scholarly journals Cleaning up our water: reducing interferences from nonhomogeneous freezing of “pure” water in droplet freezing assays of ice-nucleating particles

2018 ◽  
Vol 11 (9) ◽  
pp. 5315-5334 ◽  
Author(s):  
Michael Polen ◽  
Thomas Brubaker ◽  
Joshua Somers ◽  
Ryan C. Sullivan
Keyword(s):  
Pure Water ◽  
Freezing Temperature ◽  
Cloud Microphysics ◽  
Ice Nucleation ◽  
Great Promise ◽  
Water Control ◽  
Temperature Spectra ◽  
Droplet Sizes ◽  
Mixed Phase Clouds ◽  
Insight Into

Abstract. Droplet freezing techniques (DFTs) have been used for half a century to measure the concentration of ice-nucleating particles (INPs) in the atmosphere and determine their freezing properties to understand the effects of INPs on mixed-phase clouds. The ice nucleation community has recently adopted droplet freezing assays as a commonplace experimental approach. These droplet freezing experiments are often limited by contamination that causes nonhomogeneous freezing of the “pure” water used to generate the droplets in the heterogeneous freezing temperature regime that is being measured. Interference from the early freezing of water is often overlooked and not fully reported, or measurements are restricted to analyzing the more ice-active INPs that freeze well above the temperature of the background water. However, this avoidance is not viable for analyzing the freezing behavior of less active INPs in the atmosphere that still have potentially important effects on cold-cloud microphysics. In this work we review a number of recent droplet freezing techniques that show great promise in reducing these interferences, and we report our own extensive series of measurements using similar methodologies. By characterizing the performance of different substrates on which the droplets are placed and of different pure water generation techniques, we recommend best practices to reduce these interferences. We tested different substrates, water sources, droplet matrixes, and droplet sizes to provide deeper insight into what methodologies are best suited for DFTs. Approaches for analyzing droplet freezing temperature spectra and accounting and correcting for the background “pure” water control spectrum are also presented. Finally, we propose experimental and data analysis procedures for future homogeneous and heterogeneous ice nucleation studies to promote a more uniform and reliable methodology that facilitates the ready intercomparison of ice-nucleating particles measured by DFTs.

scholarly journals Cleaning up our water: reducing interferences from non-homogeneous freezing of pure water in droplet freezing assays of ice nucleating particles

2018 ◽  
Author(s):  
Michael Polen ◽  
Thomas Brubaker ◽  
Joshua Somers ◽  
Ryan C. Sullivan
Keyword(s):  
Pure Water ◽  
Freezing Temperature ◽  
Cloud Microphysics ◽  
Ice Nucleation ◽  
Great Promise ◽  
Water Control ◽  
Temperature Spectra ◽  
Droplet Sizes ◽  
Homogeneous Freezing ◽  
Mixed Phase Clouds

Abstract. Droplet freezing techniques (DFTs) have been used for half a century to measure the concentration of ice nucleating particles (INP) in the atmosphere and determine their freezing properties to understand the effects of INPs on mixed phase clouds. The ice nucleation community has recently adopted droplet freezing assays as a commonplace experimental approach. These droplet freezing experiments are often plagued by contamination that causes non-homogeneous freezing of the pure water used to generate the droplets in the heterogeneous freezing temperature regime that is being measured. Interference from the early freezing of water is often overlooked and not fully reported, or measurements are restricted to analyzing the more ice-active INPs that freeze well above the temperature of the background water. However, this avoidance is not viable for analyzing the freezing behavior of less active INP in the atmosphere that still have potentially important effects on cold-cloud microphysics. In this work we review a number of recent droplet freezing techniques showing great promise in reducing these interferences and report our own extensive series of measurements using similar methodologies. By characterizing the performance of different substrates on which the droplets are placed and of different pure water generation techniques, we recommend best practices to reduce these interferences. We tested different substrates, water sources, droplet matrixes, and droplet sizes to provide deeper insight into what methodologies are best suited for DFTs. Approaches for analyzing droplet freezing temperature spectra and accounting and correcting for the background pure water control spectrum are also presented. Finally, we propose experimental and data analysis procedures for future homogeneous and heterogeneous ice nucleation studies to promote a more uniform and reliable methodology that facilitates the ready intercomparison of ice nucleating particles measured by DFTs.

