scholarly journals Ice nucleation properties of volcanic ash from Eyjafjallajökull

2011 ◽  
Vol 11 (6) ◽  
pp. 17201-17243 ◽  
Author(s):  
C. R. Hoyle ◽  
V. Pinti ◽  
A. Welti ◽  
B. Zobrist ◽  
C. Marcolli ◽  
...  
Keyword(s):  
Volcanic Ash ◽  
Freezing Point ◽  
Ice Nucleation ◽  
Cloud Formation ◽  
Atmospheric Conditions ◽  
Model Calculations ◽  
Airborne Measurements ◽  
Ice Cloud ◽  
Ice Nuclei ◽  
Homogeneous Freezing

Abstract. The ice nucleation ability of volcanic ash particles collected close to the Icelandic volcano Eyjafjallajökull during its eruptions in April and May 2010 is investigated experimentally, in the immersion and deposition modes, and applied to atmospheric conditions by comparison with airborne measurements and microphysical model calculations. The number of ash particles which are active as ice nuclei (IN) is strongly temperature dependent, with a very small minority being active in the immersion mode at temperatures of 250–263 K. Average ash particles show only a moderate effect on ice nucleation, by inducing freezing at temperatures between 236 K and 240 K (i.e. approximately 3–4 K higher than temperatures required for homogeneous ice nucleation, measured with the same instrument). By scaling the results to aircraft and lidar measurements of the conditions in the ash plume days down wind of the eruption and by applying a simple microphysical model, it was found that the IN active in the immersion mode in the range 250–263 K generally occurred in atmospheric number densities at the lower end of those required to have an impact on ice cloud formation. However, 3–4 K above the homogeneous freezing point, immersion mode IN number densities a few days down wind of the eruption were sufficiently high to have a moderate influence on ice cloud formation. The efficiency of IN in the deposition mode was found to be poor except at very cold conditions (< 238 K), when they reach an efficiency similar to that of mineral dust with the onset of freezing at 10 % supersaturation with respect to ice, and with the frozen fraction nearing its maximum value at a supersaturation 20 %. In summary, these investigations suggest volcanic ash particles to have only moderate effects on atmospheric ice formation.

scholarly journals Ice nucleation properties of volcanic ash from Eyjafjallajökull

2011 ◽  
Vol 11 (18) ◽  
pp. 9911-9926 ◽  
Author(s):  
C. R. Hoyle ◽  
V. Pinti ◽  
A. Welti ◽  
B. Zobrist ◽  
C. Marcolli ◽  
...  
Keyword(s):  
Volcanic Ash ◽  
Freezing Point ◽  
Ice Nucleation ◽  
Cloud Formation ◽  
Atmospheric Conditions ◽  
Model Calculations ◽  
Airborne Measurements ◽  
Ice Cloud ◽  
Ice Nuclei ◽  
Homogeneous Freezing

Abstract. The ice nucleation ability of volcanic ash particles collected close to the Icelandic volcano Eyjafjallajökull during its eruptions in April and May 2010 is investigated experimentally, in the immersion and deposition modes, and applied to atmospheric conditions by comparison with airborne measurements and microphysical model calculations. The number of ash particles which are active as ice nuclei (IN) is strongly temperature dependent, with a very small minority being active in the immersion mode at temperatures of 250–263 K. Average ash particles show only a moderate effect on ice nucleation, by inducing freezing at temperatures between 236 K and 240 K (i.e. approximately 3–4 K higher than temperatures required for homogeneous ice nucleation, measured with the same instrument). By scaling the results to aircraft and lidar measurements of the conditions in the ash plume days down wind of the eruption, and by applying a simple microphysical model, it was found that the IN active in the immersion mode in the range 250–263 K generally occurred in atmospheric number densities at the lower end of those required to have an impact on ice cloud formation. However, 3–4 K above the homogeneous freezing point, immersion mode IN number densities a few days down wind of the eruption were sufficiently high to have a moderate influence on ice cloud formation. The efficiency of IN in the deposition mode was found to be poor except at very cold conditions (<238 K), when they reach an efficiency similar to that of mineral dust with the onset of freezing at 10 % supersaturation with respect to ice, and with the frozen fraction nearing its maximum value at a supersaturation 20 %. In summary, these investigations suggest volcanic ash particles to have only moderate effects on atmospheric ice formation.

scholarly journals Condensed-phase biogenic–anthropogenic interactions with implications for cold cloud formation

