scholarly journals Immersion freezing of water and aqueous ammonium sulfate droplets initiated by humic-like substances as a function of water activity

2013 ◽  
Vol 13 (13) ◽  
pp. 6603-6622 ◽  
Author(s):  
Y. J. Rigg ◽  
P. A. Alpert ◽  
D. A. Knopf
Keyword(s):  
Water Activity ◽  
Active Sites ◽  
Melting Curve ◽  
Contact Angles ◽  
Ice Nucleation ◽  
Nucleation Theory ◽  
Classical Nucleation Theory ◽  
Surface Areas ◽  
Immersion Freezing ◽  
Freezing Temperatures

Abstract. Immersion freezing of water and aqueous (NH4)2SO4 droplets containing leonardite (LEO) and Pahokee peat (PP) serving as surrogates for humic-like substances (HULIS) has been investigated. Organic aerosol containing HULIS are ubiquitous in the atmosphere; however, their potential for ice cloud formation is uncertain. Immersion freezing has been studied for temperatures as low as 215 K and solution water activity, aw, from 0.85 to 1.0. The freezing temperatures of water and aqueous solution droplets containing LEO and PP are 5–15 K warmer than homogeneous ice nucleation temperatures. Heterogeneous freezing temperatures can be represented by a horizontal shift of the ice melting curve as a function of solution aw by Δaw = 0.2703 and 0.2466, respectively. Corresponding hetrogeneous ice nucleation rate coefficients, Jhet, are (9.6 ± 2.5)×104 and (5.4 ± 1.4)×104 cm−2 s−1 for LEO and PP containing droplets, respectively, and remain constant along freezing curves characterized by Δaw. Consequently predictions of freezing temperatures and kinetics can be made without knowledge of the solute type when relative humidity and ice nuclei (IN) surface areas are known. The acquired ice nucleation data are applied to evaluate different approaches to fit and reproduce experimentally derived frozen fractions. In addition, we apply a basic formulation of classical nucleation theory (α(T)-model) to calculate contact angles and frozen fractions. Contact angles calculated for each ice nucleus as a function of temperature, α(T)-model, reproduce exactly experimentally derived frozen fractions without involving free-fit parameters. However, assigning the IN a single contact angle for the entire population (single-α model) is not suited to represent the frozen fractions. Application of α-PDF, active sites, and deterministic model approaches to measured frozen fractions yield similar good representations. Furthermore, when using a single parameterization of α-PDF or active sites distribution to fit all individual aw immersion freezing data simultaneously, frozen fraction curves are not reproduced. This implies that these fitting formulations cannot be applied to immersion freezing of aqueous solutions, and suggests that derived fit parameters do not represent independent particle properties. Thus, from fitting frozen fractions only, the underlying ice nucleation mechanism and nature of the ice nucleating sites cannot be inferred. In contrast to using fitted functions obtained to represent experimental conditions only, we suggest to use experimentally derived Jhet as a function of temperature and aw that can be applied to conditions outside of those probed in laboratory. This is because Jhet(T) is independent of time and IN surface areas in contrast to the fit parameters obtained by representation of experimentally derived frozen fractions.

scholarly journals Immersion freezing of water and aqueous ammonium sulphate droplets initiated by Humic Like Substances as a function of water activity

2013 ◽  
Vol 13 (2) ◽  
pp. 4917-4961
Author(s):  
Y. J. Rigg ◽  
P. A. Alpert ◽  
D. A. Knopf
Keyword(s):  
Water Activity ◽  
Active Sites ◽  
Melting Curve ◽  
Contact Angles ◽  
Ice Nucleation ◽  
Nucleation Theory ◽  
Classical Nucleation Theory ◽  
Surface Areas ◽  
Immersion Freezing ◽  
Freezing Temperatures

