Ho Fraction Research Articles

Development and commissioning of the ion implanter for the HOLMES experiment

The HOLMES experiment aims to directly measure the ν mass studying the 163Ho electron capture decay spectrum, using arrays of TES-based micro-calorimeters implanted with O(102 Bq/detector) 163Ho atoms. The neutron irradiation of 162Er enriched samples, used for the production of 163Ho, also generates radioactive contaminants. Chemical processes have been developed to extract the Ho fraction with high efficiency. The radioactive Ho isotope 166mHo is also present in the final sample and could significantly contribute to background events in the final spectrum. For this reason a dedicated implantation/mass separator has been set up and commissioned. It is designed to achieve more than 5 σ separation 163/166 u simultaneously allowing an efficient 163Ho atoms embedding inside micro-calorimeter absorbers. Its main components are a 50 kV sputter-based ion source, a magnetic dipole and a target chamber. A specially designed co-evaporation system has been implemented to deposit gold on the detector absorbers during implantation to overcome the 163Ho source saturation problem. The system performances in terms of achievable beam current, profile and mass separation have been evaluated by means of calibration runs using Cu, Mo, Au and 165Ho beams. An intensive study was done to optimize the target containing 163Ho in order to achieve a large fraction of ionized Ho in the beam. In this work, the machine development and commissioning will be described.

Investigations on mineralogy and geochemistry of a black shale profile on the northern Yangtze platform, China: Weathering fate of rare earth elements and yttrium (REY) and its implications

The mineralogical and geochemical compositions of an 11.5 m black shale weathering profile on the northern Yangtze platform, China, are documented to investigate the fate of rare earth elements and yttrium (REY) and their potential implications during chemical weathering. Our results signify that the profile has experienced an expected weathering trend in initial to moderate weathering stages based on the chemical index of alteration (CIA) values (54.45–75.50). Major elements (Ca, Mg, Na and K) have been leached to various degrees throughout the profile, roughly coupled with the weathering trends of primary minerals. Most REY were released from the black shale matrix, transported downwards and partially precipitated in a fracture. Lanthanum, Ce and Pr exhibited less mobility than other REY, which was connected with the weathering-resistant rhabdophane ((La,Ce)PO4). The REY enrichment in the fracture resulted from the adsorption of Fe–Mn oxyhydroxides and kaolinite. Negligible Ce anomalies (Ce/Ce* values of 0.95–1.12) and prominent negative Eu anomalies (Eu/Eu* values of 0.40–0.77) are observed throughout the profile. The Ce/Ce* values are covariant with the sizes of the first tetrad effect, while Eu anomalies were inherited from the bedrock, and the variation in Eu/Eu* values is related to plagioclase dissolution. LREE/HREE, Sm/Nd and Y/Ho fractionation occurred during black shale weathering. The LREE/HREE fractionation was mainly controlled by primary and secondary minerals (e.g., rhabdophane and kaolinite). The Sm–Nd and Y–Ho fractionation was affected by the tetrad effect. Fracture zones in black shales would be new potential targets for REY prospecting and extraction. Highly REY-enriched zones may occur in soils and underground aquifers, which would pose potential threats to crops and humans. Our findings, combined with previous studies, suggest that LREE/HREE fractionation is indicative of the weathering degree of continental rocks.

Characteristics of hydrophobic and hydrophilic acid fractions in drainage waters of undisturbed soil lysimeters

