Heterogeneous ice nucleation on dust particles sourced from nine deserts worldwide – Part 1: Immersion freezing

Abstract. Desert dust is one of the most abundant ice nucleating particle types in the atmosphere. Traditionally, clay minerals were assumed to determine the ice nucleation ability of desert dust and constituted the focus of ice nucleation studies over several decades. Recently some feldspar species were identified to be ice active at much higher temperatures than clay minerals, redirecting studies to investigate the contribution of feldspar to ice nucleation on desert dust. However, so far no study has shown the atmospheric relevance of this mineral phase.For this study four dust samples were collected after airborne transport in the troposphere from the Sahara to different locations (Crete, the Peloponnese, Canary Islands, and the Sinai Peninsula). Additionally, 11 dust samples were collected from the surface from nine of the biggest deserts worldwide. The samples were used to study the ice nucleation behavior specific to different desert dusts. Furthermore, we investigated how representative surface-collected dust is for the atmosphere by comparing to the ice nucleation activity of the airborne samples. We used the IMCA-ZINC setup to form droplets on single aerosol particles which were subsequently exposed to temperatures between 233 and 250 K. Dust particles were collected in parallel on filters for offline cold-stage ice nucleation experiments at 253–263 K. To help the interpretation of the ice nucleation experiments the mineralogical composition of the dusts was investigated. We find that a higher ice nucleation activity in a given sample at 253 K can be attributed to the K-feldspar content present in this sample, whereas at temperatures between 238 and 245 K it is attributed to the sum of feldspar and quartz content present. A high clay content, in contrast, is associated with lower ice nucleation activity. This confirms the importance of feldspar above 250 K and the role of quartz and feldspars determining the ice nucleation activities at lower temperatures as found by earlier studies for monomineral dusts. The airborne samples show on average a lower ice nucleation activity than the surface-collected ones. Furthermore, we find that under certain conditions milling can lead to a decrease in the ice nucleation ability of polymineral samples due to the different hardness and cleavage of individual mineral phases causing an increase of minerals with low ice nucleation ability in the atmospherically relevant size fraction. Comparison of our data set to an existing desert dust parameterization confirms its applicability for climate models. Our results suggest that for an improved prediction of the ice nucleation ability of desert dust in the atmosphere, the modeling of emission and atmospheric transport of the feldspar and quartz mineral phases would be key, while other minerals are only of minor importance.

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Heterogeneous ice nucleation on dust particles sourced from 9 deserts worldwide – Part 1: Immersion freezing

10.5194/acp-2016-438 ◽

2016 ◽

Cited By ~ 1

Author(s):

Yvonne Boose ◽

André Welti ◽

James Atkinson ◽

Fabiola Ramelli ◽

Anja Danielczok ◽

...

Keyword(s):

Clay Minerals ◽

Mineralogical Composition ◽

Ice Nucleation ◽

Dust Particles ◽

Data Set ◽

Nucleation Behavior ◽

Desert Dust ◽

Mineral Phases ◽

Nucleation Activity ◽

Ice Nucleation Activity

Abstract. Desert dust is one of the most abundant ice nucleating particle types in the atmosphere. Traditionally, clay minerals were assumed to determine the ice nucleation ability of desert dust and constituted the focus of ice nucleation studies. Only recently some feldspar species were identified to be ice-active at much higher temperatures than clay minerals, redirecting studies to investigate the contribution of feldspar to ice nucleation on desert dust. However, so far no study has shown the atmospheric relevance of this mineral phase. For this study four dust samples were collected after airborne transport in the troposphere from the Sahara to different locations (Crete, the Peloponnese, Canary Islands and the Sinai Peninsula). Additionally, eleven dust samples were collected from the surface from nine of the biggest deserts worldwide. The samples were used to study the ice nucleation behavior specific to different desert dusts. Furthermore we investigated how representative ice nucleation on surface-collected dust is for that in the atmosphere by comparing to the ice nucleation activity of the airborne samples. We used the IMCA-ZINC set-up to form droplets on single aerosol particles which were subsequently exposed to temperatures between 233–250 K. Dust particles were collected in parallel on filters for offline cold stage ice nucleation experiments at 253–263 K. To help the interpretation of the results from the ice nucleation experiments the mineralogical composition of the dusts was investigated. We found that a higher ice nucleation activity in a given sample can be attributed at 253 K to the K-feldspar content present in this sample whereas at temperatures between 238–245 K it is attributed to the sum of feldspar and quartz content present. A high clay content on the other hand is associated with a lower ice nucleation activity of a sample. This confirms the importance of feldspar at T > 250 K and the role of quartz and feldspars determining the ice nucleation activities at lower T as found by earlier studies for monomineral dust surrogates. Furthermore, we find that milling may lead to a decrease in the ice nucleation ability of polymineral samples due to a change in mineralogical composition in the atmospherically relevant size fraction arising from the different hardness and cleavage of individual mineral phases. Comparison of our comprehensive data set to an existing desert dust parameterization confirms its applicability for climate models. Our results suggest that for an improved prediction of the ice nucleation ability of desert dust in the atmosphere, the modelling of emission and atmospheric transport of the feldspar and quartz mineral phases would be key while other minerals are only of minor importance.

