Frustration driven structural distortion in VOMoO4

Nuclear magnetic resonance (NMR), electron paramagnetic resonance (EPR), magnetization measurements and electronic structure calculations in VOMoO4 are presented. It is found that VOMoO4 is a frustrated two-dimensional antiferromagnet on a square lattice with competing exchange interactions along the side J1 and the diagonal J2 of the square. From magnetization measurements J1+J2 is estimated around 155 K, in satisfactory agreement with the values derived from electronic structure calculations. Around 100 K a structural distortion, possibly driven by the frustration, is evidenced. This distortion induces significant modifications in the NMR and EPR spectra which can be accounted for by valence fluctuations. The analysis of the spectra suggests that the size of the domains where the lattice is distorted progressively grows as the temperature approaches the transition to the magnetic ground state at Tc=42 K

Loop algorithm for Heisenberg models with biquadratic interaction and phase transitions in two dimensions

We present a new algorithm for quantum Monte Carlo simulation based on global updating with loops. While various theoretical predictions are confirmed in one dimension, we find, for S=1 systems on a square lattice with an antiferromagnetic biquadratic interaction, that the intermediate phase between the antiferromagnetic and the ferromagnetic phases is disordered and that the two phase transitions are both of the first order in contrast to the one-dimensional case. It is strongly suggested that the transition points coincide those at which the algorithm changes qualitatively.Comment: 4 pages including 4 figures, to appear in JPS

Global atmospheric budget of acetaldehyde: 3-D model analysis and constraints from in-situ and satellite observations

We construct a global atmospheric budget for acetaldehyde using a 3-D model of atmospheric chemistry (GEOS-Chem), and use an ensemble of observations to evaluate present understanding of its sources and sinks. Hydrocarbon oxidation provides the largest acetaldehyde source in the model (128 Tg a<sup>−1</sup>, a factor of 4 greater than the previous estimate), with alkanes, alkenes, and ethanol the main precursors. There is also a minor source from isoprene oxidation. We use an updated chemical mechanism for GEOS-Chem, and photochemical acetaldehyde yields are consistent with the Master Chemical Mechanism. We present a new approach to quantifying the acetaldehyde air-sea flux based on the global distribution of light absorption due to colored dissolved organic matter (CDOM) derived from satellite ocean color observations. The resulting net ocean emission is 57 Tg a<sup>−1</sup>, the second largest global source of acetaldehyde. A key uncertainty is the acetaldehyde turnover time in the ocean mixed layer, with quantitative model evaluation over the ocean complicated by known measurement artifacts in clean air. Simulated concentrations in surface air over the ocean generally agree well with aircraft measurements, though the model tends to overestimate the vertical gradient. PAN:NO<sub>x</sub> ratios are well-simulated in the marine boundary layer, providing some support for the modeled ocean source. We introduce the Model of Emissions of Gases and Aerosols from Nature (MEGANv2.1) for acetaldehyde and ethanol and use it to quantify their net flux from living terrestrial plants. Including emissions from decaying plants the total direct acetaldehyde source from the land biosphere is 23 Tg a<sup>−1</sup>. Other terrestrial acetaldehyde sources include biomass burning (3 Tg a<sup>−1</sup>) and anthropogenic emissions (2 Tg a<sup>−1</sup>). Simulated concentrations in the continental boundary layer are generally unbiased and capture the spatial gradients seen in observations over North America, Europe, and tropical South America. However, the model underestimates acetaldehyde levels in urban outflow, suggesting a missing source in polluted air. Ubiquitous high measured concentrations in the free troposphere are not captured by the model, and based on present understanding are not consistent with concurrent measurements of PAN and NO<sub>x</sub>: we find no compelling evidence for a widespread missing acetaldehyde source in the free troposphere. We estimate the current US source of ethanol and acetaldehyde (primary + secondary) at 1.3 Tg a<sup>−1</sup> and 7.8 Tg a<sup>−1</sup>, approximately 60{%} and 480% of the corresponding increases expected for a national transition from gasoline to ethanol fuel

Rate of Convergence of Space Time Approximations for stochastic evolution equations

Stochastic evolution equations in Banach spaces with unbounded nonlinear drift and diffusion operators driven by a finite dimensional Brownian motion are considered. Under some regularity condition assumed for the solution, the rate of convergence of various numerical approximations are estimated under strong monotonicity and Lipschitz conditions. The abstract setting involves general consistency conditions and is then applied to a class of quasilinear stochastic PDEs of parabolic type.Comment: 33 page