Dissolved organic matter (DOM), a heterogeneous mixture of low concentrations of organic matter draining from soils, plays a significant role in soil C cycling and in nutrient and pollutant transport. DOM from undisturbed soil profiles has rarely been studied. Hydrophobic acids (Ho) and hydrophilic acids (Hi), the major components of DOM, were recovered, using XAD-8 and XAD-4 resins in series, from waters draining in winter and in spring periods from well-drained and poorly drained Irish grassland soil profiles in lysimeters. Waters were collected from 45 soil undisturbed lysimeters at the Teagasc Research Centre, Johnstown Castle, Wexford, Ireland. Four Irish representative soils had been collected as undisturbed 1.0-m-deep monoliths, transported to the experiment site and arranged randomly in an experimental facility. Water collections were carried out in winter and spring periods. The DOM was isolated and fractionated using an XAD-8 and XAD-4 resins in-tandem procedure, and hydrophobic acids (Ho) and hydrophilic acids (Hi) were isolated. The amounts of DOM recovered in the winter period were much greater than those in the spring period, and the soil types had only minor influences on the DOM concentrations recovered. The Ho and Hi fraction contents ranged from 62 to 90 and 10 to 28%, respectively, of the total DOM content extracted. The Hi acids were most enriched in 13C, and considered to reflect greater microbial inputs. The neutral sugar (NS) contents for the Ho and Hi fractions were in the range of 15 to 52 μg mg−1, with the Hi fraction most enriched. The amino acids (AAs) for the Ho and Hi fractions varied from 0.6 to 2.4%, and the total AAs and NS of the Ho acids were well correlated. The DOM fractions from the drainage waters contained much less AAs and NS than the corresponding fractions in the parent soils. The solid- and liquid-state NMR data indicated organic structures with low aromaticity, significant amounts of carbohydrate and with lesser amounts of peptide structures, and with long-chain methylene (CH2)n and methine (-CH-) groups. The application of a variety of wet chemistry and of spectroscopy procedures has given a more in-depth awareness of the compositions of the DOM in the drainage waters from four different soils in 1.0-m-deep lysimeter arrangements. Based on wet chemistry analyses, and FTIR and liquid- and solid-state NMR spectrometry, it is clear that there are some differences between the compositions of the DOM fractions recovered. Alkyl functionalities dominated the structures. These included significant amounts or O-alkyl (predominantly carbohydrate), and with lesser (and variable amounts in the different fractions) aromatic structures (to which aromatic amino acid components were considered to be significant contributors), and with no evidence for lignin-derived structures The results suggest that, during residence in the soil solution, microbiological processes transform the SOM components released into products that are greatly different from their materials of origin in the SOM.

Seasonal and storm-driven changes in chemical composition of dissolved organic matter: a case study of a reservoir and its forested tributaries

Many drinking water supplies are located in forested watersheds, which operate as an important source of dissolved organic matter (DOM). In this study, monthly sampling campaigns were conducted from a reservoir (Daecheong Reservoir, South Korea) and its forested tributaries for five consecutive months (June to October) to examine the variations of DOM composition. Excitation-emission matrix fluorescence spectroscopy combined with parallel factor analysis (EEM-PARAFAC) was applied to track the variations of different fluorescent components within bulk DOM. Selected samples were further separated into hydrophobic (Ho) and hydrophilic (Hi) fractions. Water quality and DOM composition varied greatly with the sampling locations including the upstream and the downstream tributary sites, and the reservoir. Non-metric multidimensional scaling (NMDS) provided the information on the DOM sources and the potential processes leading to the observed DOM changes. Four of the five fluorescent components, identified by EEM-PARAFAC, were well correlated with the flow rates of the tributaries, suggesting hydrological control on DOM composition. The greatest effects were found on two terrestrial humic-like components (C1 and C2). The Ho fraction of DOM was more abundant for the post-storm samples versus the non-storm samples, supporting the important roles of hydrology on the changes in chemical composition of DOM. The amounts of the DOM resin fractions, either Ho or Hi, showed strong relationships with C1 and C2, suggesting that DOM fluorescence could be successfully applied to estimate different DOM chemical constituents in forested watersheds.

Isolation of 163Ho from dysprosium target material by HPLC for neutrino mass measurements

Abstract The rare earth isotope 163Ho is of interest for neutrino mass measurements. This report describes the isolation of 163Ho from a proton-irradiated dysprosium target and its purification. A Dy metal target was irradiated with 16 MeV protons for 10 h. After target dissolution, 163Ho was separated from the bulk Dy via cation-exchange high performance liquid chromatography using 70 mmol dm –3 α-hydroxyisobutyric acid as the mobile phase. Subsequent purification of the collected Ho fraction was performed to remove the α-hydroxyisobutyrate chelating agent and to concentrate the Ho in a low ionic strength aqueous matrix. The final solution was characterized by MC-ICP-MS to determine the 163Ho/165Ho ratio, 163Ho and the residual Dy content. The HPLC purification process resulted in a decontamination factor 1.4E5 for Dy. The isolated Ho fraction contained 24.8 ± 1.3 ng of 163Ho corresponding to holmium recovery of 72 ± 3%.

