244 research outputs found
First ice core records of NO3− stable isotopes from Lomonosovfonna, Svalbard
Samples from two ice cores drilled at Lomonosovfonna, Svalbard, covering the period 1957–2009, and 1650–1995, respectively, were analyzed for NO3− concentrations, and NO3− stable isotopes (δ15N and δ18O). Post-1950 δ15N has an average of (−6.9 ± 1.9) ‰, which is lower than the isotopic signal known for Summit, Greenland, but agrees with values observed in recent Svalbard snow and aerosol. Pre-1900 δ15N has an average of (4.2 ± 1.6) ‰ suggesting that natural sources, enriched in the 15 N-isotope, dominated before industrialization. The post-1950 δ18O average of (75.1 ± 4.1) ‰ agrees with data from low and polar latitudes, suggesting similar atmospheric NOy (NOy = NO + NO2 + HNO3) processing pathways. The combination of anthropogenic source δ15N and transport isotope effect was estimated as −29.1 ‰ for the last 60 years. This value is below the usual range of NOx (NOx = NO + NO2) anthropogenic sources which is likely the result of a transport isotope effect of –32 ‰. We suggest that the δ15N recorded at Lomonosovfonna is influenced mainly by fossil fuel combustion, soil emissions and forest fires; the first and second being responsible for the marked decrease in δ15N observed in the post-1950s record with soil emissions being associated to the decreasing trend in δ15N observed up to present time, and the third being responsible for the sharp increase of δ15N around 2000
Evaluation of a coupled dispersion and aerosol process model against measurements near a major road
International audienceA field measurement campaign was conducted near a major road "Itäväylä" in an urban area in Helsinki in 17?20 February 2003. Aerosol measurements were conducted using a mobile laboratory "Sniffer" at various distances from the road, and at an urban background location. Measurements included particle size distribution in the size range of 7 nm?10 ?m (aerodynamic diameter) by the Electrical Low Pressure Impactor (ELPI) and in the size range of 3?50 nm (mobility diameter) by Scanning Mobility Particle Sizer (SMPS), total number concentration of particles larger than 3 nm detected by an ultrafine condensation particle counter (UCPC), temperature, relative humidity, wind speed and direction, driving route of the mobile laboratory, and traffic density on the studied road. In this study, we have compared measured concentration data with the predictions of the road network dispersion model CAR-FMI used in combination with an aerosol process model MONO32. The vehicular exhaust emissions, and atmospheric dispersion and transformation of fine and ultrafine particles was evaluated within the distance scale of 200 m (corresponding to a time scale of a couple of minutes). We computed the temporal evolution of the number concentrations, size distributions and chemical compositions of various particle size classes. The atmospheric dilution rate of particles is obtained from the roadside dispersion model CAR-FMI. Considering the evolution of total number concentration, dilution was shown to be the most important process. The influence of coagulation and condensation on the number concentrations of particle size modes was found to be negligible at this distance scale. Condensation was found to affect the evolution of particle diameter in the two smallest particle modes. The assumed value of the concentration of condensable organic vapour of 1012 molecules cm?3 was shown to be in a disagreement with the measured particle size evolution, while the modelling runs with the concentration of condensable organic vapour of 109?1010 molecules cm?3 resulted in particle sizes that were closest to the measured values
Evaluation and modelling of the size fractionated aerosol particle number concentration measurements nearby a major road in Helsinki ? Part I: Modelling results within the LIPIKA project
International audienceA field measurement campaign was conducted near a major road "Itäväylä" in an urban area in Helsinki in 17?20 February 2003. Aerosol measurements were conducted using a mobile laboratory "Sniffer" at various distances from the road, and at an urban background location. Measurements included particle size distribution in the size range of 7 nm?10 ?m (aerodynamic diameter) by the Electrical Low Pressure Impactor (ELPI) and in the size range of 3?50 nm (mobility diameter) by Scanning Mobility Particle Sizer (SMPS), total number concentration of particles larger than 3 nm detected by an ultrafine condensation particle counter (UCPC), temperature, relative humidity, wind speed and direction, driving route of the mobile laboratory, and traffic density on the studied road. In this study, we have compared measured concentration data with the predictions of the road network dispersion model CAR-FMI used in combination with an aerosol process model MONO32. For model comparison purposes, one of the cases was additionally computed using the aerosol process model UHMA, combined with the CAR-FMI model. The vehicular exhaust emissions, and atmospheric dispersion and transformation of fine and ultrafine particles was evaluated within the distance scale of 200 m (corresponding to a time scale of a couple of minutes). We computed the temporal evolution of the number concentrations, size distributions and chemical compositions of various particle size classes. The atmospheric dilution rate of particles is obtained from the roadside dispersion model CAR-FMI. Considering the evolution of total number concentration, dilution was shown to be the most important process. The influence of coagulation and condensation on the number concentrations of particle size modes was found to be negligible on this distance scale. Condensation was found to affect the evolution of particle diameter in the two smallest particle modes. The assumed value of the concentration of condensable organic vapour of 1012 molecules cm?3 was shown to be in a disagreement with the measured particle size evolution, while the modelling runs with the concentration of condensable organic vapour of 109?1010 molecules cm?3 resulted in particle sizes that were closest to the measured values
