12,296 research outputs found
Near-Infrared Kinetic Spectroscopy of the HO_2 and C_2H_5O_2 Self-Reactions and Cross Reactions
The self-reactions and cross reactions of the peroxy radicals HO_2 and C_2H_5O_2 and HO_2 were monitored using simultaneous independent spectroscopic probes to observe each radical species. Wavelength modulation (WM) near-infrared (NIR) spectroscopy was used to detect HO_2, and UV absorption monitored HO_2 and C_2H_5O_2. The temperature dependences of these reactions were investigated over a range of interest to tropospheric chemistry, 221−296 K. The Arrhenius expression determined for the cross reaction, k_2(T) = (6.01^(+1.95)_(−1.47)) × 10^(−13) exp((638 ± 73)/T) cm^3 molecules^(−1) s^(−1) is in agreement with other work from the literature. The measurements of the HO_2 self-reaction agreed with previous work from this lab and were not further refined.(1) The C_2H_5O_2 self-reaction is complicated by secondary production of HO_2. This experiment performed the first direct measurement of the self-reaction rate constant, as well as the branching fraction to the radical channel, in part by measurement of the secondary HO_2. The Arrhenius expression for the self-reaction rate constant is k_3(T) = (1.29^(+0.34)_(−0.27)) × 10^(−13)exp((−23 ± 61)/T) cm^3 molecules^(−1) s^(−1), and the branching fraction value is α = 0.28 ± 0.06, independent of temperature. These values are in disagreement with previous measurements based on end product studies of the branching fraction. The results suggest that better characterization of the products from RO_2 self-reactions are required
Application of processed organic municipal solid waste on agricultural land - a scenario analysis
Source separation, composting and anaerobic digestion, with associated land application, are increasingly being considered as alternative waste management strategies to landfilling and incineration of municipal solid waste (MSW). Environmental life cycle
assessments are a useful tool in political decision-making about waste management strategies. However, due to the diversity of processed organic MSW and the situations in which it can be applied, the environmental impacts of land application are very hard to determine by experimental means. In the current study, we used the agroecosystem model Daisy to simulate a range of different scenarios representing different geographical areas, farm and soil types under Danish conditions and legislation. Generally, the application of processed organic MSW resulted in increased emissions compared with the corresponding standard scenarios, but with large differences between scenarios.
Emission coefficients for nitrogen leaching to the groundwater ranged from 0.03 to 0.87, while those for nitrogen lost to surface waters through tile drains ranged from 0 to 0.30. Emission coefficients for N2O formation ranged from 0.013 to 0.022 and for ammonia
volatilization from 0.016 to 0.11. These estimates are within reasonable range of observed values under similar conditions. Furthermore, a sensitivity analysis showed that the estimates were not very sensitive to the mineralization dynamics of the processed organic MSW. The results show that agroecosystem models can be powerful tools to estimate the environmental impacts of land application of processed MSW under different conditions. Despite this, agroecosystem models have only been used to a very limited degree for this purpose
Computation of nucleation of a non-equilibrium first-order phase transition using a rare-event algorithm
We introduce a new Forward-Flux Sampling in Time (FFST) algorithm to
efficiently measure transition times in rare-event processes in non-equilibrium
systems, and apply it to study the first-order (discontinuous) kinetic
transition in the Ziff-Gulari-Barshad model of catalytic surface reaction. The
average time for the transition to take place, as well as both the spinodal and
transition points, are clearly found by this method.Comment: 12 pages, 10 figure
A combined HST and XMM-Newton campaign for the magnetic O9.7 V star HD 54879: towards constraining the weak-wind problem of massive stars
Context: HD 54879 (O9.7 V) is one of a dozen O-stars for which an organized
atmospheric magnetic field has been detected. To gain insights into the
interplay between atmospheres, winds, and magnetic fields of massive stars, we
acquired UV and X-ray data of HD 54879 using the Hubble Space Telescope and the
XMM-Newton satellite. In addition, 35 optical amateur spectra were secured to
study the variability of HD 54879. A multiwavelength (X-ray to optical)
spectral analysis is performed using the Potsdam Wolf-Rayet (PoWR) model
atmosphere code and the xspec software.
Results: The photospheric parameters are typical for an O9.7 V star. The
microturbulent, macroturbulent, and projected rotational velocities are lower
than previously suggested (<4 km/s). An initial mass of 16 and an
age of 5 Myr are inferred from evolutionary tracks. We derive a mean X-ray
emitting temperature of [K] and an X-ray luminosity of
[erg/s]. Short- and long-scale variability is seen in
the H-alpha line, but only a very long period of yr could be
estimated. Assessing the circumstellar density of HD 54879 using UV spectra, we
can roughly estimate the mass-loss rate HD 54879 would have in the absence of a
magnetic field as . The
magnetic field traps the stellar wind up to the Alfv\'en radius >
, implying that its true mass-loss rate is . Hence, density enhancements around magnetic stars
can be exploited to estimate mass-loss rates of non-magnetic stars of similar
spectral types, essential for resolving the weak wind problem.