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Heterogeneous ice nucleation on dust particles sourced from nine deserts worldwide – Part 2: Deposition nucleation and condensation freezing

Atmospheric Chemistry and Physics ◽

10.5194/acp-19-1059-2019 ◽

2019 ◽

Vol 19 (2) ◽

pp. 1059-1076 ◽

Cited By ~ 9

Author(s):

Yvonne Boose ◽

Philipp Baloh ◽

Michael Plötze ◽

Johannes Ofner ◽

Hinrich Grothe ◽

...

Keyword(s):

Active Sites ◽

Mineral Dust ◽

Atmospheric Transport ◽

Chemical Imaging ◽

Ice Nucleation ◽

Dust Particles ◽

Full Potential ◽

Nucleation Behavior ◽

Crystal Water ◽

Desert Dust

Abstract. Mineral dust particles from deserts are amongst the most common ice nucleating particles in the atmosphere. The mineralogy of desert dust differs depending on the source region and can further fractionate during the dust emission processes. Mineralogy to a large extent explains the ice nucleation behavior of desert aerosol, but not entirely. Apart from pure mineral dust, desert aerosol particles often exhibit a coating or are mixed with small amounts of biological material. Aging on the ground or during atmospheric transport can deactivate nucleation sites, thus strong ice nucleating minerals may not exhibit their full potential. In the partner paper of this work, it was shown that mineralogy determines most but not all of the ice nucleation behavior in the immersion mode found for desert dust. In this study, the influence of semi-volatile organic compounds and the presence of crystal water on the ice nucleation behavior of desert aerosol is investigated. This work focuses on the deposition and condensation ice nucleation modes at temperatures between 238 and 242 K of 18 dust samples sourced from nine deserts worldwide. Chemical imaging of the particles' surface is used to determine the cause of the observed differences in ice nucleation. It is found that, while the ice nucleation ability of the majority of the dust samples is dominated by their quartz and feldspar content, in one carbonaceous sample it is mostly caused by organic matter, potentially cellulose and/or proteins. In contrast, the ice nucleation ability of an airborne Saharan sample is found to be diminished, likely by semi-volatile species covering ice nucleation active sites of the minerals. This study shows that in addition to mineralogy, other factors such as organics and crystal water content can alter the ice nucleation behavior of desert aerosol during atmospheric transport in various ways.

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A comprehensive laboratory study on the immersion freezing behavior of illite NX particles: a comparison of 17 ice nucleation measurement techniques

Atmospheric Chemistry and Physics ◽

10.5194/acp-15-2489-2015 ◽

2015 ◽

Vol 15 (5) ◽

pp. 2489-2518 ◽

Cited By ~ 123

Author(s):

N. Hiranuma ◽

S. Augustin-Bauditz ◽

H. Bingemer ◽

C. Budke ◽

J. Curtius ◽

...

Keyword(s):

