2,852 research outputs found
Yields of oxidized volatile organic compounds during the OH radical initiated oxidation of isoprene, methyl vinyl ketone, and methacrolein under high-NO_x conditions
We present first-generation and total production yields of glyoxal, methylglyoxal, glycolaldehyde, and hydroxyacetone from the oxidation of isoprene, methyl vinyl ketone (MVK), and methacrolein (MACR) with OH under high NO_x conditions. Several of these first-generation yields are not included in commonly used chemical mechanisms, such as the Leeds Master Chemical Mechanism (MCM) v. 3.2. The first-generation yield of glyoxal from isoprene was determined to be 2.1 (±0.6)%. Inclusion of first-generation production of glyoxal, glycolaldehyde and hydroxyacetone from isoprene greatly improves performance of an MCM based model during the initial part of the experiments. In order to further improve performance of the MCM based model, higher generation glyoxal production was reduced by lowering the first-generation yield of glyoxal from C5 hydroxycarbonyls. The results suggest that glyoxal production from reaction of OH with isoprene under high NO_x conditions can be approximated by inclusion of a first-generation production term together with secondary production only via glycolaldehyde. Analogously, methylglyoxal production can be approximated by a first-generation production term from isoprene, and secondary production via MVK, MACR and hydroxyacetone. The first-generation yields reported here correspond to less than 5% of the total oxidized yield from isoprene and thus only have a small effect on the fate of isoprene. However, due to the abundance of isoprene, the combination of first-generation yields and reduced higher generation production of glyoxal from C5 hydroxycarbonyls is important for models that include the production of the small organic molecules from isoprene
Stable coexistence of equivalent nutrient competitors through niche differentiation in the light spectrum
Signature of a three-dimensional photonic band gap observed on silicon inverse woodpile photonic crystals
We have studied the reflectivity of CMOS-compatible three-dimensional silicon
inverse woodpile photonic crystals at near-infrared frequencies.
Polarization-resolved reflectivity spectra were obtained from two orthogonal
crystal surfaces corresponding to 1.88 pi sr solid angle. The spectra reveal
broad peaks with high reflectivity up to 67 % that are independent of the
spatial position on the crystals. The spectrally overlapping reflectivity peaks
for all directions and polarizations form the signature of a broad photonic
band gap with a relative bandwidth up to 16 %. This signature is supported with
stopgaps in plane wave bandstructure calculations and with the frequency region
of the expected band gap.Comment: 9 pages, 5 figure
Exploring the phase space of multiple states in highly turbulent Taylor-Couette flow
We investigate the existence of multiple turbulent states in highly turbulent
Taylor-Couette flow in the range of to ,
by measuring the global torques and the local velocities while probing the
phase space spanned by the rotation rates of the inner and outer cylinder. The
multiple states are found to be very robust and are expected to persist beyond
. The rotation ratio is the parameter that most strongly
controls the transitions between the flow states; the transitional values only
weakly depend on the Taylor number. However, complex paths in the phase space
are necessary to unlock the full region of multiple states. Lastly, by mapping
the flow structures for various rotation ratios in a Taylor-Couette setup with
an equal radius ratio but a larger aspect ratio than before, multiple states
were again observed. Here, they are characterized by even richer roll structure
phenomena, including, for the first time observed in highly turbulent TC flow,
an antisymmetrical roll state.Comment: 9 pages, 7 figure
Inhibited spontaneous emission of quantum dots observed in a 3D photonic band gap
We present time-resolved emission experiments of semiconductor quantum dots
in silicon 3D inverse-woodpile photonic band gap crystals. A systematic study
is made of crystals with a range of pore radii to tune the band gap relative to
the emission frequency. The decay rates averaged over all dipole orientations
are inhibited by a factor of 10 in the photonic band gap and enhanced up to 2?