2017 ◽  
Vol 200 ◽  
pp. 165-194 ◽  
Author(s):  
Joseph C. Charnawskas ◽  
Peter A. Alpert ◽  
Andrew T. Lambe ◽  
Thomas Berkemeier ◽  
Rachel E. O’Brien ◽  
...  
Keyword(s):  
Freezing Temperature ◽  
Organic Aerosol ◽  
Ice Nucleation ◽  
Mixed Phase ◽  
Cloud Formation ◽  
Fossil Fuel Combustion ◽  
Soot Particles ◽  
Ice Nuclei ◽  
Deliquescence Relative Humidity ◽  
Homogeneous Freezing

Anthropogenic and biogenic gas emissions contribute to the formation of secondary organic aerosol (SOA). When present, soot particles from fossil fuel combustion can acquire a coating of SOA. We investigate SOA–soot biogenic–anthropogenic interactions and their impact on ice nucleation in relation to the particles’ organic phase state. SOA particles were generated from the OH oxidation of naphthalene, α-pinene, longifolene, or isoprene, with or without the presence of sulfate or soot particles. Corresponding particle glass transition (Tg) and full deliquescence relative humidity (FDRH) were estimated using a numerical diffusion model. Longifolene SOA particles are solid-like and all biogenic SOA sulfate mixtures exhibit a core–shell configuration (i.e.a sulfate-rich core coated with SOA). Biogenic SOA with or without sulfate formed ice at conditions expected for homogeneous ice nucleation, in agreement with respectiveTgand FDRH. α-pinene SOA coated soot particles nucleated ice above the homogeneous freezing temperature with soot acting as ice nuclei (IN). At lower temperatures the α-pinene SOA coating can be semisolid, inducing ice nucleation. Naphthalene SOA coated soot particles acted as ice nuclei above and below the homogeneous freezing limit, which can be explained by the presence of a highly viscous SOA phase. Our results suggest that biogenic SOA does not play a significant role in mixed-phase cloud formation and the presence of sulfate renders this even less likely. However, anthropogenic SOA may have an enhancing effect on cloud glaciation under mixed-phase and cirrus cloud conditions compared to biogenic SOA that dominate during pre-industrial times or in pristine areas.

scholarly journals Suspendable macromolecules are responsible for ice nucleation activity of birch and conifer pollen

2012 ◽  
Vol 12 (5) ◽  
pp. 2541-2550 ◽  
Author(s):  
B. G. Pummer ◽  
H. Bauer ◽  
J. Bernardi ◽  
S. Bleicher ◽  
H. Grothe
Keyword(s):  
Radiative Forcing ◽  
Pollen Grains ◽  
Ice Nucleation ◽  
Cloud Formation ◽  
Distribution Area ◽  
Ice Cloud ◽  
Ice Nuclei ◽  
Nucleation Activity ◽  
Ice Nucleation Activity ◽  
The Impact

Abstract. The ice nucleation of bioaerosols (bacteria, pollen, spores, etc.) is a topic of growing interest, since their impact on ice cloud formation and thus on radiative forcing, an important parameter in global climate, is not yet fully understood. Here we show that pollen of different species strongly differ in their ice nucleation behaviour. The average freezing temperatures in laboratory experiments range from 240 to 255 K. As the most efficient nuclei (silver birch, Scots pine and common juniper pollen) have a distribution area up to the Northern timberline, their ice nucleation activity might be a cryoprotective mechanism. Far more intriguingly, it has turned out that water, which has been in contact with pollen and then been separated from the bodies, nucleates as good as the pollen grains themselves. The ice nuclei have to be easily-suspendable macromolecules located on the pollen. Once extracted, they can be distributed further through the atmosphere than the heavy pollen grains and so presumably augment the impact of pollen on ice cloud formation even in the upper troposphere. Our experiments lead to the conclusion that pollen ice nuclei, in contrast to bacterial and fungal ice nucleating proteins, are non-proteinaceous compounds.

scholarly journals Ice cloud processing of ultra-viscous/glassy aerosol particles leads to enhanced ice nucleation ability

2012 ◽  
Vol 12 (18) ◽  
pp. 8589-8610 ◽  
Author(s):  
R. Wagner ◽  
O. Möhler ◽  
H. Saathoff ◽  
M. Schnaiter ◽  
J. Skrotzki ◽  
...  
Keyword(s):  
Glass Transition ◽  
Aerosol Particles ◽  
Ice Nucleation ◽  
Ice Formation ◽  
Transition Temperatures ◽  
Glass Transition Temperatures ◽  
Ice Cloud ◽  
Ice Nuclei ◽  
Freezing Cycle ◽  
Homogeneous Freezing