scholarly journals Enhanced ice nucleation efficiency of microcline immersed in dilute NH<sub>3</sub> and NH<sub>4</sub><sup>+</sup>-containing solutions

2018 ◽  
Author(s):  
Anand Kumar ◽  
Claudia Marcolli ◽  
Beiping Luo ◽  
Thomas Peter
Keyword(s):  
Volume Fraction ◽  
Pure Water ◽  
Freezing Temperature ◽  
Ice Nucleation ◽  
Ice Melting ◽  
Salt Solutions ◽  
Point Curve ◽  
Mineral Dusts ◽  
Immersion Freezing ◽  
Mixed Phase Clouds

Abstract. Potassium containing feldspars (K-feldspars) have been considered key mineral dusts for ice nucleation (IN) in mixed-phase clouds. To investigate the effect of solutes on their IN efficiency, we performed immersion freezing experiments with the K-feldspar microcline, which is highly IN active. Freezing of emulsified droplets with microcline suspended in aqueous solutions of NH3, (NH4)2SO4, NH4HSO4, NH4NO3, NH4Cl, Na2SO4, H2SO4, K2SO4 and KCl, with solute concentrations corresponding to water activities aw = 0.9–1.0, were investigated by means of a differential scanning calorimeter (DSC). The measured heterogeneous ice nucleation onset temperatures, Thet (aw) deviate strongly from Thet∆awhet (aw), the values calculated from the water-activity-based approach (where Thet∆awhet (aw) = Tmelt (aw + ∆awhet) with a constant offset ∆awhet with respect to the ice melting point curve). Surprisingly, for very dilute solutions of NH3 and NH4+-salts (molalities  ~ 0.96), we find IN temperatures raised by up to 4.5 K above the onset freezing temperature of microcline in pure water (Thet (aw = 1)) and 5.5 K above Thet∆awhet (aw), revealing NH3 and NH4+ to significantly enhance the IN of the microcline surface. Conversely, more concentrated NH3 and NH4+ solutions show a depression of the onset temperature below Thet∆awhet (aw) by as much as 13.5 K caused by a decline in IN ability accompanied with a reduction in the volume fraction of water frozen heterogeneously. All salt solutions not containing NH4+ as cation exhibit nucleation temperatures Thet (aw)  ~ 0.96) at warm (252–257 K) and NH3/NH4+-rich conditions.

scholarly journals An instrument for quantifying heterogeneous ice nucleation in multiwell plates using infrared emissions to detect freezing

2018 ◽  
Vol 11 (10) ◽  
pp. 5629-5641 ◽  
Author(s):  
Alexander D. Harrison ◽  
Thomas F. Whale ◽  
Rupert Rutledge ◽  
Stephen Lamb ◽  
Mark D. Tarn ◽  
...  
Keyword(s):  
Active Sites ◽  
Freezing Temperature ◽  
Equilibrium Temperature ◽  
Ice Nucleation ◽  
Ir Camera ◽  
Low Concentrations ◽  
Immersion Freezing ◽  
Mixed Phase Clouds ◽  
The City ◽  
Good Agreement