The Role of the Ocean in the Global Atmospheric Budget of Acetone

[1] Acetone is one of the most abundant carbonyl compounds in the atmosphere and it plays an important role in atmospheric chemistry. The role of the ocean in the global atmospheric acetone budget is highly uncertain, with past studies reaching opposite conclusions as to whether the ocean is a source or sink. Here we use a global 3-D chemical transport model (GEOS-Chem) simulation of atmospheric acetone to evaluate the role of air-sea exchange in the global budget. Inclusion of updated (slower) photolysis loss in the model means that a large net ocean source is not needed to explain observed acetone in marine air. We find that a simulation with a fixed seawater acetone concentration of 15 nM based on observations can reproduce the observed global patterns of atmospheric concentrations and air-sea fluxes. The Northern Hemisphere oceans are a net sink for acetone while the tropical oceans are a net source. On a global scale the ocean is in near-equilibrium with the atmosphere. Prescribing an ocean concentration of acetone as a boundary condition in the model assumes that ocean concentrations are controlled by internal production and loss, rather than by air-sea exchange. An implication is that the ocean plays a major role in controlling atmospheric acetone. This hypothesis needs to be tested by better quantification of oceanic acetone sources and sinks.Engineering and Applied Science

Observations of total peroxy nitrates and aldehydes: measurement interpretation and inference of OH radical concentrations

We describe measurements of total peroxy nitrates (ΣPNs), NO₂, O₃ and several aldehydes at Granite Bay, California, during the Chemistry and Transport of the Sacramento Urban Plume-2001 (CATSUP 2001) campaign, from 19 July–16 September 2001. We observed a strong photochemically driven variation of ΣPNs during the day with the median of 1.2 ppb at noon. Acetaldehyde, pentanal, hexanal and methacrolein had median abundances in the daytime of 1.2 ppb, 0.093 ppb, 0.14 ppb, and 0.27 ppb, respectively. We compare steady state and time dependent calculations of the dependence of ΣPNs on aldehydes, OH, NO and NO₂ showing that the steady state calculations are accurate to ±30% between 10:00 and 18:00 h. We use the steady state calculation to investigate the composition of ΣPNs and the concentration of OH at Granite Bay. We find that PN molecules that have never been observed before make up an unreasonably large fraction of the ΣPNs unless we assume that there exists a PAN source that is much larger than the acetaldehyde source. We calculate that OH at the site varied between 2 and 7×10⁶ molecule cm^−3 at noon during the 8 weeks of the experiment

Specification of Eight 2400 W @ 1.8 K Refrigeration Units for the LHC

The cooling capacity below 2 K for the superconducting magnets in the Large Hadron Collider (LHC), at CERN, will be provided by eight refrigeration units at 1.8 K, each of them coupled to a 4.5 K refrigerator. Taking into account the cryogenic architecture of the LHC and corresponding process design constraints, a reference solution based on a combination of cold centrifugal and warm volumetric compressors was established in 1997. The process and technical requirements expressed in the specification issued in 1998 and the procurement scenario based on pre-series acceptance prior to final series delivery between 2002 and 2004 are presented in this paper

Direct retrieval of isoprene from satellite-based infrared measurements.

Isoprene is the atmosphere's most important non-methane organic compound, with key impacts on atmospheric oxidation, ozone, and organic aerosols. In-situ isoprene measurements are sparse, and satellite-based constraints have employed an indirect approach using its oxidation product formaldehyde, which is affected by non-isoprene sources plus uncertainty and spatial smearing in the isoprene-formaldehyde relationship. Direct global isoprene measurements are therefore needed to better understand its sources, sinks, and atmospheric impacts. Here we show that the isoprene spectral signatures are detectable from space using the satellite-borne Cross-track Infrared Sounder (CrIS), develop a full-physics retrieval methodology for quantifying isoprene abundances from these spectral features, and apply the algorithm to CrIS measurements over Amazonia. The results are consistent with model output and in-situ data, and establish the feasibility of direct global space-based isoprene measurements. Finally, we demonstrate the potential for combining space-based measurements of isoprene and formaldehyde to constrain atmospheric oxidation over isoprene source regions

A low-dimensional spin S = 1/2 system at the quantum critical limit: Na2V2O7

We report the results of measurements of the dc-susceptibility and the 23Na-NMR response of Na2V2O7, a recently synthesized, non metallic low dimensional spin system. Our results indicate that upon reducing the temperature to below 100 K, the V^{4+} moments are gradually quenched, leaving only one moment out of 9 active. The NMR data reveal a phase transition at very low temperatures. With decreasing applied field H, the critical temperature shifts towards T = 0 K, suggesting that Na2V2O7 may be regarded as an insulator reaching a quantum critical point at H = 0.Comment: 4 pages, 5 figure