Cooling Rate ◽

Surface Area ◽

Mineral Dust ◽

Measurement Techniques ◽

Ice Nucleation ◽

Measurement Methods ◽

Dust Particles ◽

Immersion Freezing ◽

Nucleation Activity ◽

Ice Nucleation Activity

Abstract. Immersion freezing is the most relevant heterogeneous ice nucleation mechanism through which ice crystals are formed in mixed-phase clouds. In recent years, an increasing number of laboratory experiments utilizing a variety of instruments have examined immersion freezing activity of atmospherically relevant ice-nucleating particles. However, an intercomparison of these laboratory results is a difficult task because investigators have used different ice nucleation (IN) measurement methods to produce these results. A remaining challenge is to explore the sensitivity and accuracy of these techniques and to understand how the IN results are potentially influenced or biased by experimental parameters associated with these techniques. Within the framework of INUIT (Ice Nuclei Research Unit), we distributed an illite-rich sample (illite NX) as a representative surrogate for atmospheric mineral dust particles to investigators to perform immersion freezing experiments using different IN measurement methods and to obtain IN data as a function of particle concentration, temperature (T), cooling rate and nucleation time. A total of 17 measurement methods were involved in the data intercomparison. Experiments with seven instruments started with the test sample pre-suspended in water before cooling, while 10 other instruments employed water vapor condensation onto dry-dispersed particles followed by immersion freezing. The resulting comprehensive immersion freezing data set was evaluated using the ice nucleation active surface-site density, ns, to develop a representative ns(T) spectrum that spans a wide temperature range (−37 °C < T < −11 °C) and covers 9 orders of magnitude in ns. In general, the 17 immersion freezing measurement techniques deviate, within a range of about 8 °C in terms of temperature, by 3 orders of magnitude with respect to ns. In addition, we show evidence that the immersion freezing efficiency expressed in ns of illite NX particles is relatively independent of droplet size, particle mass in suspension, particle size and cooling rate during freezing. A strong temperature dependence and weak time and size dependence of the immersion freezing efficiency of illite-rich clay mineral particles enabled the ns parameterization solely as a function of temperature. We also characterized the ns(T) spectra and identified a section with a steep slope between −20 and −27 °C, where a large fraction of active sites of our test dust may trigger immersion freezing. This slope was followed by a region with a gentler slope at temperatures below −27 °C. While the agreement between different instruments was reasonable below ~ −27 °C, there seemed to be a different trend in the temperature-dependent ice nucleation activity from the suspension and dry-dispersed particle measurements for this mineral dust, in particular at higher temperatures. For instance, the ice nucleation activity expressed in ns was smaller for the average of the wet suspended samples and higher for the average of the dry-dispersed aerosol samples between about −27 and −18 °C. Only instruments making measurements with wet suspended samples were able to measure ice nucleation above −18 °C. A possible explanation for the deviation between −27 and −18 °C is discussed. Multiple exponential distribution fits in both linear and log space for both specific surface area-based ns(T) and geometric surface area-based ns(T) are provided. These new fits, constrained by using identical reference samples, will help to compare IN measurement methods that are not included in the present study and IN data from future IN instruments.

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A laboratory investigation of the ice nucleation efficiency of three types of mineral and soil dust

10.5194/acp-2018-543 ◽

2018 ◽

Author(s):

Mikhail Paramonov ◽

Robert O. David ◽

Ruben Kretzschmar ◽

Zamin A. Kanji

Keyword(s):

Active Sites ◽

Particle Surface ◽

Ice Nucleation ◽

Dust Particles ◽

Bet Surface Area ◽

Soil Dust ◽

Organic Components ◽

Nucleation Activity ◽

Ice Nucleation Activity ◽

Soluble Material

Abstract. Surface-collected dust from three different locations around the world was examined with respect to its ice nucleation activity (INA) with the Portable Ice Nucleation Chamber (PINC). Ice nucleation experiments were conducted with particles of 200 and 400 nm in diameter in the temperature range of 233–243 K in both deposition nucleation and condensation freezing regimes. Several treatments were performed in order to investigate the effect of mineralogical composition, as well as the presence of biological and proteinaceous, organic and soluble compounds on the INA of mineral and soil dust. The INA of untreated dust particles correlated well with the total feldspar and K-feldspar content, corroborating previously published results. The removal of heat-sensitive proteinaceous and organic components from the particle surface with heat decreased the INA of dusts. However, the decrease in the INA was not proportional to the amount of these organic components, indicating that different proteinaceous and organic species have different ice nucleation activities, and the exact speciation is required in order to determine why dusts respond differently to the heating process. The INA of certain dusts increased after the removal of soluble material from the particle surface, demonstrating the low INA of the soluble compounds and/or the exposition of the underlying active sites. Similar to the proteinaceous organic compounds, soluble compounds seem to have different effects on the INA of surface-collected dusts, and a general conclusion about how the presence of soluble material on the particle surface affects its INA is not possible. The investigation of the heated and washed dusts revealed that mineralogy alone is not able to fully explain the observed INA of surface-collected dusts at the examined temperature and relative humidity conditions. The results showed that it is not possible to predict the INA of surface-collected soil dust based on the presence and amount of certain minerals or any particular class of compounds, such as soluble or proteinaceous/organic. Instead, at temperatures of 238–243 K the ice nucleation activity of the untreated, surface-collected soil dust in condensation freezing mode can be roughly approximated by one of the existing surrogates for atmospheric mineral dust, such as illite NX. Uncertainties associated with mechanical damage and possible changes to the mineralogy during treatments, as well as with the BET surface area and its immediate impact on the number of active sites ns,BET parameterisation, are addressed.