Abstract. Immersion freezing of water and aqueous (NH4)2SO4 droplets containing Leonardite (LEO) and Pahokee peat (PP) serving as surrogates for Humic Like Substances (HULIS) has been investigated. Organic aerosol containing HULIS are ubiquitous in the atmosphere, however, their potential for ice cloud formation is uncertain. Immersion freezing has been studied for temperatures as low as 215 K and solution water activity, aw, from 0.85–1.0. The freezing temperatures of water and aqueous solution droplets containing LEO and PP are 5–15 K warmer than homogeneous ice nucleation temperatures. Heterogeneous freezing temperatures can be represented by a horizontal shift of the ice melting curve as a function of solution aw, Δaw, by 0.2703 and 0.2466, respectively. Corresponding heterogeneous ice nucleation rate coefficients, Jhet, are (9.6 ± 2.5)×104 and (5.4 ± 1.4)×104 cm−2 s−1 for LEO and PP containing droplets, respectively, and remain constant along freezing curves characterized by Δaw. Consequently predictions of freezing temperatures and kinetics can be made without knowledge of the solute type when relative humidity and IN surface areas are known. The acquired ice nucleation data are applied to evaluate different approaches to fit and reproduce experimentally derived frozen fractions. In addition, we apply a basic formulation of classical nucleation theory (α(T)-model) to calculate contact angles and frozen fractions. Contact angles calculated for each ice nucleus as a function of temperature, α(T)-model, reproduce exactly experimentally derived frozen fractions without involving free fit parameters. However, assigning the IN a single contact angle for entire population (single-α model) is not suited to represent the frozen fractions. Application of α-PDF, active sites, and deterministic model approaches to measured frozen fractions yield similar good representations. Thus, from fitting frozen fractions only, the underlying ice nucleation mechanism and nature of the ice nucleating sites cannot be inferred. In contrast to using fitted functions obtained to represent experimental conditions only, we suggest to use experimentally derived Jhet as a function of temperature and aw that can be applied to conditions outside of those probed in laboratory. This is because Jhet(T) is independent of time and IN surface areas in contrast to the fit parameters obtained by representation of experimentally derived frozen fractions.

scholarly journals Efficiency of immersion mode ice nucleation on surrogates of mineral dust

2007 ◽  
Vol 7 (19) ◽  
pp. 5081-5091 ◽  
Author(s):  
C. Marcolli ◽  
S. Gedamke ◽  
T. Peter ◽  
B. Zobrist
Keyword(s):  
Contact Angle ◽  
Active Sites ◽  
Quantitative Agreement ◽  
Ice Nucleation ◽  
Nucleation Theory ◽  
Classical Nucleation Theory ◽  
Aqueous Suspensions ◽  
Freezing Temperatures ◽  
Good Agreement ◽  
Heterogeneous Ice Nucleation

Abstract. A differential scanning calorimeter (DSC) was used to explore heterogeneous ice nucleation of emulsified aqueous suspensions of two Arizona test dust (ATD) samples with particle diameters of nominally 0–3 and 0–7 μm, respectively. Aqueous suspensions with ATD concentrations of 0.01–20 wt% have been investigated. The DSC thermograms exhibit a homogeneous and a heterogeneous freezing peak whose intensity ratios vary with the ATD concentration in the aqueous suspensions. Homogeneous freezing temperatures are in good agreement with recent measurements by other techniques. Depending on ATD concentration, heterogeneous ice nucleation occurred at temperatures as high as 256 K or down to the onset of homogeneous ice nucleation (237 K). For ATD-induced ice formation Classical Nucleation Theory (CNT) offers a suitable framework to parameterize nucleation rates as a function of temperature, experimentally determined ATD size, and emulsion droplet volume distributions. The latter two quantities serve to estimate the total heterogeneous surface area present in a droplet, whereas the suitability of an individual heterogeneous site to trigger nucleation is described by the compatibility function (or contact angle) in CNT. The intensity ratio of homogeneous to heterogeneous freezing peaks is in good agreement with the assumption that the ATD particles are randomly distributed amongst the emulsion droplets. The observed dependence of the heterogeneous freezing temperatures on ATD concentrations cannot be described by assuming a constant contact angle for all ATD particles, but requires the ice nucleation efficiency of ATD particles to be (log)normally distributed amongst the particles. Best quantitative agreement is reached when explicitly assuming that high-compatibility sites are rare and that therefore larger particles have on average more and better active sites than smaller ones. This analysis suggests that a particle has to have a diameter of at least 0.1 μm to exhibit on average one active site.