Interference through quantum dots
We discuss the effect of quantum interference on transport through a quantum
dot system. We introduce an indirect coherent coupling parameter alpha, which
provides constructive/destructive interference in the transport current
depending on its phase and the magnetic flux. We estimate the current through
the quantum dot system using the non-equilibrium Green's function method as
well as the master equation method in the sequential tunneling regime. The
visibility of the Aharonov-Bohm oscillation is evaluated. For a large inter-dot
Coulomb interaction, the current is strongly suppressed by the quantum
interference effect, while the current is restored by applying an oscillating
resonance field with the frequency of twice the inter-dot tunneling energy.Comment: 10 pages, 3 figure
Longitudinal interrelationships between dental fear and dental attendance among adult Finns in 2000-2011
Objectives: The aim of this longitudinal study was to investigate causal pathways among as well as interrelationships between changes in dental fear and dental attendance in a nationally representative sample of adult Finns aged 19 years or older in 2000, with 11 years of follow‐up.Methods: Data from the Health 2000 and 2011 Surveys (BRIF8901) in Finland were used. The Health 2000 survey used a stratified two‐stage cluster sampling design (N=9742). Of the participants in 2000, 7964 were eligible and invited to participate in 2011. Of the participants in 2011 (n=5806), 3,631 (63%) responded to both dental fear and attendance questions in both years. Both fear and attendance were assessed using single questions and dichotomized. The background variables included were age, gender and education. Path analysis and logistic regression models were used.Results: Dental fear led to nonhabitual use of dental services rather than vice versa (−0.07 to 0.04 vs. 0.00). When confounders were considered, in both age groups (29‐39 years and 40+ years) an increase in fear predicted nonhabitual dental attendance. This association was stronger among the younger age group (OR = 4.91) than among those aged 40 years and older (OR = 2.88). Among the younger age group, improved dental fear decreased the risk of nonhabitual dental attendance (OR = 0.16), while among older age group, stable fear increased the risk of nonhabitual dental attendance (OR = 2.33).Conclusions: Dental fear causes nonhabitual dental attendance, and decreasing dental fear increases habitual attendance. Oral health personnel should adapt measures to prevent and treat dental fear.</p
Universal conductance enhancement and reduction of the two-orbital Kondo effect
We investigate theoretically the linear and nonlinear conductance through a
nanostructure with two-fold degenerate single levels, corresponding to the
transport through nanostructures such as a carbon nanotube, or double dot
systems with capacitive interaction. It is shown that the presence of the
interaction asymmetry between orbits/dots affects significantly the profile of
the linear conductance at finite temperature, and, of the nonlinear
conductance, particularly around half-filling, where the two-particle Kondo
effect occurs. Within the range of experimentally feasible parameters, the
SU(4) universal behavior is suggested, and comparison with relevant experiments
is made.Comment: 10 pages, 16 figure
Enhancement of Kondo effect in quantum dots with an even number of electrons
We investigate the Kondo effect in a quantum dot with almost degenerate
spin-singlet and triplet states for an even number of electrons. We show that
the Kondo temperature as a function of the energy difference between the states
Delta reaches its maximum around Delta=0 and decreases with increasing Delta.
The Kondo effect is thus enhanced by competition between singlet and triplet
states. Our results explain recent experimental findings. We evaluate the
linear conductance in the perturbative regime.Comment: 5 pages; Phys. Rev. Lett., in pres
Kondo effect in coupled quantum dots under magnetic fields
The Kondo effect in coupled quantum dots is investigated theoretically under
magnetic fields. We show that the magnetoconductance (MC) illustrates peak
structures of the Kondo resonant spectra. When the dot-dot tunneling coupling
is smaller than the dot-lead coupling (level broadening), the
Kondo resonant levels appear at the Fermi level (). The Zeeman splitting
of the levels weakens the Kondo effect, which results in a negative MC. When
is larger than , the Kondo resonances form bonding and
anti-bonding levels, located below and above , respectively. We observe a
positive MC since the Zeeman splitting increases the overlap between the levels
at . In the presence of the antiferromagnetic spin coupling between the
dots, the sign of MC can change as a function of the gate voltage.Comment: 6 pages, 3 figure
Magnetic Field Effects on the Far-Infrared Absorption in Mn_12-acetate
We report the far-infrared spectra of the molecular nanomagnet Mn_12-acetate
(Mn_12) as a function of temperature (5-300 K) and magnetic field (0-17 T). The
large number of observed vibrational modes is related to the low symmetry of
the molecule, and they are grouped together in clusters. Analysis of the mode
character based on molecular dynamics simulations and model compound studies
shows that all vibrations are complex; motion from a majority of atoms in the
molecule contribute to most modes. Three features involving intramolecular
vibrations of the Mn_12 molecule centered at 284, 306 and 409 cm-1 show changes
with applied magnetic field. The structure near 284 cm displays the
largest deviation with field and is mainly intensity related. A comparison
between the temperature dependent absorption difference spectra, the gradual
low-temperature cluster framework distortion as assessed by neutron diffraction
data, and field dependent absorption difference spectra suggests that this mode
may involve Mn motion in the crown.Comment: 5 pages, 4 figures, PRB accepte
- …