Characterization of dissolved organic matter from surface waters with low to high dissolved organic carbon and the related disinfection byproduct formation potential

In this study, the disinfection byproduct formation potential (DBPFP) of three surface waters with the dissolved organic carbon (DOC) content of 2.5, 5.2, and 7.9mg/L was investigated. The formation and distribution of trihalomethanes and haloacetic acids were evaluated. Samples collected from three surface waters in China were fractionated based on molecular weight and hydrophobicity. The raw water containing more hydrophobic (Ho) fraction exhibited higher formation potentials of haloacetic acid and trihalomethane. The DBPFP of the surface waters did not correlate with the DOC value. The values of DBPFP per DOC were correlated with the specific ultraviolet absorbance (SUVA) for Ho and Hi fractions. The obtained results suggested that SUVA cannot reveal the ability of reactive sites to form disinfection byproducts for waters with few aromatic structures. Combined with the analysis of FTIR and nuclear magnetic resonance spectra of the raw waters and the corresponding fractions, it was concluded that the Ho fraction with phenolic hydroxyl and conjugated double bonds was responsible for the production of trichloromethanes and trichloroacetic acids. The Hi fraction with amino and carboxyl groups had the potential to form dichloroacetic acids and chlorinated trihalomethanes.

Fractionation of yttrium and holmium during basaltic soil weathering

The anomalously low affinity of yttrium (Y) for iron (Fe) (oxyhydr)oxides relative to lanthanides with similar ionic radius (e.g., Ho) has been demonstrated in experiments with isolated Fe minerals and in a variety of marine systems that contain high concentrations of solid phase Fe. However, it has not previously been demonstrated to occur during soil genesis, despite the common observation that many soils become enriched in Fe over time. We hypothesized that Y would become progressively depleted in soils relative to Ho with increased weathering. Since, trivalent Y has an anomalously low Misono softness relative to other trivalent ions included in the rare earth element and yttrium group (REY3+), we also investigated whether soil REY fractionation reflects variation in Misono softness. To test this, we measured trends in total REY concentrations for Hawaiian soils derived from basaltic parent materials aged 0.3–4100ky, and measured REYs released from the same samples during short-time (3h) dissolution experiments conducted as part of a previous investigation linking dissolution with surface charge properties (Chorover et al., 2004). The chondrite-normalized Y/Ho ratios in the parent Hawaiian basalt (Chond[Y/Ho]=0.998) and continental dust (Chond[Y/Ho]=0.994) inputs are remarkably similar, and thus we can interpret deviations from Chond[Y/Ho]∼1.0 to result from soil biogeochemical processes and not source mixing. Between 0.3 and 20ky, the Chond[Y/Ho] ratio of the subsurface soils decreased from 0.96±0.07(2σ) to 0.71±0.05, and then remained unchanged across the rest of the weathering sequence. In contrast, the Chond[Y/Ho] ratio of the surface soils decreased from 0.99±0.07 to 0.76±0.05 at 150ky and then, most likely due to continued dust inputs, increased to 1.04±0.07 in the oldest soils. Analysis of the short-time dissolution experiments revealed preferential release of Y relative to Ho (and also La relative Pr) at intermediate pH where aqueous REY concentrations are governed by proton competition for adsorption sites. Proton-competition-control over REY release is bounded at high pH by the onset of colloidal dispersion—represented by the point of minimum dissolution (p.m.d.) of Al—and at low pH by the soil’s point of zero net charge (p.z.n.c.) and/or when proton-promoted dissolution of REY-containing solids, including Fe-(oxyhydr)oxides, control REY release. Results of our dissolution experiments suggest that complexation of REYs by dissolved organic matter (DOM) does not drive Y–Ho fractionation during pedogenesis, but rather may suppress it. Synthesis of these field and laboratory experiments suggests the Y/Ho ratio decreases early in soil development (<20ky) when weathering rates are high and competitive proton adsorption affects REY fractionation. Given that Fe-(oxyhydr)oxide sorbents exhibit greater affinity for Ho relative to Y, their prevalent neo-formation during incipient pedogenesis likely plays a central role in Y–Ho fractionation in these soils. Persistence of low Chond[Y/Ho] ratios in the subsurface soils even at 4100ky suggests Y–Ho fractionation continues, albeit at a slower rate, as weathering proceeds.