outside the gap, in agreement with theory. We discuss the effects of spatial
inhomogeneity, nonradiative decay, and transition dipole orientations on the
observed inhibition in the band gap.Comment: 5 figures, update author lis
Unbalanced reduction of nutrient loads has created an offshore gradient from phosphorus to nitrogen limitation in the North Sea
Measures to reduce eutrophication have often led to a more effective decline of phosphorus (P) than nitrogen(N) concentrations. The resultant changes in riverine nutrient loads can cause an increase in the N : Pratios of coastal waters. During four research cruises along a 450 km transect, we investigated how reductionsin nutrient inputs during the past 25 yr have affected nutrient limitation patterns in the North Sea. Thisrevealed a strong offshore gradient of dissolved inorganic N : P ratios in spring, from 375 : 1 nearshoretoward 1 : 1 in the central North Sea. This gradient was reflected in high nearshore N : P and C : P ratios ofparticulate organic matter (mainly phytoplankton), indicative of severe P deficiency of coastal phytoplankton,which may negatively affect higher trophic levels in the food web. Nutrient enrichment bioassays performedon-board showed P and Si limitation of phytoplankton growth nearshore, co-limitation of N and P ina transitional region, and N limitation in the outer-shore waters, confirming the existence of an offshore gradientfrom P to N limitation. Different species were limited by different nutrients, indicating that furtherreductions of P loads without concomitant reductions of N loads will suppress colonial Phaeocystis blooms,but will be less effective in diminishing harmful algal blooms by dino- and nanoflagellates. Hence, our resultsprovide evidence that de-eutrophication efforts in northwestern Europe have led to a large imbalance in theN : P stoichiometry of coastal waters of the North Sea, with major consequences for the growth, species composition,and nutritional quality of marine phytoplankton communities
Modelling of point and non-point source pollution of nitrate with SWAT in the river Dill, Germany
International audienceWe used the Soil and Water Assessment Tool (SWAT) to simulate point and non-point source pollution of nitrate in a mesoscale mountainous catchment. The results show that the model efficiency for daily discharge is 0.81 for the calibration period (November 1990 to December 1993) and 0.56 for the validation period (April 2000 to January 2003). The model efficiency for monthly nitrate load is 0.66 and 0.77 for the calibration period (April 2000 to March 2002) and validation period (April 2002 to January 2003), respectively. However, the model efficiency for daily loads is low (0.15), which cannot only be attributed to the quality of input data of point source effluents. An analysis of the internal fluxes and cycles of nitrogen pointed out considerable weaknesses in the models conceptualisation of the nitrogen modules which will be improved in future research
Assessing the model performance of an integrated hydrological and biogeochemical model for discharge and nitrate load predictions
International audienceIn this study, we evaluate the performance of the SWAT-N model, a modified version of the widely used SWAT version, for discharge and nitrate predictions at the mesoscale Dill catchment (Germany) for a 5-year period. The underlying question is, whether the model efficiency is sufficient for scenario analysis of land-use changes on both water quantity and quality. The Shuffled Complex Evolution (SCE-UA) algorithm is used to calibrate the model for daily discharge at the catchments outlet. Model performance is assessed with a split-sampling as well as a proxy-basin test using recorded hydrographs of four additional gauges located within the catchment. The efficiency regarding nitrate load simulation is assessed without further calibration on a daily, log-daily, weekly, and monthly basis as compared to observations derived from an intensive sampling campaign conducted at the catchments outlet. A new approach is employed to test the spatial consistency of the model, where simulated longitudinal profiles of nitrate concentrations were compared with observed longitudinal profiles. It is concluded that the model efficiency of SWAT-N is sufficient for the assessment of scenarios for daily discharge predictions. SWAT-N can be employed without further calibration for nitrate load simulations on both a weekly and monthly basis with an acceptable degree of accuracy. However, the model efficiency for daily nitrate load is insufficient, which can be attributed to both data uncertainty (i.e. point-source effluents and actual farming practise) as well as structural errors. The simulated longitudinal profiles meet the observations reasonably well, which suggests that the model is spatially consistent
Spin Relaxation in Graphene with self-assembled Cobalt Porphyrin Molecules
In graphene spintronics, interaction of localized magnetic moments with the
electron spins paves a new way to explore the underlying spin relaxation
mechanism. A self-assembled layer of organic cobalt-porphyrin (CoPP) molecules
on graphene provides a desired platform for such studies via the magnetic
moments of porphyrin-bound cobalt atoms. In this work a study of spin transport
properties of graphene spin-valve devices functionalized with such CoPP
molecules as a function of temperature via non-local spin-valve and Hanle spin
precession measurements is reported. For the functionalized (molecular)
devices, we observe a slight decrease in the spin relaxation time ({\tau}s),
which could be an indication of enhanced spin-flip scattering of the electron
spins in graphene in the presence of the molecular magnetic moments. The effect
of the molecular layer is masked for low quality samples (low mobility),
possibly due to dominance of Elliot-Yafet (EY) type spin relaxation mechanisms
Once-daily tiotropium and olodaterol fixed-dose combination via the Respimat® improves outcomes versus mono-components in COPD in two 1-year studies
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