scholarly journals Efficiency of immersion mode ice nucleation on surrogates of mineral dust

2007 ◽  
Vol 7 (4) ◽  
pp. 9687-9716
Author(s):  
C. Marcolli ◽  
S. Gedamke ◽  
T. Peter ◽  
B. Zobrist
Keyword(s):  
Contact Angle ◽  
Active Sites ◽  
Quantitative Agreement ◽  
Ice Nucleation ◽  
Nucleation Theory ◽  
Classical Nucleation Theory ◽  
Aqueous Suspensions ◽  
Freezing Temperatures ◽  
Good Agreement ◽  
Heterogeneous Ice Nucleation

Abstract. A differential scanning calorimeter (DSC) was used to explore heterogeneous ice nucleation of emulsified aqueous suspensions of two Arizona test dust (ATD) samples with particle diameters of nominally 0–3 and 0–7 μm, respectively. Aqueous suspensions with ATD concentrations of 0.01–20 wt% have been investigated. The DSC thermograms exhibit a homogeneous and a heterogeneous freezing peak whose intensity ratios vary with the ATD concentration in the aqueous suspensions. Homogeneous freezing temperatures are in good agreement with recent measurements by other techniques. Depending on ATD concentration, heterogeneous ice nucleation occurred at temperatures as high as 256 K or down to the onset of homogeneous ice nucleation (237 K). For ATD-induced ice formation Classical Nucleation Theory (CNT) offers a suitable framework to parameterize nucleation rates as a function of temperature, experimentally determined ATD size, and emulsion droplet volume distributions. The latter two quantities serve to estimate the total heterogeneous surface area present in a droplet, whereas the suitability of an individual heterogeneous site to trigger nucleation is described by the compatibility function (or contact angle) in heterogeneous CNT. The intensity ratio of homogeneous to heterogeneous freezing peaks is in good agreement with the assumption that the ATD particles are randomly distributed amongst the emulsion droplets. The observed dependence of the heterogeneous freezing temperatures on ATD concentrations cannot be described by assuming a constant contact angle for all ATD particles, but requires the ice nucleation efficiency of ATD particles to be (log)normally distributed amongst the particles. Best quantitative agreement is reached when explicitly assuming that high-compatibility sites are rare and that therefore larger particles have on average more and better active sites than smaller ones. This analysis suggests that a particle has to have a diameter of at least 0.1 μm to exhibit on average one active site.

scholarly journals Time dependence of immersion freezing: an experimental study on size selected kaolinite particles

2012 ◽  
Vol 12 (20) ◽  
pp. 9893-9907 ◽  
Author(s):  
A. Welti ◽  
F. Lüönd ◽  
Z. A. Kanji ◽  
O. Stetzer ◽  
U. Lohmann
Keyword(s):  
Time Dependence ◽  
Residence Time ◽  
Particle Surface ◽  
Laboratory Data ◽  
Contact Angles ◽  
Ice Nucleation ◽  
Mixed Phase ◽  
Nucleation Theory ◽  
Classical Nucleation Theory ◽  
Immersion Freezing