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Heterogeneous ice nucleation on dust particles sourced from 9 deserts worldwide – Part 2: Deposition nucleation and condensation freezing

10.5194/acp-2018-1036 ◽

2018 ◽

Author(s):

Yvonne Boose ◽

Philipp Baloh ◽

Michael Plötze ◽

Johannes Ofner ◽

Hinrich Grothe ◽

...

Keyword(s):

Active Sites ◽

Mineral Dust ◽

Atmospheric Transport ◽

Chemical Imaging ◽

Ice Nucleation ◽

Dust Particles ◽

Full Potential ◽

Nucleation Behavior ◽

Crystal Water ◽

Desert Dust

Abstract. Mineral dust particles from deserts are amongst the most common ice-nucleating particles in the atmosphere. The mineralogy of desert dust differs depending on the source region and can further fractionate during the dust emission processes. Mineralogy to a large extent explains the ice nucleation behavior of desert aerosol, but not entirely. Apart from pure mineral dust, desert aerosol particles are often mixed with small amounts of biological material or particles exhibit a coating. Aging on the ground or during atmospheric transport can deactivate nucleation sites and thus strong ice-nucleating minerals may not exhibit their full potential. In the partner paper of this work, it was shown that mineralogy determines most but not all of the ice nucleation behavior in the immersion mode found for desert dust. In this study, the influence of semi-volatile organic compounds and the presence of crystal water on the ice nucleation behavior of desert aerosol in addition to mineralogy is investigated. This work focuses on the deposition and condensation ice nucleation modes at temperatures between 238 to 242 K of 18 dust samples sources from 9 deserts worldwide. Chemical imaging of the particles' surface is used to determine the cause of the observed differences in ice nucleation. It is found that while the ice nucleation ability of the majority of the dust samples is dominated by their quartz and feldspar content, in one carbonaceous sample it is mostly caused by organic matter, potentially cellulose and/or proteins. On the other hand, the ice nucleation ability of an airborne Saharan sample is found to be diminished likely by semi-volatile species covering ice nucleation active sites of the minerals. This study shows that in addition to mineralogy, other factors such as organics and crystal water content can alter the ice nucleation behavior of desert aerosol during atmospheric transport in various ways.

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Ice nucleation efficiency of natural dust samples in the immersion mode

Atmospheric Chemistry and Physics ◽

10.5194/acp-16-11177-2016 ◽

2016 ◽

Vol 16 (17) ◽

pp. 11177-11206 ◽

Cited By ~ 30

Author(s):

Lukas Kaufmann ◽

Claudia Marcolli ◽

Julian Hofer ◽

Valeria Pinti ◽

Christopher R. Hoyle ◽

...

Keyword(s):