Abstract. The ice nucleation potential of airborne glassy aqueous aerosol particles has been investigated by controlled expansion cooling cycles in the AIDA aerosol and cloud chamber of the Karlsruhe Institute of Technology at temperatures between 247 and 216 K. Four different solutes were used as proxies for oxygenated organic matter found in the atmosphere: raffinose, 4-hydroxy-3-methoxy-DL-mandelic acid (HMMA), levoglucosan, and a multi-component mixture of raffinose with five dicarboxylic acids and ammonium sulphate. Similar to previous experiments with citric acid aerosols, all particles were found to nucleate ice heterogeneously before reaching the homogeneous freezing threshold provided that the freezing cycles were started well below the respective glass transition temperatures of the compounds; this is discussed in detail in a separate article. In this contribution, we identify a further mechanism by which glassy aerosols can promote ice nucleation below the homogeneous freezing limit. If the glassy aerosol particles are probed in freezing cycles started only a few degrees below their respective glass transition temperatures, they enter the liquid regime of the state diagram upon increasing relative humidity (moisture-induced glass-to-liquid transition) before being able to act as heterogeneous ice nuclei. Ice formation then only occurs by homogeneous freezing at elevated supersaturation levels. When ice forms the remaining solution freeze concentrates and re-vitrifies. If these ice cloud processed glassy aerosol particles are then probed in a second freezing cycle at the same temperature, they catalyse ice formation at a supersaturation threshold between 5 and 30% with respect to ice. By analogy with the enhanced ice nucleation ability of insoluble ice nuclei like mineral dusts after they nucleate ice once, we refer to this phenomenon as pre-activation. We propose a number of possible explanations for why glassy aerosol particles that have re-vitrified in contact with the ice crystals during the preceding homogeneous freezing cycle exhibit pre-activation: they may retain small ice embryos in pores, have footprints on their surface which match the ice lattice, or simply have a much greater surface area or different surface microstructure compared to the unprocessed glassy aerosol particles. Pre-activation must be considered for the correct interpretation of experimental results on the heterogeneous ice nucleation ability of glassy aerosol particles and may provide a mechanism of producing a population of extremely efficient ice nuclei in the upper troposphere.

scholarly journals Insights into the role of soot aerosols in cirrus cloud formation

2007 ◽  
Vol 7 (16) ◽  
pp. 4203-4227 ◽  
Author(s):  
B. Kärcher ◽  
O. Möhler ◽  
P. J. DeMott ◽  
S. Pechtl ◽  
F. Yu
Keyword(s):  
Environmental Parameters ◽  
Supercooled Liquid ◽  
Critical Discussion ◽  
Cloud Formation ◽  
Cirrus Cloud ◽  
Atmospheric Conditions ◽  
Soot Particles ◽  
Ice Nuclei ◽  
Homogeneous Freezing

Abstract. Cirrus cloud formation is believed to be dominated by homogeneous freezing of supercooled liquid aerosols in many instances. Heterogeneous ice nuclei such as mineral dust, metallic, and soot particles, and some crystalline solids within partially soluble aerosols are suspected to modulate cirrus properties. Among those, the role of ubiquitous soot particles is perhaps the least understood. Because aviation is a major source of upper tropospheric soot particles, we put emphasis on ice formation in dispersing aircraft plumes. The effect of aircraft soot on cirrus formation in the absence of contrails is highly complex and depends on a wide array of emission and environmental parameters. We use a microphysical-chemical model predicting the formation of internally mixed, soot-containing particles up to two days after emission, and suggest two principal scenarios: high concentrations of original soot emissions could slightly increase the number of ice crystals; low concentrations of particles originating from coagulation of emitted soot with background aerosols could lead to a significant reduction in ice crystal number. Both scenarios assume soot particles to be moderate ice nuclei relative to cirrus formation by homogeneous freezing in the presence of few efficient dust ice nuclei. A critical discussion of laboratory experiments reveals that the ice nucleation efficiency of soot particles depends strongly on their source, and, by inference, on atmospheric aging processes. Mass and chemistry of soluble surface coatings appear to be crucial factors. Immersed soot particles tend to be poor ice nuclei, some bare ones nucleate ice at low supersaturations. However, a fundamental understanding of these studies is lacking, rendering extrapolations to atmospheric conditions speculative. In particular, we cannot yet decide which indirect aircraft effect scenario is more plausible, and options suggested to mitigate the problem remain uncertain.

