scholarly journals Dust in brown dwarfs and extra-solar planets

2018 ◽  
Vol 614 ◽  
pp. A126 ◽  
Author(s):  
G. K. H. Lee ◽  
J. Blecic ◽  
Ch. Helling
Keyword(s):  
Stellar Atmospheres ◽  
Nucleation Theory ◽  
Cloud Formation ◽  
Classical Nucleation Theory ◽  
Seed Particle ◽  
Aerosol Layer ◽  
Gas Temperature ◽  
Seed Formation ◽  
Seed Particles ◽  
Cloud Models

Context. The cloud formation process starts with the formation of seed particles, after which, surface chemical reactions grow or erode the cloud particles. If seed particles do not form, or are not available by another means, an atmosphere is unable to form a cloud complex and will remain cloud free. Aims. We aim to investigate which materials may form cloud condensation seeds in the gas temperature and pressure regimes (Tgas = 100–2000 K, pgas = 10−8–100 bar) expected to occur in planetary and brown dwarf atmospheres. Methods. We have applied modified classical nucleation theory which requires surface tensions and vapour pressure data for each solid species, which are taken from the literature. Input gas phase number densities are calculated assuming chemical equilibrium at solar metallicity. Results. We calculated the seed formation rates of TiO2[s] and SiO[s] and find that they efficiently nucleate at high temperatures of Tgas = 1000–1750 K. Cr[s], KCl[s] and NaCl[s] are found to efficiently nucleate across an intermediate temperature range of Tgas = 500–1000 K. We find CsCl[s] may serve as the seed particle for the water cloud layers in cool sub-stellar atmospheres. The nucleation rates of four low temperature ice species (Tgas = 100–250 K), H2O[s/l], NH3[s], H2S[s/l], and CH4[s], are also investigated for the coolest sub-stellar and planetary atmospheres. Conclusions. Our results suggest a possibly (Tgas, pgas) distributed hierarchy of seed particle formation regimes throughout the substellar and planetary atmospheric temperature-pressure space. With TiO2[s] providing seed particles for the most refractory cloud formation species (e.g. Al2O3[s], Fe[s], MgSiO3[s], Mg2SiO4[s]), Cr[s] providing the seed particles for MnS[s], Na2S[s], and ZnS[s] sulfides, and K/Na/Rb/Cs/NH4-Cl binding solid species providing the seed particles for H2O[s/l] and NH4-H2PO4/SH[s] clouds. A detached, high-altitude aerosol layer may form in some sub-stellar atmospheres from the nucleation process, dependent on the upper atmosphere temperature, pressure and availability of volatile elements. In order to improve the accuracy of the nucleation rate calculation, further research into the small cluster thermochemical data for each cloud species is warranted. The validity of these seed particle scenarios will be tested by applying it to more complete cloud models in the future.

Thermodynamic and Kinetic controls to the ice nucleation rate

2020 ◽  
Author(s):  
Donifan Barahona
Keyword(s):  
Nucleation Rate ◽  
Anthropogenic Activities ◽  
Ice Nucleation ◽  
Nucleation Theory ◽  
Cloud Formation ◽  
Classical Nucleation Theory ◽  
Limited Information ◽  
Atmospheric Conditions ◽  
Theoretical Approaches ◽  
Cloud Models