Abstract. The time dependence of immersion freezing was studied for temperatures between 236 K and 243 K. Droplets with single immersed, size-selected 400 nm and 800 nm kaolinite particles were produced at 300 K, cooled down to supercooled temperatures, and the fraction of frozen droplets with increasing residence time was detected. To simulate the conditions of immersion freezing in mixed-phase clouds we used the Zurich Ice Nucleation Chamber (ZINC) and its vertical extension, the Immersion Mode Cooling chAmber (IMCA). We observed that the frozen fraction of droplets increased with increasing residence time in the chamber. This suggests that there is a time dependence of immersion freezing and supports the importance of a stochastic component in the ice nucleation process. The rate at which droplets freeze was observed to decrease towards higher temperatures and smaller particle sizes. Comparison of the laboratory data with four different ice nucleation models, three based on classical nucleation theory with different representations of the particle surface properties and one singular, suggest that the classical, stochastic approach combined with a distribution of contact angles is able to reproduce the ice nucleation observed in these experiments most accurately. Using the models to calculate the increase in frozen fraction at typical mixed-phase cloud temperatures over an extended period of time, yields an equivalent effect of −1 K temperature shift for an increase in times scale by one order of magnitude. This suggests that temperature is more important than time.

scholarly journals Time dependence of immersion freezing

2012 ◽  
Vol 12 (5) ◽  
pp. 12623-12662 ◽  
Author(s):  
A. Welti ◽  
F. Lüönd ◽  
Z. A. Kanji ◽  
O. Stetzer ◽  
U. Lohmann
Keyword(s):  
Time Dependence ◽  
Residence Time ◽  
Particle Surface ◽  
Laboratory Data ◽  
Contact Angles ◽  
Ice Nucleation ◽  
Mixed Phase ◽  
Nucleation Theory ◽  
Classical Nucleation Theory ◽  
Immersion Freezing

Abstract. The time dependence of immersion freezing was studied for temperatures between 236 K and 243 K. Droplets with single immersed, size-selected 400 nm and 800 nm kaolinite particles were produced at 300 K, cooled down to supercooled temperatures typical for mixed-phase cloud conditions, and the fraction of frozen droplets with increasing residence time was detected. To simulate the conditions of immersion freezing in mixed-phase clouds we used the Zurich Ice Nucleation Chamber (ZINC) and its vertical extension, the Immersion Mode Cooling chAmber (IMCA). We observed that the frozen fraction of droplets increased with increasing residence time in the chamber. This suggests that there is a time dependence of immersion freezing and supports the importance of a stochastic component in the ice nucleation process. The rate at which droplets freeze was observed to decrease towards higher temperatures and smaller particle sizes. Comparison of the laboratory data with four different ice nucleation models, three based on classical nucleation theory with different representations of the particle surface properties and one singular, suggest that the classical, stochastic approach combined with a distribution of contact angles is able to reproduce the ice nucleation observed in these experiments most accurately. Using the models to calculate the increase in frozen fraction at typical mixed-phase cloud temperatures over an extended period of time, yields an equivalent effect of −1 K temperature shift and an increase in time scale by a factor of ~10.

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 An approximation for homogeneous freezing temperature of water droplets

2015 ◽  
Vol 15 (21) ◽  
pp. 31867-31889
Author(s):  
K.-T. O ◽  
R. Wood
Keyword(s):  
Experimental Studies ◽  
Freezing Temperature ◽  
Ice Nucleation ◽  
Nucleation Theory ◽  
Classical Nucleation Theory ◽  
Water Droplets ◽  
Wide Range ◽  
The Mean ◽  
Freezing Temperatures ◽  
Homogeneous Freezing

Abstract. In this work, based on the well-known formulae of classical nucleation theory (CNT), the temperature TNc = 1 at which the mean number of critical embryos inside a droplet is unity is derived and proposed as a new approximation for homogeneous freezing temperature of water droplets. Without consideration of time dependence and stochastic nature of the ice nucleation process, the approximation TNc = 1 is able to reproduce the dependence of homogeneous freezing temperature on drop size and water activity of aqueous drops observed in a wide range of experimental studies. We use the TNc = 1 approximation to argue that the distribution of homogeneous freezing temperatures observed in the experiments may largely be explained by the spread in the size distribution of droplets used in the particular experiment. It thus appears that this approximation is useful for predicting homogeneous freezing temperatures of water droplets in the atmosphere.