A study on microstructure and luminescent properties of oxyfluoride silicate glass-ceramics with (Ho 3+,Yb 3+):NaYF 4 crystallites

Glass-ceramics containing NaYF 4 nanocrystals were prepared by heat-treatment from oxyfluoride silicate-based glass doped with Ho 3+ and Yb 3+ ions. The formation of crystalline fluoride phase was confirmed by X-ray diffraction and transmission electron microscopy. Absorption and emission spectra revealed that a fraction of Ho 3+ and Yb 3+ ions is incorporated into the NaYF 4 ordered lattice influencing spectroscopic features of glass-ceramics in comparison with those of precursor glass. Green up-conversion emission (545 nm) originating in the 5S 2 level in glass-ceramics and up-converted red emission (650 nm) originating in the 5F 5 level in as-melted glass were observed under optical pumping into Yb 3+ absorption band and analyzed. Although both emissions in both materials are achieved by two-photon excitations, the relation between green and red emission intensity in glass-ceramics and glass implies that processes relevant to up-conversion phenomena are different. Based on a careful analysis of relaxation dynamics of Ho 3+ and Yb 3+ excited states, the mechanisms involved in conversion of the infrared radiation into the visible emission in these materials are proposed and discussed.

Application of Rare Earth Elements for Water Source Discrimination in Renlou Coal Mine, Northern Anhui Province, China

Rare earth element (REEs) concentrations were measured for groundwater and wall rock samples from two aquifers (sandstone and limestone aquifers) in Renlou coal mine, northern Anhui Province, China. The results indicate that groundwater from these two aquifers are rich in REEs as expressed by their Nd concentrations (0.012 and 0.008 μg/L). They all show heavy REEs enrichment with low NdRN/YbRN ratios (0.35 and 0.40) when normalized to their aquifer rocks. Y- Ho fractionation are only observed for groundwater from limestone aquifer with high Y/Ho ratio (97 in average), whereas groundwater from sandstone aquifer have low Y/Ho ratios (24 in average). The former is considered to be related to the release of Y and Ho from calcite with different partition coefficient. The similarity of some REEs between groundwater and aquifer rock, as well as the different rock composition of aquifers, provide possibility for discrimination of water sources by using REEs, and Y/Ho ratios and Eu anomalies are expected to be useful in Renlou coal mine.

Local structure of Y and Ho in calcite and its relevance to Y fractionation from Ho in partitioning between calcite and aqueous solution

Among rare earth elements (REEs), the behavior of Y and Ho in most igneous activities is very close due to the similarity in their ionic radii, while Y fractionates from Ho in marine systems. In this study, in order to elucidate Y–Ho fractionation observed in marine systems in terms of structural chemistry, we examined Ho L III-edge and Y K-edge EXAFS study for two partitioning systems, namely, 1) calcite-aqueous solution (Y–Ho fractionation system) and 2) strong acid cation exchange resin-aqueous solution (non-Y–Ho fractionation system). The results of the EXAFS analysis did not show significant differences in interatomic distances to the most neighboring O atoms between Ho and Y for all the samples. However, it was found that the first shell Ho–O and Y–O distances in the Y–Ho doped calcite were shorter than those in the aqua ion. In contrast, the first shell Ho–O and Y–O distances in the Y–Ho doped resin were similar to those in the aqua ion. Previous studies have suggested that lanthanide (Ln) is more covalent due to 4f orbital participation in bonding relative to Y. Spectroscopic studies on various Ln 3+ complexes show that Racah parameters for 4f electron repulsion in Ln 3+ ions decrease with an increase in covalency of bonding of Ln 3+ ions with anionic ligands. Therefore, our EXAFS results suggest that Y–Ho fractionation in partitioning is possibly attributed to the difference of change in covalency associated with the ligand exchange between Y and Ho, which we have observed as differences in partition coefficients between calcite and aqueous solution.

Alteration effects of volcanic ash in seawater: Anomalous Y/Ho ratios in coastal waters of the Central Mediterranean sea

This paper presents the results of a study based on data collected during the oceanographic cruise ANSIC 2001 carried out in the Ionian Sea during the explosive activity of Mount Etna in the summer of 2001. Anomalous low values of Y/Ho ratios in seawater suggest extensive scavenging processes on the surfaces of smectitic alteration products, with Y and Ho fractionation controlled by the differences in their electronic configurations and behaviour during solution/surface complexation equilibria. These processes can also be traced through the presence of significant tetrad effects recorded in the chondrite-normalised Rare Earth Elements and Yttrium (YREEs) patterns of suspended particulate matter. This suggests that the preferential Y scavenging from seawater is due to the formation of inner-sphere complexes with OH − groups on montmorillonite crystal surfaces. The preliminary results of kinetic experiments of YREE released from volcanic ash to coexisting seawater, and the related effects on Y/Ho ratios and Ce anomalies, are consistent with the fractionation of Light Rare Earth Elements (LREEs) with respect to Heavy Rare Earth Elements (HREEs) observed in dissolved phase. They suggest a behaviour of Y similar to that reported for LREEs, particularly for Ce and Pr.

