11,017 research outputs found
Biological control of the terrestrial carbon sink
This lecture reviews the past (since 1964 when the International Biological Program began) and the future of our understanding of terrestrial carbon fluxes with focus on photosynthesis, respiration, primary-, ecosystem-, and biomeproductivity. Photosynthetic capacity is related to the nitrogen concentration of leaves, but the capacity is only rarely reached under field conditions. Average rates of photosynthesis and stomatal conductance are closely correlated and operate near 50% of their maximal rate, with light being the limiting factor in humid regions and air humidity and soil water the limiting factor in arid climates. Leaf area is the main factor to extrapolate from leaves to canopies, with maximum surface conductance being dependent on leaf level stomatal conductance. Additionally, gas exchange depends also on rooting depth which determines the water and nutrient availability and on mycorrhizae which regulate the nutrient status. An important anthropogenic disturbance is the nitrogen uptake from air pollutants, which is not balanced by cation uptake from roots and this may lead to damage and breakdown of the plant cover. Photosynthesis is the main carbon input into ecosystems, but it alone does not represent the ecosystem carbon balance, which is determined by respiration of various kinds. Plant respiration and photosynthesis determine growth (net primary production) and microbial respiration balances the net ecosystem flux. In a spruce forest, 30% of the assimilatory carbon gain is used for respiration of needles, 20% is used for respiration in stems. Soil respiration is about 50% the carbon gain, half of which is root respiration, half is microbial respiration. In addition, disturbances lead to carbon losses, where fire, harvest and grazing bypass the chain of respiration. In total, the carbon balance at the biome level is only about 1% of the photosynthetic carbon input, or may indeed become negative. The recent observed increase in plant growth has different reasons depending on the region of the world: anthropogenic nitrogen deposition is the controlling factor in Europe, increasing global temperatures is the main factor in Siberia, and maybe rising CO2 the factor controlling the carbon fluxes in Amazonia. However, this has not lead to increases in net biome productivity, due to associated losses. Also important is the interaction between biodiversity and biogeochemical processes. It is shown that net primary productivity increases with plant species diversity (50% species loss equals 20% loss in productivity). However, in this extrapolation the action of soil biota is poorly understood although soils contribute the largest number of species and of taxonomic groups to an ecosystem. The global terrestrial carbon budget strongly depends on areas with pristine old growth forests which are carbon sinks. The management options are very limited, mostly short term, and usually associated with high uncertainty. Unmanaged grasslands appear to be a carbon sink of similar magnitude as forest, but generally these ecosystems lost their C with grazing and agricultural use. Extrapolation to the future of Earth climate shows that the biota will not be able to balance fossil fuel emissions, and that it will be essential to develop a carbon free energy system in order to maintain the living conditions on earth. [References: 68
The effect of ambipolar electric fields on the electron heating in capacitive RF plasmas
We investigate the electron heating dynamics in electropositive argon and
helium capacitively coupled RF discharges driven at 13.56 MHz by Particle in
Cell simulations and by an analytical model. The model allows to calculate the
electric field outside the electrode sheaths, space and time resolved within
the RF period. Electrons are found to be heated by strong ambipolar electric
fields outside the sheath during the phase of sheath expansion in addition to
classical sheath expansion heating. By tracing individual electrons we also
show that ionization is primarily caused by electrons that collide with the
expanding sheath edge multiple times during one phase of sheath expansion due
to backscattering towards the sheath by collisions. A synergistic combination
of these different heating events during one phase of sheath expansion is
required to accelerate an electron to energies above the threshold for
ionization. The ambipolar electric field outside the sheath is found to be time
modulated due to a time modulation of the electron mean energy caused by the
presence of sheath expansion heating only during one half of the RF period at a
given electrode. This time modulation results in more electron heating than
cooling inside the region of high electric field outside the sheath on time
average. If an electric field reversal is present during sheath collapse, this
time modulation and, thus, the asymmetry between the phases of sheath expansion
and collapse will be enhanced. We propose that the ambipolar electron heating
should be included in models describing electron heating in capacitive RF
plasmas
Derivation of diagnostic models based on formalized process knowledge
© IFAC.Industrial systems are vulnerable to faults. Early and accurate detection and diagnosis in production systems can minimize down-time, increase the safety of the plant operation, and reduce manufacturing costs. Knowledge- and model-based approaches to automated fault detection and diagnosis have been demonstrated to be suitable for fault cause analysis within a broad range of industrial processes and research case studies. However, the implementation of these methods demands a complex and error-prone development phase, especially due to the extensive efforts required during the derivation of models and their respective validation. In an effort to reduce such modeling complexity, this paper presents a structured causal modeling approach to supporting the derivation of diagnostic models based on formalized process knowledge. The method described herein exploits the Formalized Process Description Guideline VDI/VDE 3682 to establish causal relations among key-process variables, develops an extension of the Signed Digraph model combined with the use of fuzzy set theory to allow more accurate causality descriptions, and proposes a representation of the resulting diagnostic model in CAEX/AutomationML targeting dynamic data access, portability, and seamless information exchange
Nuclear Pairing in the T=0 channel revisited
Recent published data on the isoscalar gap in symmetric nuclear matter using
the Paris force and the corresponding BHF single particle dispersion are
corrected leading to an extremely high proton-neutron gap of
MeV at . Arguments whether this value can be reduced due
to screening effects are discussed. A density dependent delta interaction with
cut off is adjusted so as to approximately reproduce the nuclear matter values
with the Paris force.Comment: 4 pages, 4 figure
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