<p>Ice nucleation is a necessary step for the formation of ice clouds in the atmosphere. It has become clear that its correct representation is critical for the accurate description of atmospheric processes, and for the reliable prediction of the effect of anthropogenic activities on climate. This is accomplished in most cases using empirical models. Although a simple way to parameterize ice nucleation they provide limited information on the nature of ice formation and may not represent all atmospheric conditions.  Theoretical approaches used in cloud models are typically based on the Classical Nucleation Theory (CNT).  There is however a large uncertainty in key parameters of the theory which in most cases are fitted to reproduce observed rates. This talk details recent efforts to go beyond the formulation of CNT to describe ice nucleation. It shows that it is possible to define uncertain parameters like the ice-liquid interfacial tension and the activation energy over a pure thermodynamic basis, hence only as a function of the bulk thermodynamic properties of water. This approach is extended to describe heterogeneous ice nucleation mediated by immersed ice nucleating particles (INPs). It is shown that INPs that significantly reduce the work of ice nucleation also pose strong limitations to the growth of the nascent ice germs. This leads to the onset of a new ice nucleation regime, called spinodal ice nucleation, where the dynamics of ice germ growth instead of the ice germ size determines the nucleation rate. Nucleation in this regime is characterized by an enhanced sensitivity to particle area and cooling rate. Finally a new approach to extract intrinsic nucleation rates from droplet-freezing experiments is used to compare of predicted ice nucleation rates against experimental measurements, for a diverse set of species relevant to cloud formation. This comparison suggests that spinodal ice nucleation may be common in nature, and shows a considerable skill of the new theory in predicting measured ice nucleation rates. </p>

scholarly journals On the onset of dust formation in AGB stars

2018 ◽  
Vol 14 (S343) ◽  
pp. 119-128
Author(s):  
David Gobrecht ◽  
Stefan T. Bromley ◽  
John M. C. Plane ◽  
Leen Decin ◽  
Sergio Cristallo
Keyword(s):  
Emission Feature ◽  
Stellar Atmospheres ◽  
Nucleation Theory ◽  
Classical Nucleation Theory ◽  
Quantum Mechanical ◽  
Dust Formation ◽  
Agb Stars ◽  
Promising Candidate ◽  
Bending Modes ◽  
Structure Calculations

AbstractA promising candidate to initiate dust formation in oxygen-rich AGB stars is alumina (Al2O3) showing an emission feature around ∼13μm attributed to Al−O stretching and bending modes (Posch+99,Sloan+03). The counterpart to alumina in carbon-rich AGB atmospheres is the highly refractory silicon carbide (SiC) showing a characteristic feature around 11.3μm (Treffers74). Alumina and SiC grains are thought to represent the first condensates to emerge in AGB stellar atmospheres. We follow a bottom-up approach, starting with the smallest stoichiometric clusters (i.e. Al4O6, Si2C2), successively building up larger-sized clusters. We present new results of quantum-mechanical structure calculations of (Al2O3)n, n = 1−10 and (SiC)n clusters with n = 1−16, including potential energies, rotational constants, and structure-specific vibrational spectra. We demonstrate the energetic viability of homogeneous nucleation scenarios where monomers (Al2O3 and SiC) or dimers (Al4O6 and Si2C2) are successively added. We find significant differences between our quantum theory based results and nanoparticle properties derived from (classical) nucleation theory.

scholarly journals Analysis of the effect of water activity on ice formation using a new thermodynamic framework

2014 ◽  
Vol 14 (14) ◽  
pp. 7665-7680 ◽  
Author(s):  
D. Barahona
Keyword(s):  
Water Activity ◽  
Nucleation Rate ◽  
Ice Nucleation ◽  
Nucleation Theory ◽  
Cloud Formation ◽  
Classical Nucleation Theory ◽  
Solid Matrix ◽  
Effect Of Water ◽  
Thermodynamic Framework ◽  
New Framework

Abstract. In this work a new thermodynamic framework is developed and used to investigate the effect of water activity on the formation of ice within supercooled droplets. The new framework is based on a novel concept where the interface is assumed to be made of liquid molecules "trapped" by the solid matrix. It also accounts for the change in the composition of the liquid phase upon nucleation. Using this framework, new expressions are developed for the critical ice germ size and the nucleation work with explicit dependencies on temperature and water activity. However unlike previous approaches, the new model does not depend on the interfacial tension between liquid and ice. The thermodynamic framework is introduced within classical nucleation theory to study the effect of water activity on the ice nucleation rate. Comparison against experimental results shows that the new approach is able to reproduce the observed effect of water activity on the nucleation rate and the freezing temperature. It allows for the first time a phenomenological derivation of the constant shift in water activity between melting and nucleation. The new framework offers a consistent thermodynamic view of ice nucleation, simple enough to be applied in atmospheric models of cloud formation.