Optical properties of Ho3+–Yb3+ co‐doped nanostructured SiO2–LaF3 glass‐ceramics prepared by sol–gel method

AbstractTransparent glass‐ceramics with composition of 95SiO2–5LaF3 doped with 0.1 mol% of Ho3+ or co‐doped with 0.1 mol% of Ho3+ and 0.3 mol% of Yb3+ were synthesized by thermal treatment of precursor sol–gel glasses. Segregation of LaF3 nanocrystals in the matrix was confirmed from a structural analysis by means of X‐ray diffraction. Blue, green and red efficient up‐conversion emissions were observed under 980 nm excitation at room temperature. These results could be attributed to the partition of a fraction of Ho3+ ions into the precipitated LaF3 nanocrystals. Moreover, near infrared down‐conversion at 1.2 μm is also observed. The mechanisms involved in the up‐conversion emissions could be ascribed to a two‐ and three‐ photon process. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

The behaviour of REEs in Thailand's Mae Klong estuary: Suggestions from the Y/Ho ratios and lanthanide tetrad effects

The concentrations of Rare Earth Elements and yttrium (REY) were measured in dissolved phase, in suspended particulate matter (SPM) and in sediments in seven sampling stations in the Mae Klong estuarine system (Inner Thailand Gulf) in order to study their behaviour and distribution pattern. The analysed samples generally show high Rare Earth Element (REE) content in the dissolved phase, with high Medium Rare Earth Elements (MREEs) and Y enrichments in the shale-normalized pattern (versus PAAS). These chemical features are interpreted in terms of direct influences of weathering processes of REE-rich minerals (e.g., phosphates), which abundantly out-crop in the Mae Klong watershed. These large MREE enrichments also suggest that dissolved sulphate complexes play an important role in the dissolved REE fate close to the marine coastal system. The shale normalized REEs and Y/Ho ratios additionally indicate a direct REY co-precipitation with Fe-oxyhydroxides along the river path and consequent adsorption, as coating, onto FeOOH surfaces. The REY behaviour in SPM appears strongly influenced by Al, Fe and Mn content, suggesting that the formation of an authigenic Fe, Mn oxyhydroxides and aluminosilicate particulate leads to REY adsorption and/or co-precipitation. The observed variations in the Y/Ho ratios suggest that different Y–Ho fractionation processes take place in the studied estuarine system. Strong similarities among chondrite-normalized REE patterns in both sediments and granitic rocks out-cropping in the surrounding regions suggest a generic relationship between the two groups of materials.

Phenols and Amine Induced HO• Generation During the Initial Phase of Natural Water Ozonation

The initial phase of ozone decomposition in natural water (t < 20 s) is poorly understood. It has recently been shown to result in very high transient HO* concentrations and, thereby, plays an essential role during processes such as bromate formation or contaminants oxidation. Phenols and amines are ubiquitous moieties of natural organic matter. Naturally occurring concentrations of primary, secondary, and tertiary amines, amino acids, and phenol were added to surface water, and ozone decomposition as well as HO* generation were measured starting 350 milliseconds after ozone addition. Six seconds into the process, 5 microM of dimethylamine and phenol had generated integral of HO* dt = 1 x 10(-10) M*s and 1.8 x 10(-10) M*s, respectively. With 10 microM dimethylamine and 1.5 mg O3/L, R(ct), (integral of HO*dt/ integral of O3dt) reached 10(-6), which is larger than in advanced oxidation processes (AOP) such as O3/H2O2. Experiments in the presence of HO*-scavengers indicated that a significant fraction of phenol-induced ozone decomposition and HO* generation results from a direct electron transfer to ozone. For dimethylamine, the main mechanism of HO* generation is direct formation of O2*- which reacts selectively with O3 to form O3*-. Pretreatment of phenol-containing water with HOCl or HOBr did not decrease HO* generation, while the same treatment of dimethylamine-containing water considerably reduced HO* generation.

