}{\mathrm{O}}_{4}\text{\ensuremath{-}}II$ and $\mathrm{Pb}\mathrm{W}{\mathrm{O}}_{4}\text{\ensuremath{-}}III$ (space group $P{2}_{1}∕n$). We have also performed ab initio total-energy calculations which support the stability of this structure at high pressures in both compounds. The theoretical calculations further find that upon increase of pressure the scheelite phases become locally unstable and transform displacively into the fergusonite structure. The fergusonite structure is, however, metastable and can only occur if the transition to the $P{2}_{1}∕n$ phases were kinetically inhibited. Our experiments in $\mathrm{Ba}\mathrm{W}{\mathrm{O}}_{4}$ indicate that it becomes amorphous beyond $47\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$.

Angle-dispersive x-ray-diffraction and x-ray-absorption near-edge structure measurements have been performed on ${\mathrm{CaWO}}_{4}$ and ${\mathrm{SrWO}}_{4}$ up to pressures of approximately 20 GPa. Both materials display similar behavior in the range of pressures investigated in our experiments. As in the previously reported case of ${\mathrm{CaWO}}_{4}$, under hydrostatic conditions ${\mathrm{SrWO}}_{4}$ undergoes a pressure-induced scheelite-to-fergusonite transition around 10 GPa. Our experimental results are compared to those found in the literature and are further supported by ab initio total-energy calculations, from which we also predict the instability at larger pressures of the fergusonite phases against an orthorhombic structure with space group Cmca. Finally, a linear relationship between the charge density in the $A{\mathrm{O}}_{8}$ polyhedra of $AB{\mathrm{O}}_{4}$ scheelite-related structures and their bulk modulus is discussed and used to predict the bulk modulus of other materials, like hafnon.