scholarly journals On the ice nucleation spectrum

2011 ◽  
Vol 11 (11) ◽  
pp. 29601-29646 ◽  
Author(s):  
D. Barahona
Keyword(s):  
Surface Composition ◽  
Ice Nucleation ◽  
Atmospheric Modeling ◽  
Cloud Formation ◽  
Volume Distribution ◽  
Ice Crystal ◽  
Ice Nuclei ◽  
New Formulation ◽  
Homogeneous Freezing ◽  
Heterogeneous Ice Nucleation

Abstract. This work presents a novel formulation of the ice nucleation spectrum, i.e. the function relating the ice crystal concentration to cloud formation conditions and aerosol properties. The new formulation relies on a statistical view of the ice nucleation process and explicitly accounts for the dependency of the ice crystal concentration on temperature, supersaturation, cooling rate, and particle size, and, in the case of heterogeneous ice nucleation, on the distributions of particle area and surface composition. The new formulation is used to generate ice nucleation parameterizations for the homogeneous freezing of cloud droplets and the heterogeneous deposition ice nucleation on dust and soot ice nuclei. For homogeneous freezing, it was found that by increasing the dispersion in the droplet volume distribution the fraction of supercooled droplets in the population increases. For heterogeneous ice nucleation it was found that ice nucleation on efficient ice nuclei (IN) shows features consistent with the singular hypothesis (characterized by a lack of temporal dependency of the ice nucleation spectrum) whereas less efficient IN tend to display stochastic behavior. Analysis of empirical nucleation spectra suggested that inferring the aerosol heterogeneous ice nucleation properties from measurements of the onset supersaturation and temperature may carry significant error as the variability in ice nucleation properties within the aerosol population is not accounted for. This work provides a simple and rigorous ice nucleation framework were theoretical predictions, laboratory measurements and field campaign data can be reconciled, and that is suitable for application in atmospheric modeling studies.

scholarly journals Ice cloud processing of ultra-viscous/glassy aerosol particles leads to enhanced ice nucleation ability

2012 ◽  
Vol 12 (4) ◽  
pp. 8921-8977 ◽  
Author(s):  
R. Wagner ◽  
O. Möhler ◽  
H. Saathoff ◽  
M. Schnaiter ◽  
J. Skrotzki ◽  
...  
Keyword(s):  
Glass Transition ◽  
Aerosol Particles ◽  
Ice Nucleation ◽  
Ice Formation ◽  
Transition Temperatures ◽  
Glass Transition Temperatures ◽  
Ice Cloud ◽  
Ice Nuclei ◽  
Freezing Cycle ◽  
Homogeneous Freezing

Abstract. The ice nucleation potential of airborne glassy aqueous aerosol particles has been investigated by controlled expansion cooling cycles in the AIDA aerosol and cloud chamber of the Karlsruhe Institute of Technology at temperatures between 247 and 216 K. Four different solutes were used as proxies for oxygenated organic matter found in the atmosphere: raffinose, 4-hydroxy-3-methoxy-DL-mandelic acid (HMMA), levoglucosan, and a multi-component mixture of raffinose with five dicarboxylic acids and ammonium sulphate. Similar to previous experiments with citric acid aerosols, all particles were found to nucleate ice heterogeneously before reaching the homogeneous freezing threshold provided that the freezing cycles were started well below the respective glass transition temperatures of the compounds; this is discussed in detail in a separate article. In this contribution, we identify a further mechanism by which glassy aerosols can promote ice nucleation below the homogeneous freezing limit. If the glassy aerosol particles are probed in freezing cycles started only a few degrees below their respective glass transition temperatures, they enter the liquid regime of the state diagram upon increasing relative humidity (moisture-induced glass-to-liquid transition) before being able to act as heterogeneous ice nuclei. Ice formation then only occurs by homogeneous freezing at elevated supersaturation levels. When ice forms the remaining solution freeze concentrates and re-vitrifies. If these ice cloud processed glassy aerosol particles are then probed in a second freezing cycle at the same temperature, they catalyse ice formation at a supersaturation threshold between 5 and 30% with respect to ice. By analogy with the enhanced ice nucleation ability of insoluble ice nuclei like mineral dusts after they nucleate ice once, we refer to this phenomenon as pre-activation. We propose a number of possible explanations for why glassy aerosols that have re-vitrified in contact with the ice crystals during the preceding homogeneous freezing cycle exhibit pre-activation: they may retain small ice embryos in pores, have footprints on their surface which match the ice lattice, or simply have a much greater surface area or different surface microstructure compared to the unprocessed glassy aerosol particles. Pre-activation must be considered for the correct interpretation of experimental results on the heterogeneous ice nucleation ability of glassy aerosol particles and may provide a mechanism of producing a population of extremely efficient ice nuclei in the upper troposphere.

