551 research outputs found
Clouds in the atmospheres of extrasolar planets. IV. On the scattering greenhouse effect of CO2 ice particles: Numerical radiative transfer studies
Owing to their wavelengths dependent absorption and scattering properties,
clouds have a strong impact on the climate of planetary atmospheres.
Especially, the potential greenhouse effect of CO2 ice clouds in the
atmospheres of terrestrial extrasolar planets is of particular interest because
it might influence the position and thus the extension of the outer boundary of
the classic habitable zone around main sequence stars.
We study the radiative effects of CO2 ice particles obtained by different
numerical treatments to solve the radiative transfer equation. The comparison
between the results of a high-order discrete ordinate method and simpler
two-stream approaches reveals large deviations in terms of a potential
scattering efficiency of the greenhouse effect. The two-stream methods
overestimate the transmitted and reflected radiation, thereby yielding a higher
scattering greenhouse effect. For the particular case of a cool M-type dwarf
the CO2 ice particles show no strong effective scattering greenhouse effect by
using the high-order discrete ordinate method, whereas a positive net
greenhouse effect was found in case of the two-stream radiative transfer
schemes. As a result, previous studies on the effects of CO2 ice clouds using
two-stream approximations overrated the atmospheric warming caused by the
scattering greenhouse effect. Consequently, the scattering greenhouse effect of
CO2 ice particles seems to be less effective than previously estimated. In
general, higher order radiative transfer methods are necessary to describe the
effects of CO2 ice clouds accurately as indicated by our numerical radiative
transfer studies.Comment: accepted for publication in A&
Low-temperature nucleation in a kinetic Ising model under different stochastic dynamics with local energy barriers
Using both analytical and simulational methods, we study low-temperature
nucleation rates in kinetic Ising lattice-gas models that evolve under two
different Arrhenius dynamics that interpose between the Ising states a
transition state representing a local energy barrier. The two dynamics are the
transition-state approximation [T. Ala-Nissila, J. Kjoll, and S. C. Ying, Phys.
Rev. B 46, 846 (1992)] and the one-step dynamic [H. C. Kang and W. H. Weinberg,
J. Chem. Phys. 90, 2824 (1989)]. Even though they both obey detailed balance
and are here applied to a situation that does not conserve the order parameter,
we find significant differences between the nucleation rates observed with the
two dynamics, and between them and the standard Glauber dynamic [R. J. Glauber,
J. Math. Phys. 4, 294 (1963)], which does not contain transition states. Our
results show that great care must be exercised when devising kinetic Monte
Carlo transition rates for specific physical or chemical systems.Comment: 14 pages RevTex, 6 embedded figures. Minor revisions. J. Chem. Phys.,
in pres
On the climatic impact of CO2 ice particles in atmospheres of terrestrial exoplanets
Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugÀnglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.Clouds play a significant role for the energy budget in planetary atmospheres. They can scatter incident stellar radiation back to space, effectively cooling the surface of terrestrial planets. On the other hand, they may contribute to the atmospheric greenhouse effect by trapping outgoing thermal radiation. For exoplanets near the outer boundary of the habitable zone, condensation of CO2 can occur due to the low atmospheric temperatures. These CO2 ice clouds may play an important role for the surface temperature and, therefore, for the question of habitability of those planets. However, the optical properties of CO2 ice crystals differ significantly from those of water droplets or water ice particles. Except for a small number of strong absorption bands, they are almost transparent with respect to absorption. Instead, they are highly effective scatterers at long and short wavelengths. Therefore, the climatic effect of a CO2 ice cloud will depend on how much incident stellar radiation is scattered to space in comparison to the amount of thermal radiation scattered back towards the planetary surface. This contribution aims at the potential greenhouse effect of CO2 ice particles. Their scattering and absorption properties are calculated for assumed particle size distributions with different effective radii and particle densities. An accurate radiative transfer model is used to determine the atmospheric radiation field affected by such CO2 particles. These results are compared to less detailed radiative transfer schemes employed in previous studies
FastChem Cond: Equilibrium chemistry with condensation and rainout for cool planetary and stellar environments
Cool astrophysical objects, such as (exo)planets, brown dwarfs, or asymptotic
giant branch stars, can be strongly affected by condensation. Condensation does
not only directly affect the chemical composition of the gas phase by removing
elements but the condensed material also influences other chemical and physical
processes in these object. This includes, for example, the formation of clouds
in planetary atmospheres and brown dwarfs or the dust-driven winds of evolved
stars. In this study we introduce FastChem Cond, a new version of the FastChem
equilibrium chemistry code that adds a treatment of equilibrium condensation.
Determining the equilibrium composition under the impact of condensation is
complicated by the fact that the number of condensates that can exist in
equilibrium with the gas phase is limited by a phase rule. However, this phase
rule does not directly provide information on which condensates are stable. As
a major advantage of FastChem Cond is able to automatically select the set
stable condensates satisfying the phase rule. Besides the normal equilibrium
condensation, FastChem Cond can also be used with the rainout approximation
that is commonly employed in atmospheres of brown dwarfs or (exo)planets.