Clay Minerals ◽

Active Sites ◽

Aqueous Suspension ◽

Mineralogical Composition ◽

Dust Sample ◽

Ice Nucleation ◽

Scanning Calorimetry ◽

Dust Collection ◽

Nucleation Activity ◽

Ice Nucleation Activity

Abstract. A total of 12 natural surface dust samples, which were surface-collected on four continents, most of them in dust source regions, were investigated with respect to their ice nucleation activity. Dust collection sites were distributed across Africa, South America, the Middle East, and Antarctica. Mineralogical composition has been determined by means of X-ray diffraction. All samples proved to be mixtures of minerals, with major contributions from quartz, calcite, clay minerals, K-feldspars, and (Na, Ca)-feldspars. Reference samples of these minerals were investigated with the same methods as the natural dust samples. Furthermore, Arizona test dust (ATD) was re-evaluated as a benchmark. Immersion freezing of emulsion and bulk samples was investigated by differential scanning calorimetry. For emulsion measurements, water droplets with a size distribution peaking at about 2 µm, containing different amounts of dust between 0.5 and 50 wt % were cooled until all droplets were frozen. These measurements characterize the average freezing behaviour of particles, as they are sensitive to the average active sites present in a dust sample. In addition, bulk measurements were conducted with one single 2 mg droplet consisting of a 5 wt % aqueous suspension of the dusts/minerals. These measurements allow the investigation of the best ice-nucleating particles/sites available in a dust sample. All natural dusts, except for the Antarctica and ATD samples, froze in a remarkably narrow temperature range with the heterogeneously frozen fraction reaching 10 % between 244 and 250 K, 25 % between 242 and 246 K, and 50 % between 239 and 244 K. Bulk freezing occurred between 255 and 265 K. In contrast to the natural dusts, the reference minerals revealed ice nucleation temperatures with 2–3 times larger scatter. Calcite, dolomite, dolostone, and muscovite can be considered ice nucleation inactive. For microcline samples, a 50 % heterogeneously frozen fraction occurred above 245 K for all tested suspension concentrations, and a microcline mineral showed bulk freezing temperatures even above 270 K. This makes microcline (KAlSi3O8) an exceptionally good ice-nucleating mineral, superior to all other analysed K-feldspars, (Na, Ca)-feldspars, and the clay minerals. In summary, the mineralogical composition can explain the observed freezing behaviour of 5 of the investigated 12 natural dust samples, and partly for 6 samples, leaving the freezing efficiency of only 1 sample not easily explained in terms of its mineral reference components. While this suggests that mineralogical composition is a major determinant of ice-nucleating ability, in practice, most natural samples consist of a mixture of minerals, and this mixture seems to lead to remarkably similar ice nucleation abilities, regardless of their exact composition, so that global models, in a first approximation, may represent mineral dust as a single species with respect to ice nucleation activity. However, more sophisticated representations of ice nucleation by mineral dusts should rely on the mineralogical composition based on a source scheme of dust emissions.

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Bio-Surfaces of Frost Resistant Plants as Source of Ice-Nucleating Macromolecules

10.5194/egusphere-egu21-15236 ◽

2021 ◽

Author(s):

Teresa M. Seifried ◽

Paul Bieber ◽

Anna T. Kunert ◽

David G. Schmale III ◽

Karin Whitmore ◽

...

Keyword(s):

Betula Pendula ◽

Field Experiments ◽

Aerosol Particles ◽

Chem Phys ◽

Ice Nucleation ◽

Number Concentration ◽

Silver Birch ◽

Dust Particles ◽

Nucleation Activity ◽

Ice Nucleation Activity

The ice nucleation activity of pollen from silver birch (Betula pendula), pines (e.g. Pinus sylvestris) and other trees has been assigned not only to pollen grains but also to subpollen particles (SPP) and extractable macromolecules, i.e. ice-nucleating macromolecules (INMs) (Pummer et al., 2012). The number concentration of pollen in comparison to other ice-nucleating particles suggests a minor impact to atmospheric cloud glaciation (Hoose et al., 2010). When focusing on macromolecules, the importance of INMs from vegetation, however, needs to be re-evaluated in respect to atmospheric ice nucleation. It has been shown that INMs are present in nearly every tissue of birches (Felgitsch et al., 2018) and furthermore, that the macromolecules are extracted from the surface, when they come into contact with water (Seifried et al., 2020). We hypothesize that extractable INMs from tree surfaces are emitted during rainfall by splash induced emissions and field experiments were performed to evaluate the amount of INMs extracted by rain-droplets. Sampled rainwater, which was splashed off from birch surfaces, revealed INMs in high number concentration (108 cm-2) and can be attributed to the vegetation surface (Seifried et al., 2020). To further investigate emission sources an aerosol sampling tool (including an impinger and an impactor) has been developed and mounted on two rotary-wing drones (Bieber et al., 2020). Aerosol samples were collected in an alpine environment on ground level and above the canopy of birches and pines. We found that the bioaerosol concentration increased after rainfall and collected INMs show a similar onset freezing temperature as birch surface extracts (around -20&#176;C). Microscopic images revealed a fluorescent organic film on aerosol particles, which might be linked to extractable material from bio-surfaces.&#160; We suggest splash induced aerosolization of INMs during rainfall to be an underestimated source for atmospheric cloud glaciation, since INMs can easily be carried on larger aerosol particles, e.g. on SPP or on mineral dust particles.&#160;References:Pummer, B. G., Bauer, H., Bernardi, J., Bleicher, S., and Grothe, H.: Suspendable macromolecules are responsible for ice nucleation activity of birch and conifer pollen, Atmos. Chem. Phys., 12, 2541&#8211;2550, https://doi.org/10.5194/acp-12-2541-2012, 2012.Hoose, C., J. E. Kristj&#225;nsson, and S. M. Burrows.: How important is biological ice nucleation in clouds on a global scale?, Environ. Res. Lett., 5, https://doi.org/10.1088/1748-9326/5/2/024009, 2010.Felgitsch, L., Baloh, P., Burkart, J., Mayr, M., Momken, M. E., Seifried, T. M., Winkler, P., Schmale III, D. G., and Grothe, H.: Birch leaves and branches as a source of ice-nucleating macromolecules, Atmos. Chem. Phys., 18, 16063&#8211;16079, https://doi.org/10.5194/acp-18-16063-2018, 2018.Seifried, T. M., Bieber, P., Felgitsch, L., Vlasich, J., Reyzek, F., Schmale III, D. G., and Grothe, H.: Surfaces of silver birch (Betula pendula) are sources of biological ice nuclei: in vivo and in situ investigations, Biogeosciences, 17, 5655&#8211;5667, https://doi.org/10.5194/bg-17-5655-2020, 2020.Bieber, P.; Seifried, T.M.; Burkart, J.; Gratzl, J.; Kasper-Giebl, A.; Schmale, D.G., III; Grothe, H. A Drone-Based Bioaerosol Sampling System to Monitor Ice Nucleation Particles in the Lower Atmosphere. Remote Sens., 12, 552, 2020.