Local flame structure in the well-stirred reactor regime

Direct numerical simulations of hydrogen/air turbulent premixed flame progagating in the three-dimensional turbulence are conducted to investigate local flame structures in the well-stirred reactor regime. A detailed kinetic mechanism including 12 reactive species and 27 elementary reactions is used to represent the H 2 /air reaction in turbulence. Although the flame condition is classified into the well-stirred reactor regime, the geometry of the regions with high heat release rate shows thin sheetlike structure. The fluctuation of the heat release rate along the flame surface is relatively high, and the maximum heat release rate reaches up to 1,3 times the corresponding laminar flame. The heat release rate tends to be high in the regions convex toward the burned side. The flame structure in the case of the well-stirred reactor regime shows a double-layered feature. One may conclude that reaction zone becomes thick in the well-stirred reactor regimes only from temperature, H, and OH distributions, while the heat release rate, mass fraction of O atoms, and reaction rates of O atoms and OH radicals are fluctuating significantly in that region in fact. Specifically, reaction rates of O atoms and OH radicals show characteristic behaviors in the burned side due to their chemical characteristics. In the preheat zone, mass fraction and reaction rate of HO 2 show quite thin and smooth distributions compared to other properties such as the heat release rate. The distribution of H 2 O 2 reaction rate reflects the double-layered feature of the well-stirred reactor regime very well. It is shown that the double-layered feature can be explained by discussing the balances of the elementary reactions in detail. As the flame front can be defined even in the well-stirred reactor regime, statistics of the local flame elements are also discussed.

Geochemistry of low-temperature water–rock interaction: evidence from natural waters, andesite, and iron-oxyhydroxide precipitates at Nishiki-numa iron-spring, Hokkaido, Japan

Cold, mildly acidic, and slightly oxidizing spring water from Nishiki-numa iron-spring, Hokkaido, Japan, has been sampled in two consecutive years, and analyzed for anions and thirty cations. The chemical composition of the acid–sulphate water is independent of filtration (0.2 μm–0.8 μm), and shows only minor temporal variation. The most striking feature of the Fe-rich water is the high abundance of dissolved rare-earth elements (ΣREE: 385 nM) and Y (281 nM). Chondrite-normalized Rare-Earths and Y (REY CN; Y inserted between Dy and Ho) patterns show a strong negative Eu CN anomaly, increase slightly from La to Pr, and decrease systematically from Pr to Lu. REY speciation in the spring water is dominated by sulphate complexes, preventing any significant change of the solution complexation along the REY series. Leaching experiments performed on the andesitic country-rock at Nishiki-numa allowed us to characterize the easily accessible fraction of its total element content. Alkali and alkali–earth elements are significantly less easily accessible than U and REY, the latter being the most easily accessible of all elements considered. This suggests that the often observed minor to negligible mobility of REY during alteration is almost entirely due to the formation of REY-bearing secondary minerals. Within the REY series accessibilities are constant from La to Nd and decrease from Nd to Lu, except for Eu which is considerably less easily accessible than its REY neighbours. This indicates that the easily accessible element fraction is not derived from the rock-forming minerals, but from boundary films and/or glass in the fine-grained matrix. The negative Eu CN anomaly of Nishiki-numa spring water is a reflection of this anomalously low accessibility of Eu. Further comparison of water composition to results from leaching experiments shows striking similarities, suggesting that at Nishiki-numa removal by secondary phases is of minor importance for most of the elements studied. Exceptions are Ba and U, because of the formation of barite and less soluble U(IV) species, respectively. Once the spring water is expelled at the surface, elements are partially removed from solution due to scavenging by ferrihydrite precipitates. Apparent bulk coefficients for `partitioning' of REY between the ferrihydrite and the mildly acidic spring water, app K REY D , give a sigmoidal pattern with a strong negative deviation from the general trend for Y, indicating strong fractionation of Y from its geochemical twin Ho. Considering that speciation (solution complexation) is similar for all REY, the differences between the individual app K REY D values must be related to complexation with functional groups on the ferrihydrite surface. Our data show that Y–Ho fractionation is not confined to the mildly alkaline marine environment, but the typical result of interaction of dissolved REY with iron-oxyhydroxides.