scholarly journals Deposition-mode ice nucleation reexamined at temperatures below 200 K

2015 ◽  
Vol 15 (4) ◽  
pp. 1621-1632 ◽  
Author(s):  
E. S. Thomson ◽  
X. Kong ◽  
P. Papagiannakopoulos ◽  
J. B. C. Pettersson
Keyword(s):  
Substrate Surface ◽  
Ice Nucleation ◽  
Nucleation Theory ◽  
Cloud Formation ◽  
Classical Nucleation Theory ◽  
Careful Examination ◽  
Contradictory Result ◽  
Experimental Conditions ◽  
Water Ice ◽  
Atmospheric Nucleation

Abstract. The environmental chamber of a molecular beam apparatus is used to study deposition nucleation of ice on graphite, alcohols and acetic and nitric acids at temperatures between 155 and 200 K. The critical supersaturations necessary to spontaneously nucleate water ice on six different substrate materials are observed to occur at higher supersaturations than are theoretically predicted. This contradictory result motivates more careful examination of the experimental conditions and the underlying basis of the current theories. An analysis based on classical nucleation theory supports the view that at these temperatures nucleation is primarily controlled by the rarification of the vapor and the strength of water's interaction with the substrate surface. The technique enables a careful probing of the underlying processes of ice nucleation and the substrate materials of study. The findings are relevant to atmospheric nucleation processes that are intrinsically linked to cold cloud formation and lifetime.

scholarly journals An aerosol chamber investigation of the heterogeneous ice nucleating potential of refractory nanoparticles

2010 ◽  
Vol 10 (3) ◽  
pp. 1227-1247 ◽  
Author(s):  
R. W. Saunders ◽  
O. Möhler ◽  
M. Schnaiter ◽  
S. Benz ◽  
R. Wagner ◽  
...  
Keyword(s):  
Silicon Oxide ◽  
Cloud Model ◽  
Active Surface ◽  
Accommodation Coefficient ◽  
Contact Angles ◽  
Nucleation Theory ◽  
Cloud Formation ◽  
Classical Nucleation Theory ◽  
Ice Nuclei

Abstract. Nanoparticles of iron oxide (crystalline and amorphous), silicon oxide and magnesium oxide were investigated for their propensity to nucleate ice over the temperature range 180–250 K, using the AIDA chamber in Karlsruhe, Germany. All samples were observed to initiate ice formation via the deposition mode at threshold ice super-saturations (RHithresh) ranging from 105% to 140% for temperatures below 220 K. Approximately 10% of amorphous Fe2O3 particles (modal diameter = 30 nm) generated in situ from a photochemical aerosol reactor, led to ice nucleation at RHithresh = 140% at an initial chamber temperature of 182 K. Quantitative analysis using a singular hypothesis treatment provided a fitted function [ns(190 K)=10(3.33×sice)+8.16] for the variation in ice-active surface site density (ns:m−2) with ice saturation (sice) for Fe2O3 nanoparticles. This was implemented in an aerosol-cloud model to determine a predicted deposition (mass accommodation) coefficient for water vapour on ice of 0.1 at temperatures appropriate for the upper atmosphere. Classical nucleation theory was used to determine representative contact angles (θ) for the different particle compositions. For the in situ generated Fe2O3 particles, a slight inverse temperature dependence was observed with θ = 10.5° at 182 K, decreasing to 9.0° at 200 K (compared with 10.2° and 11.4° respectively for the SiO2 and MgO particle samples at the higher temperature). These observations indicate that such refractory nanoparticles are relatively efficient materials for the nucleation of ice under the conditions studied in the chamber which correspond to cirrus cloud formation in the upper troposphere. The results also show that Fe2O3 particles do not act as ice nuclei under conditions pertinent for tropospheric mixed phase clouds, which necessarily form above ~233 K. At the lower temperatures (<150 K) where noctilucent clouds form during summer months in the high latitude mesosphere, higher contact angles would be expected, which may reduce the effectiveness of these particles as ice nuclei in this part of the atmosphere.