Conclusions: Our study confirms that strongly magnetized stars lose little or
no mass, and supplies important constraints on the weak-wind problem of massive
main sequence stars.Comment: Accepted for publication in A&A on Aug. 9, 2017, 12 + 1 pages, 15
figures. Paper replaced due to typos and missing acknowledgment
Stellar population of the superbubble N206 in the LMC I. Analysis of the Of-type stars
Massive stars are the key agents of feedback. Consequently, quantitative
analysis of massive stars are required to understand how the feedback of these
objects shapes/ creates the large scale structures of the ISM. The giant HII
region N206 in the Large Magellanic Cloud contains an OB association that
powers a X-ray superbubble, serving as an ideal laboratory in this context. We
obtained optical spectra with the muti-object spectrograph FLAMES at the
ESO-VLT. When possible, the optical spectroscopy was complemented by UV spectra
from the HST, IUE, and FUSE archives. Detailed spectral classifications are
presented for our sample Of-type stars. For the quantitative spectroscopic
analysis we use the Potsdam Wolf-Rayet (PoWR) model atmosphere code. The
physical parameters and nitrogen abundances of our sample stars are determined
by fitting synthetic spectra to the observations. The stellar and wind
parameters of nine Of-type stars are used to construct wind momentum,luminosity
relationship. We find that our sample follows a relation close to the
theoretical prediction, assuming clumped winds. The most massive star in the
N206 association is an Of supergiant which has a very high mass-loss rate. Two
objects in our sample reveal composite spectra, showing that the Of primaries
have companions of late O subtype. All stars in our sample have an evolutionary
age less than 4 million years, with the O2-type star being the youngest. All
these stars show a systematic discrepancy between evolutionary and
spectroscopic masses. All stars in our sample are nitrogen enriched. Nitrogen
enrichment shows a clear correlation with increasing projected rotational
velocities. The mechanical energy input from the Of stars alone is comparable
to the energy stored in the N206 superbubble as measured from the observed
X-ray and H alpha emission.Comment: Accepted for the pubblication in Astronomy & Astrophysic
Memory specificity is linked to repetition effects in event-related potentials across the lifespan
Spectral pattern similarity analysis: Tutorial and application in developmental cognitive neuroscience
The human brain encodes information in neural activation patterns. While standard approaches to analyzing neural data focus on brain (de-)activation (e.g., regarding the location, timing, or magnitude of neural responses), multivariate neural pattern similarity analyses target the informational content represented by neural activity. In adults, a number of representational properties have been identified that are linked to cognitive performance, in particular the stability, distinctiveness, and specificity of neural patterns. However, although growing cognitive abilities across childhood suggest advancements in representational quality, developmental studies still rarely utilize information-based pattern similarity approaches, especially in electroencephalography (EEG) research. Here, we provide a comprehensive methodological introduction and step-by-step tutorial for pattern similarity analysis of spectral (frequency-resolved) EEG data including a publicly available pipeline and sample dataset with data from children and adults. We discuss computation of single-subject pattern similarities and their statistical comparison at the within-person to the between-group level as well as the illustration and interpretation of the results. This tutorial targets both novice and more experienced EEG researchers and aims to facilitate the usage of spectral pattern similarity analyses, making these methodologies more readily accessible for (developmental) cognitive neuroscientists
Atmospheric Bromine and Ozone Perturbations in the Lower Stratosphere
The role of bromine compounds in the photochemistry of the natural and perturbed stratosphere has been reexamined using an expanded reaction scheme and the results of recent laboratory studies of several key reactions. The most important finding is that through the reaction BrO + CIO → Br + Cl + O2, there is a synergistic effect between bromine and chlorine which results in an efficient catalytic destruction of ozone in the lower stratosphere. One-dimensional photochemical model results indicate that BrO is the major bromine species throughout the stratosphere, followed by BrONO2, HBr, HOBr and Br. We show from the foregoing that bromine is more efficient than chlorine as a catalyst for destroying ozone, and discuss the implications for stratospheric ozone of possible future growth in the industrial and agricultural use of bromine. Bromine concentrations of 20 pptv (2 × 10^−11), as suggested by recent observations, can decrease the present-day integrated ozone column density by 2.4%, and can enhance ozone depletion from steady-state chlorofluoromethane release at 1973 rates by a factor of 1.1–1.2
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