scholarly journals A New Ice Nucleation Active Site Parameterization for Desert Dust and Soot

2017 ◽  
Vol 74 (3) ◽  
pp. 699-717 ◽  
Author(s):  
Romy Ullrich ◽  
Corinna Hoose ◽  
Ottmar Möhler ◽  
Monika Niemand ◽  
Robert Wagner ◽  
...  
Keyword(s):  
Active Surface ◽  
Ice Nucleation ◽  
Nucleation Theory ◽  
Classical Nucleation Theory ◽  
Negative Slope ◽  
Organic Carbon Content ◽  
Desert Dust ◽  
Soot Aerosol ◽  
Immersion Freezing ◽  
Heterogeneous Ice Nucleation

Abstract Based on results of 11 yr of heterogeneous ice nucleation experiments at the Aerosol Interaction and Dynamics in the Atmosphere (AIDA) chamber in Karlsruhe, Germany, a new empirical parameterization framework for heterogeneous ice nucleation was developed. The framework currently includes desert dust and soot aerosol and quantifies the ice nucleation efficiency in terms of the ice nucleation active surface site (INAS) approach. The immersion freezing INAS densities nS of all desert dust experiments follow an exponential fit as a function of temperature, well in agreement with an earlier analysis of AIDA experiments. The deposition nucleation nS isolines for desert dust follow u-shaped curves in the ice saturation ratio–temperature (Si–T) diagram at temperatures below about 240 K. The negative slope of these isolines toward lower temperatures may be explained by classical nucleation theory (CNT), whereas the behavior toward higher temperatures may be caused by a pore condensation and freezing mechanism. The deposition nucleation measured for soot at temperatures below about 240 K also follows u-shaped isolines with a shift toward higher Si for soot with higher organic carbon content. For immersion freezing of soot aerosol, only upper limits for nS were determined and used to rescale an existing parameterization line. The new parameterization framework is compared to a CNT-based parameterization and an empirical framework as used in models. The comparison shows large differences in shape and magnitude of the nS isolines especially for deposition nucleation. For the application in models, implementation of this new framework is simple compared to that of other expressions.

scholarly journals Heterogeneous ice nucleation on atmospheric aerosols: a review of results from laboratory experiments

2012 ◽  
Vol 12 (20) ◽  
pp. 9817-9854 ◽  
Author(s):  
C. Hoose ◽  
O. Möhler
Keyword(s):  
Laboratory Experiments ◽  
Mineral Dust ◽  
Fungal Spores ◽  
Pollen Grains ◽  
Contact Angles ◽  
Ice Nucleation ◽  
Nucleation Theory ◽  
Classical Nucleation Theory ◽  
Small Subset ◽  
Heterogeneous Ice Nucleation