scholarly journals Parameterizing the competition between homogeneous and heterogeneous freezing in ice cloud formation – polydisperse ice nuclei

2009 ◽  
Vol 9 (16) ◽  
pp. 5933-5948 ◽  
Author(s):  
D. Barahona ◽  
A. Nenes
Keyword(s):  
Number Concentration ◽  
Nucleation Theory ◽  
Cloud Formation ◽  
Classical Nucleation Theory ◽  
Average Error ◽  
Ice Crystal ◽  
Ice Cloud ◽  
Ice Nuclei ◽  
Wide Range ◽  
Model Equations

Abstract. This study presents a comprehensive ice cloud formation parameterization that computes the ice crystal number, size distribution, and maximum supersaturation from precursor aerosol and ice nuclei. The parameterization provides an analytical solution of the cloud parcel model equations and accounts for the competition effects between homogeneous and heterogeneous freezing, and, between heterogeneous freezing in different modes. The diversity of heterogeneous nuclei is described through a nucleation spectrum function which is allowed to follow any form (i.e., derived from classical nucleation theory or from observations). The parameterization reproduces the predictions of a detailed numerical parcel model over a wide range of conditions, and several expressions for the nucleation spectrum. The average error in ice crystal number concentration was −2.0±8.5% for conditions of pure heterogeneous freezing, and, 4.7±21% when both homogeneous and heterogeneous freezing were active. The formulation presented is fast and free from requirements of numerical integration.

Kinetic Analysis on Nanoparticle Condensation by Molecular Dynamics

2013 ◽  
Vol 135 (10) ◽  
Author(s):  
Donguk Suh ◽  
Kenji Yasuoka
Keyword(s):  
Molecular Dynamics ◽  
Growth Rate ◽  
Free Energy ◽  
Kinetic Analysis ◽  
Seed Size ◽  
Homogeneous Nucleation ◽  
Cluster Formation ◽  
Nucleation Theory ◽  
Classical Nucleation Theory ◽  
Seed Particle

Condensation on a cubic seed particle was simulated by classical molecular dynamics (MD). Seed size and supersaturation ratio of the system were the factors that were examined in order to observe the effects of the dimension of seeds and thermodynamic conditions. Two stages of nucleation were observed in the phenomenon, where the first stage is from the seed growth and the second from homogeneous nucleation. Therefore, the nucleation rate and growth rate were each calculated by the Yasuoka–Matsumoto (YM) method. As the seed size increased, the growth rate decreased, but there was no clear seed influence on the homogeneous nucleation characteristics. Besides, the classical nucleation theory (CNT), cluster formation free energy and kinetic analysis were conducted. The free energy in the exponential term of the classical nucleation theory and that obtained from the cluster formation free energy showed different characteristics.

scholarly journals Exploring the Mechanisms of Ice Nucleation on Kaolinite: From Deposition Nucleation to Condensation Freezing

2013 ◽  
Vol 71 (1) ◽  
pp. 16-36 ◽  
Author(s):  
André Welti ◽  
Zamin A. Kanji ◽  
F. Lüönd ◽  
Olaf Stetzer ◽  
Ulrike Lohmann
Keyword(s):  
Water Saturation ◽  
Particle Surface ◽  
Ice Nucleation ◽  
Mixed Phase ◽  
Nucleation Theory ◽  
Cloud Formation ◽  
Classical Nucleation Theory ◽  
Appreciable Amount ◽  
Nucleation Mechanisms ◽  
Ice Nucleation Activity