scholarly journals Insights into the role of soot aerosols in cirrus cloud formation

2007 ◽  
Vol 7 (3) ◽  
pp. 7843-7905 ◽  
Author(s):  
B. Kärcher ◽  
O. Möhler ◽  
P. J. DeMott ◽  
S. Pechtl ◽  
F. Yu
Keyword(s):  
Environmental Parameters ◽  
Supercooled Liquid ◽  
Critical Discussion ◽  
Cloud Formation ◽  
Cirrus Cloud ◽  
Atmospheric Conditions ◽  
Soot Particles ◽  
Ice Nuclei ◽  
Homogeneous Freezing

Abstract. Cirrus cloud formation is believed to be domi\\-nated by homogeneous freezing of supercooled liquid aerosols in many instances. Heterogeneous ice nuclei such as mineral dust, metallic, and soot particles, and some crystalline solids within partially soluble aerosols are suspected to modulate cirrus properties. Among those, the role of ubiqui\\-tous soot particles is perhaps the least understood. Because aviation is a major source of upper tropospheric soot particles, we put emphasis on ice formation in dispersing aircraft plumes. The effect of aircraft soot on cirrus formation in the absence of contrails is highly complex and depends on a wide array of emission and environmental parameters. We use a microphysical-chemical model predicting the formation of internally mixed, soot-containing particles up to two days after emission, and suggest two principal scenarios, both assuming soot particles to be moderate ice nuclei relative to cirrus formation by homogeneous freezing in the presence of few efficient dust ice nuclei: high concentrations of original soot emissions could slightly increase the number of ice crystals; low concentrations of particles originating from coagulation of emitted soot with background aerosols could lead to a significant reduction in ice crystal number. A critical discussion of laboratory experiments reveals that the ice nucleation efficiency of soot particles depends strongly on their source, and, by inference, on atmospheric aging processes. Mass and chemistry of soluble surface coatings appear to be crucial factors. Immersed soot particles tend to be poor ice nuclei, some bare ones nucleate ice at low supersaturations. However, a fundamental understanding of these studies is lacking, rendering extrapolations to atmospheric conditions speculative. In particular, we cannot yet decide which indirect aircraft effect scenario is more plausible, and options suggested to mitigate the problem remain uncertain.

scholarly journals Pre-activation of aeosol particles by pore condensation and freezing

2016 ◽  
Author(s):  
Claudia Marcolli
Keyword(s):  
Relative Humidity ◽  
Volcanic Ash ◽  
Freezing Point ◽  
Ice Nucleation ◽  
Cloud Formation ◽  
Ice Crystal ◽  
Pore Width ◽  
Pore Characteristics ◽  
Ice Growth ◽  
Pore Condensation

Abstract. Pre-activation denotes the capability of particles or materials to nucleate ice at lower relative humidities or higher temperatures compared to their intrinsic ice nucleation efficiency after having experienced an ice nucleation event or low temperature before. This review presumes a pore condensation and freezing (PCF) mechanism to analyze studies on pre-activation. Idealized trajectories of air parcels are used to discuss the pore characteristics needed for ice to persist in pores and to induce macroscopic ice-growth out of the pores. The pore width needed to keep pores filled with water decreases with decreasing relative humidity as described by the inverse Kelvin equation. Thus, narrow pores remain filled with ice well below ice saturation. However, the smaller the pore width, the larger the melting and freezing point depressions within the pores. Therefore, pre-activation by PCF is constrained by the melting of ice in narrow pores and the sublimation of ice from wide pores imposing severe restrictions on the temperature and relative humidity range of pre-activation for cylindrical pores. Ice is better protected in ink-bottle-shaped pores with a narrow opening leading to a large cavity. However, whether pre-activation is efficient also depends on the capability of ice to grow macroscopically, i.e. out of the pore. A strong effect of pre-activation is expected for swelling pores, because at low relative humidity (RH) their openings narrow and protect the ice within them against sublimation. At high relative humidities, they open up and the ice can grow to macrosopical size and form an ice crystal. Similarly, ice protected in pockets are perfectly sheltered against sublimation but needs the dissolution of the surrounding matrix to be effective. Pores partially filled with condensable material may also show pre-activation. In this case, complete filling occurs at lower RH than for empty pores and freezing shifts to lower temperatures. Pre-activation experiments confirm that materials susceptible to pre-activation are indeed porous. Pre-activation was observed for clay minerals like illite, kaolinite and montmorillonite with inherent porosity. The largest effect was observed for the swelling clay mineral montmorillonite. Some materials may acquire porosity depending on the formation and processing conditions. Particles of CaCO3, meteoritic material, and volcanic ash showed pre-activation for some samples or in some studies but not in other ones. Quartz and silver iodide were not susceptible to pre-activation. Atmospheric relevance of pre-activation by a PCF mechanism may not be generally given but depend on the atmospheric scenario. Lower-level cloud seeding by pre-activated particles released from high-level clouds crucially depends on the ability of pores to retain ice at the relative humidities and temperatures of the air masses they pass through. Porous particles that are recycled in wave clouds may show pre-activation with subsequent ice growth as soon as ice saturation is exceeded after having passed a first cloud event. Volcanic ash particles and meteoritic material likely influence ice cloud formation by pre-activation. Therefore, pre-activation needs to be considered when ice crystal number densities in clouds exceed the number of ice-nucleating particles measured at the cloud forming temperature.