FastChem Cond is available as open-source code, released under the GPLv3
licence. In addition to the C++ code, FastChem Cond also offers a Python
interface. Together with the code update we also add about 290 liquid and solid
condensate species to FastChem.Comment: submitted to MNRAS, code available at
https://github.com/exoclime/FastChe
The extrasolar planet Gliese 581 d: a potentially habitable planet? (Corrigendum to arXiv:1009.5814)
We report here that the equation for H2O Rayleigh scattering was incorrectly
stated in the original paper [arXiv:1009.5814]. Instead of a quadratic
dependence on refractivity r, we accidentally quoted an r^4 dependence. Since
the correct form of the equation was implemented into the model, scientific
results are not affected.Comment: accepted to Astronomy&Astrophysic
Heavy Metals Removal Using Natural Jordanian Volcanic Tuff
The removal performance and the selectivity sequence of separate metal ions (Fe2+, Cr3+, Cu2+, Zn2+ and Ni2+) in aqueous solution were studied by adsorption process on untreated and natural volcanic tuff. A series of experiments were conducted in batch-wise and fixed-bed columns to investigate the removal efficiency of natural Jordanian volcanic tuff as low cost and an effective adsorbent for heavy metal ions and to examine its economical application in water purification and treatment practices.Water and wastewater samples containing metal ions with concentrations ranging from 1 to 15 mg/L were used. The plexi glas columns were filled with natural occurring volcanic tuff particles ranging between (0.350 â 3.000) mm. Photometric methods were used for laboratory analysis of samples.The experiments were carried out under changing conditions as a function of different pH-values (2,4,6 and 7), initial solute concentrations (1, 5, 10, 15) mg/L, and different room temperatures (20, 25 and 30 CÂș ), and varying tuff particle sizes (0.35 -3.0) mm. The breakthrough curves were derived by plotting the normalized effluent metal concentrations (C/C0) versus bed volume.Obtained results showed that natural Jordanian volcanic tuff has an adsorption capacity of 0.417 mg/g for Fe 2+ and 0.151mg/g for C 2+. Factors in the reaction medium such as pH and ionic strength influenced the adsorption process. The quantity of particular ionic species (Cu2+, Pb2+, Cr2+ ,Fe2+, Zn2+) bound in dependence on the initial concentrations, indicates that the removal efficiency from the liquid phase follows the sequence Fe2+>Cu2+>Pb2+> Cr2+>Zn2+ when keeping the pH at 4 and follows the sequence Cu2+>Zi2+>Fe2+>Cr2+>Pb2+ when keeping the pH at 6. Equilibrium modeling of the removal showed that the adsorption of the metal cations Cr2+, Pb2+, Zn2+ , Cu2+ and Fe2+ were fitted to one of the adsorption isotherms
Low-temperature nucleation in a kinetic Ising model with soft stochastic dynamics
We study low-temperature nucleation in kinetic Ising models by analytical and
simulational methods, confirming the general result for the average metastable
lifetime, = A*exp(beta*Gamma) (beta = 1/kT) [E. Jordao Neves and R.H.
Schonmann, Commun. Math. Phys. 137, 209 (1991)]. Contrary to common belief, we
find that both A and Gamma depend significantly on the stochastic dynamic. In
particular, for a ``soft'' dynamic, in which the effects of the interactions
and the applied field factorize in the transition rates, Gamma does NOT simply
equal the energy barrier against nucleation, as it does for the standard
Glauber dynamic, which does not have this factorization property.Comment: 4 pages RevTex4, 2 figures. Phys. Rev. Lett., in pres
Quantum approach to nucleation times of kinetic Ising ferromagnets
Low temperature dynamics of Ising ferromagnets under finite magnetic fields
are studied in terms of quantum spin representations of stochastic evolution
operators. These are constructed for the Glauber dynamic as well as for a
modification of this latter, introduced by K. Park {\it et al.} in Phys. Rev.
Lett. {\bf 92}, 015701 (2004). In both cases the relaxation time after a field
quench is evaluated both numerically and analytically using the spectrum gap of
the corresponding operators. The numerical work employs standard recursive
techniques following a symmetrization of the evolution operator accomplished by
a non-unitary spin rotation. The analytical approach uses low temperature
limits to identify dominant terms in the eigenvalue problem. It is argued that
the relaxation times already provide a measure of actual nucleation lifetimes
under finite fields. The approach is applied to square, triangular and
honeycomb lattices.Comment: 14 pages, 6 figure
Clouds in the atmospheres of extrasolar planets. II. Thermal emission spectra of Earth-like planets influenced by low and high-level clouds
We study the impact of multi-layered clouds (low-level water and high-level
ice clouds) on the thermal emission spectra of Earth-like planets orbiting
different types of stars. Clouds have an important influence on such planetary
emission spectra due to their wavelength dependent absorption and scattering
properties. We also investigate the influence of clouds on the ability to
derive information about planetary surface temperatures from low-resolution
spectra.Comment: accepted for publication in A&
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