Abstract To identify the temperature and humidity conditions at which different ice nucleation mechanisms are active, the authors conducted experiments on 200-, 400-, and 800-nm size-selected kaolinite particles, exposing them to temperatures between 218 and 258 K and relative humidities with respect to ice (RHi) between 100% and 180%, including the typical conditions for cirrus and mixed-phase-cloud formation. Measurements of the ice active particle fraction as a function of temperature and relative humidity with respect to ice are reported. The authors find enhanced activated fractions when water saturation is reached at mixed-phase-cloud temperatures between 235 and 241 K and a distinct increase in the activated fraction below 235 K at conditions below water saturation. To provide a functional description of the observed ice nucleation mechanisms, the experimental results are analyzed by two different particle-surface models within the framework of classical nucleation theory. Describing the ice nucleation activity of kaolinite particles by assuming deposition nucleation to be the governing mechanism below water saturation was found to be inadequate to represent the experimental data in the whole temperature range investigated. The observed increase in the activated fraction below water saturation and temperatures below 235 K corroborate the assumption that an appreciable amount of adsorbed or capillary condensed water is present on kaolinite particles, which favors ice nucleation.

scholarly journals Parameterizing the competition between homogeneous and heterogeneous freezing in ice cloud formation – polydisperse ice nuclei

2009 ◽  
Vol 9 (3) ◽  
pp. 10957-11004 ◽  
Author(s):  
D. Barahona ◽  
A. Nenes
Keyword(s):  
Size Distribution ◽  
Nucleation Theory ◽  
Cloud Formation ◽  
Classical Nucleation Theory ◽  
Average Error ◽  
Ice Crystal ◽  
Ice Cloud ◽  
Ice Nuclei ◽  
Wide Range ◽  
Model Equations

Abstract. This study presents a comprehensive ice cloud formation parameterization that computes the ice crystal number, size distribution, and maximum supersaturation from precursor aerosol and ice nuclei with any size distribution and chemical composition. The parameterization provides an analytical solution of the cloud parcel model equations and accounts for the competition effects between homogeneous and heterogeneous freezing, and, between heterogeneous freezing in different modes. The diversity of heterogeneous nuclei is described through a nucleation spectrum function which is allowed to follow any form (i.e., derived from classical nucleation theory or from empirical observations). The parameterization reproduced the predictions of a detailed numerical parcel model over a wide range of conditions, and several expressions for the nucleation spectrum. The average error in ice crystal number concentration was −2.0±8.5% for conditions of pure heterogeneous freezing, and, 4.7±21% when both homogeneous and heterogeneous freezing were active. Apart from its rigor, excellent performance and versatility, the formulation is extremely fast and free from requirements of numerical integration.

Kinetic Analysis on Nanoparticle Condensation by Molecular Dynamics

2012 ◽  
Author(s):  
Donguk Suh ◽  
Kenji Yasuoka
Keyword(s):  
Molecular Dynamics ◽  
Growth Rate ◽  
Free Energy ◽  
Kinetic Analysis ◽  
Seed Size ◽  
Homogeneous Nucleation ◽  
Cluster Formation ◽  
Nucleation Theory ◽  
Classical Nucleation Theory ◽  
Seed Particle

Condensation on a cubic seed particle was simulated by classical molecular dynamics. Seed size and supersaturation ratio of the system were the factors that were examined in order to observe the effects of the dimension of seeds and thermodynamic conditions. Two stages of nucleation were observed in the phenomenon, where the first stage is from the seed growth and the second from homogeneous nucleation. Therefore, the nucleation rate and growth rate were each calculated by the Yasuoka-Matsumoto method. As seed size increased the growth rate decreased, but there was no clear seed influence on the homogeneous nucleation characteristics. Besides the classical nucleation theory, cluster formation free energy and kinetic analysis were conducted. The free energy in the exponential term of the classical nucleation theory and that obtained from the cluster formation free energy showed different characteristics.

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