scholarly journals Pre-activation of aerosol particles by ice preserved in pores

2017 ◽  
Vol 17 (3) ◽  
pp. 1595-1622 ◽  
Author(s):  
Claudia Marcolli
Keyword(s):  
Relative Humidity ◽  
Volcanic Ash ◽  
Freezing Point ◽  
Ice Nucleation ◽  
Cloud Formation ◽  
Ice Crystal ◽  
Pore Width ◽  
Pore Characteristics ◽  
Ice Growth ◽  
Surrounding Matrix

Abstract. Pre-activation denotes the capability of particles or materials to nucleate ice at lower relative humidities or higher temperatures compared to their intrinsic ice nucleation efficiency after having experienced an ice nucleation event or low temperature before. This review presumes that ice preserved in pores is responsible for pre-activation and analyses pre-activation under this presumption. Idealized trajectories of air parcels are used to discuss the pore characteristics needed for ice to persist in pores and to induce macroscopic ice growth out of the pores. The pore width needed to keep pores filled with water decreases with decreasing relative humidity as described by the inverse Kelvin equation. Thus, narrow pores remain filled with ice well below ice saturation. However, the smaller the pore width, the larger the melting and freezing point depressions within the pores. Therefore, pre-activation due to pore ice is constrained by the melting of ice in narrow pores and the sublimation of ice from wide pores imposing restrictions on the temperature and relative humidity range of pre-activation for cylindrical pores. Ice is better protected in ink-bottle-shaped pores with a narrow opening leading to a large cavity. However, whether pre-activation is efficient also depends on the capability of ice to grow macroscopically, i.e. out of the pore. A strong effect of pre-activation is expected for swelling pores, because at low relative humidity (RH) their openings narrow and protect the ice within them against sublimation. At high relative humidities, they open up and the ice can grow to macroscopic size and form an ice crystal. Similarly, ice protected in pockets is perfectly sheltered against sublimation but needs the dissolution of the surrounding matrix to be effective. Pores partially filled with condensable material may also show pre-activation. In this case, complete filling occurs at lower RH than for empty pores and freezing shifts to lower temperatures.Pre-activation experiments confirm that materials susceptible to pre-activation are indeed porous. Pre-activation was observed for clay minerals like illite, kaolinite, and montmorillonite with inherent porosity. The largest effect was observed for the swelling clay mineral montmorillonite. Some materials may acquire porosity, depending on the formation and processing conditions. Particles of CaCO3, meteoritic material, and volcanic ash showed pre-activation for some samples or in some studies but not in other ones. Quartz and silver iodide were not susceptible to pre-activation.Atmospheric relevance of pre-activation by ice preserved in pores may not be generally given but depend on the atmospheric scenario. Lower-level cloud seeding by pre-activated particles released from high-level clouds crucially depends on the ability of pores to retain ice at the relative humidities and temperatures of the air masses they pass through. Porous particles that are recycled in wave clouds may show pre-activation with subsequent ice growth as soon as ice saturation is exceeded after having passed a first cloud event. Volcanic ash particles and meteoritic material likely influence ice cloud formation by pre-activation. Therefore, the possibility of pre-activation should be considered when ice crystal number densities in clouds exceed the number of ice-nucleating particles measured at the cloud forming temperature.

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