The photochemistry of acetone in the upper troposphere: A source of odd‐hydrogen radicals

This paper summarizes measured photodissociation quantum yields for acetone in the 290‐320 nm wavelength region for pressures and temperatures characteristic of the upper troposphere. Calculations combine this laboratory data with trace gas concentrations obtained during the NASA and NOAA sponsored Stratospheric Tracers of Atmospheric Transport (STRAT) field campaign, in which measurements of OH, HO2, odd‐nitrogen, and other compounds were collected over Hawaii, and west of California during fall and winter of 1995/1996. OH and HO2 concentrations within 2 to 5 km layers just below the tropopause are ∼50% larger than expected from O3, CH4, and H2O chemistry alone. Although not measured during STRAT, acetone is inferred from CO measurements and acetone‐CO correlations from a previous field study. These inferred acetone levels are a significant source of odd‐hydrogen radicals that can explain a large part of the discrepancy in the upper troposphere. For lower altitudes, the inferred acetone makes a negligible contribution to HOx (HO+HO2), but influences NOy partitioning. A major fraction of HOx production by acetone is through CH2O formation, and the HOx discrepancy can also be explained by CH2O levels in the 20 to 50 pptv range, regardless of the source.

Magnetization studies of Ho/Y superlattices: Role of magnetoelastic coupling effects.

Ho films and Ho/Y superlattices were fabricated and characterized structurally and magnetically. Anomalies in the magnetization vs field applied along the easy axis identify magnetic phase transitions, as found in bulk Ho, but with enhanced critical fields. The critical fields required to magnetize the samples vary systematically with the total Ho fraction in a manner that is shown to be consistent with the assumption that the Ho and Y layers are magnetoelastically coupled. \textcopyright{} 1996 The American Physical Society.

Mathematically reduced reaction mechanisms applied to adiabatic flat hydrogen/air flames

Several hydrogen/air reaction systems are reduced mathematically to one-step schemes, using the method introduced by Maas and Pope [1]. The reduction is obtained by assuming fast reaction groups of the reaction system to be in steady state. We developed a method to apply the reduced schemes to adiabatic flat flames. The results are compared with those of detailed chemistry calculations. The accuracy of the results of reduction of the most simple reaction system, (which does not include the species HO 2 and H 2O 2) is quite well. The other reaction systems, however, give appreciable errors in burning velocity. This is mainly caused by large deviations in HO 2 mole fractions between reduced and full scheme calculations, while the reaction rate and the burning velocity are sensitive to variations in the mole fraction of HO 2. Considering the time scales of the reaction system and the time scales of convection and diffusion it is shown that at low temperatures, where the mole fraction of HO 2 reaches its maximum, the basic assumptions applied to reduce the mechanisms are not justified. It is concluded that the hydrogen/air system can only be reduced to an accurate one-step reduced scheme, if reaction schemes without HO 2 and H 2O 2 are used. This reduction technique also indicates, in accordance with conclusions of Peters et al. [3], that a two-step reduced scheme has to be used if more realistic hydrogen/air schemes, including HO 2 and H 2O 2, are considered.

A continuous stirred tank reactor investigation of the gas‐phase reaction of hydroxyl radicals and toluene

AbstractA continuous stirred tank reactor (CSTR) was used to study the gas‐phase reaction between HO⋅ and toluene. HO⋅ was generated by the in situ photolysis of nitrous acid. Flow reactor operation at steady‐state conditions with a residence time of 20 min allowed investigation of primary and very rapid secondary reactions. CSTR and batch reactor experiments were also performed with selected products. Both gas‐phase and aerosol products were identified by chromatography and mass spectroscopy, with total product yields between 55 and 75% of reacted carbon. Toluene reaction products included cresols, nitrocresols, nitrotoluenes, 3,5‐dinitrotouluene, benzaldehyde, benzyl nitrate, nitrophenols, methyl‐p‐benzoquinone, glyoxal, methylglyoxal, formaldehyde, methyl nitrate, PAN, and CO. The fraction of HO⋅ methyl hydrogen abstraction was calculated to be 0.13 ± 0.04. The ratio of reaction rate constants for nitrotoluene versus cresol formation from the HO⋅‐adduct was calculated to be about 3.3 × 104. Also, the ratio of cresol formation versus O2 addition to the HO⋅‐adduct was estimated to be ≥0.5 for atmospheric conditions. Comparisons of these measurements with previous values and the implications with respect to photochemical kinetics modeling of the atmosphere are discussed.