Abstract. A small subset of the atmospheric aerosol population has the ability to induce ice formation at conditions under which ice would not form without them (heterogeneous ice nucleation). While no closed theoretical description of this process and the requirements for good ice nuclei is available, numerous studies have attempted to quantify the ice nucleation ability of different particles empirically in laboratory experiments. In this article, an overview of these results is provided. Ice nucleation "onset" conditions for various mineral dust, soot, biological, organic and ammonium sulfate particles are summarized. Typical temperature-supersaturation regions can be identified for the "onset" of ice nucleation of these different particle types, but the various particle sizes and activated fractions reported in different studies have to be taken into account when comparing results obtained with different methodologies. When intercomparing only data obtained under the same conditions, it is found that dust mineralogy is not a consistent predictor of higher or lower ice nucleation ability. However, the broad majority of studies agrees on a reduction of deposition nucleation by various coatings on mineral dust. The ice nucleation active surface site (INAS) density is discussed as a simple and empirical normalized measure for ice nucleation activity. For most immersion and condensation freezing measurements on mineral dust, estimates of the temperature-dependent INAS density agree within about two orders of magnitude. For deposition nucleation on dust, the spread is significantly larger, but a general trend of increasing INAS densities with increasing supersaturation is found. For soot, the presently available results are divergent. Estimated average INAS densities are high for ice-nucleation active bacteria at high subzero temperatures. At the same time, it is shown that INAS densities of some other biological aerosols, like certain pollen grains, fungal spores and diatoms, tend to be similar to those of dust. These particles may owe their high ice nucleation onsets to their large sizes. Surface-area-dependent parameterizations of heterogeneous ice nucleation are discussed. For immersion freezing on mineral dust, fitted INAS densities are available, but should not be used outside the temperature interval of the data they were based on. Classical nucleation theory, if employed with only one fitted contact angle, does not reproduce the observed temperature dependence for immersion nucleation, the temperature and supersaturation dependence for deposition nucleation, and the time dependence of ice nucleation. Formulations of classical nucleation theory with distributions of contact angles offer possibilities to overcome these weaknesses.

scholarly journals Ice nucleation from aqueous NaCl droplets with and without marine diatoms

2011 ◽  
Vol 11 (3) ◽  
pp. 8291-8336 ◽  
Author(s):  
P. A. Alpert ◽  
J. Y. Aller ◽  
D. A. Knopf
Keyword(s):  
Surface Area ◽  
Water Activity ◽  
Nucleation Rate ◽  
Particle Production ◽  
Ice Nucleation ◽  
Nucleation Theory ◽  
Rate Coefficients ◽  
Freezing Temperatures ◽  
Aqueous Volume ◽  
Heterogeneous Ice Nucleation

Abstract. Ice formation in the atmosphere by homogeneous and heterogeneous nucleation is one of the least understood processes in cloud microphysics and climate. Here we describe our investigation of the marine environment as a potential source of atmospheric IN by experimentally observing homogeneous ice nucleation from aqueous NaCl droplets and comparing against heterogeneous ice nucleation from aqueous NaCl droplets containing intact and fragmented diatoms. Homogeneous and heterogeneous ice nucleation are studied as a function of temperature and water activity, aw. Additional analyses are presented on the dependence of diatom surface area and aqueous volume on heterogeneous freezing temperatures, ice nucleation rates, ωhet, ice nucleation rate coefficients, Jhet, and differential and cumulative ice nuclei spectra, k(T) and K(T), respectively. Homogeneous freezing temperatures and corresponding nucleation rate coefficients are in agreement with the water activity based homogeneous ice nucleation theory within experimental and predictive uncertainties. Our results confirm, as predicted by classical nucleation theory, that a stochastic interpretation can be used to describe this nucleation process. Heterogeneous ice nucleation initiated by intact and fragmented diatoms can be adequately represented by a modified water activity based ice nucleation theory. A horizontal shift in water activity, Δaw, het = 0.2303, of the ice melting curve can describe median heterogeneous freezing temperatures. Individual freezing temperatures showed no dependence on available diatom surface area and aqueous volume. Determined at median diatom freezing temperatures for aw from 0.8 to 0.99, ωhet ~ 0.11+0.06−0.05 s−1, Jhet ~ 1.0+1.16−0.61 × 104 cm−2 s−1, and K ~ 6.2+3.5−4.1 × 104 cm−2. The experimentally derived ice nucleation rates and nuclei spectra allow us to estimate ice particle production which we subsequently use for a comparison with observed ice crystal concentrations typically found in cirrus and polar marine mixed-phase clouds. Differences in application of time-dependent and time-independent analyses to predict ice particle production are discussed.

Sign in / Sign up

Export Citation Format

Share Document