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Ice nucleation efficiency of natural dust samples in the immersion mode

10.5194/acp-2016-337 ◽

2016 ◽

Author(s):

Lukas Kaufmann ◽

Claudia Marcolli ◽

Julian Hofer ◽

Valeria Pinti ◽

Christopher R. Hoyle ◽

...

Keyword(s):

Clay Minerals ◽

Active Sites ◽

Aqueous Suspension ◽

Mineralogical Composition ◽

Ice Nucleation ◽

Scanning Calorimetry ◽

Dust Collection ◽

Source Regions ◽

Nucleation Activity ◽

Ice Nucleation Activity

Abstract. Twelve natural dust samples from eight dust source regions on four continents were investigated with respect to their ice nucleation activity, revealing no significant differences between source regions. Dust collection sites were distributed across Africa, South America, the Middle East and Antarctica. Mineralogical compositions have been determined by means of X-ray diffraction. All samples proved to be mixtures of minerals, with major contributions from quartz, calcite, clay minerals, K-feldspars and (Na, Ca)-feldspars. Reference samples of these minerals were investigated with the same methods as the natural dust samples. Furthermore, Arizona Test Dust (ATD) was re-evaluated as a benchmark. Immersion freezing of emulsion and bulk samples was investigated by differential scanning calorimetry. For emulsion measurements, water droplets with a diameter of about 2 µm, containing different amounts of dust between 0.5 wt% and 50 wt% were cooled until all droplets were frozen. These measurements characterize the average freezing behaviour of particles, as they are sensitive to the average active sites present in a dust sample. In addition, bulk measurements were conducted with one single 1 mm diameter droplet consisting of a 5 wt% aqueous suspension of the dusts/minerals. These measurements allow the investigation of the best particles/sites available in a dust. All natural dusts except for the Antarctica and ATD samples froze in a remarkably narrow temperature range with the heterogeneously frozen fraction reaching 10 % between 244 and 250 K, 25 % between 242 and 246 K, and 50 % between 239 and 244 K. Bulk freezing occurred between 255 and 265 K. In contrast to the natural dusts, the reference minerals reveal ice nucleation temperatures with 2–3 times larger scatter. Calcite, dolomite, dolostone and muscovite can be considered inactive as ice nuclei (IN). For microcline samples 50 % heterogeneously frozen fraction occurred above 245 K for all tested suspension concentrations, and a microcline mineral showed bulk freezing temperatures even above 270 K. This makes microcline (KAlSi3O8) an exceptionally good IN, superior to all other analysed K-feldspars, (Na, Ca)-feldspars and the clay minerals. However, microcline is not abundant in the investigated natural dust samples. While K-feldspars were identified in five of the seven investigated natural source regions, only one sample contained microcline, and then only as a minor fraction. In summary, the mineralogical composition can explain the observed freezing behaviour of five of the investigated 12 natural dust samples, and partly for six samples, leaving the freezing efficiency of only one sample not easily explained in terms of its mineral reference components. While this suggests that mineralogical composition is a major determinant of ice nucleation ability, in practice most natural samples consist of a mixture of minerals, and this mixture seems to lead to remarkably similar ice nucleation abilities, regardless of their exact composition, so that global models, in a first approximation, may represent mineral dust as a single species with respect to ice nucleation activity. However, more sophisticated representations of ice nucleation by mineral dusts should rely on the mineralogical composition based on a source scheme of dust emissions.

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