Abstract. Low concentrations of ice-nucleating particles (INPs) are thought to be important for the properties of mixed-phase clouds, but their detection is challenging. Hence, there is a need for instruments where INP concentrations of less than 0.01 L−1 can be routinely and efficiently determined. The use of larger volumes of suspension in drop assays increases the sensitivity of an experiment to rarer INPs or rarer active sites due to the increase in aerosol or surface area of particulates per droplet. Here we describe and characterise the InfraRed-Nucleation by Immersed Particles Instrument (IR-NIPI), a new immersion freezing assay that makes use of IR emissions to determine the freezing temperature of individual 50 µL droplets each contained in a well of a 96-well plate. Using an IR camera allows the temperature of individual aliquots to be monitored. Freezing temperatures are determined by detecting the sharp rise in well temperature associated with the release of heat caused by freezing. In this paper we first present the calibration of the IR temperature measurement, which makes use of the fact that following ice nucleation aliquots of water warm to the ice–liquid equilibrium temperature (i.e. 0 ∘C when water activity is ∼1), which provides a point of calibration for each individual well in each experiment. We then tested the temperature calibration using ∼100 µm chips of K-feldspar, by immersing these chips in 1 µL droplets on an established cold stage (µL-NIPI) as well as in 50 µL droplets on IR-NIPI; the results were consistent with one another, indicating no bias in the reported freezing temperature. In addition we present measurements of the efficiency of the mineral dust NX-illite and a sample of atmospheric aerosol collected on a filter in the city of Leeds. NX-illite results are consistent with literature data, and the atmospheric INP concentrations were in good agreement with the results from the µL-NIPI instrument. This demonstrates the utility of this approach, which offers a relatively high throughput of sample analysis and access to low INP concentrations.

scholarly journals Sensitivity of cirrus and mixed-phase clouds to the ice nuclei spectra in McRAS-AC: single column model simulations

2012 ◽  
Vol 12 (6) ◽  
pp. 14927-14957
Author(s):  
R. Morales Betancourt ◽  
D. Lee ◽  
L. Oreopoulos ◽  
Y. C. Sud ◽  
D. Barahona ◽  
...  
Keyword(s):  
Cloud Microphysics ◽  
Ice Nucleation ◽  
Mixed Phase ◽  
Cloud Particle ◽  
Water Path ◽  
Ice Water Path ◽  
Single Column ◽  
Mixed Phase Clouds ◽  
Ice Water ◽  
The Impact

Abstract. The salient features of mixed-phase and ice clouds in a GCM cloud scheme are examined using the ice formation parameterizations of Liu and Penner (LP) and Barahona and Nenes (BN). The performance of LP and BN ice nucleation parameterizations were assessed in the GEOS-5 AGCM using the McRAS-AC cloud microphysics framework in single column mode. Four dimensional assimilated data from the intensive observation period of ARM TWP-ICE campaign was used to drive the fluxes and lateral forcing. Simulation experiments where established to test the impact of each parameterization in the resulting cloud fields. Three commonly used IN spectra were utilized in the BN parameterization to described the availability of IN for heterogeneous ice nucleation. The results show large similarities in the cirrus cloud regime between all the schemes tested, in which ice crystal concentrations were within a factor of 10 regardless of the parameterization used. In mixed-phase clouds there are some persistent differences in cloud particle number concentration and size, as well as in cloud fraction, ice water mixing ratio, and ice water path. Contact freezing in the simulated mixed-phase clouds contributed to transfer liquid to ice efficiently, so that on average, the clouds were fully glaciated at T~260 K, irrespective of the ice nucleation parameterization used. Comparison of simulated ice water path to available satellite derived observations were also performed, finding that all the schemes tested with the BN parameterization predicted average values of IWP within ±15% of the observations.

scholarly journals Ice nucleation activity of silicates and aluminosilicates in pure water and aqueous solutions – Part 1: The K-feldspar microcline

2018 ◽  
Vol 18 (10) ◽  
pp. 7057-7079 ◽  
Author(s):  
Anand Kumar ◽  
Claudia Marcolli ◽  
Beiping Luo ◽  
Thomas Peter
Keyword(s):  
Aqueous Solutions ◽  
Water Activity ◽  
Volume Fraction ◽  
Pure Water ◽  
Freezing Temperature ◽  
Solute Concentration ◽  
Ice Nucleation ◽  
Specific Chemical ◽  
Airborne Dust ◽  
Immersion Freezing

Abstract. Potassium-containing feldspars (K-feldspars) have been considered as key mineral dusts for ice nucleation (IN) in mixed-phase clouds. To investigate the effect of solutes on their IN efficiency, we performed immersion freezing experiments with the K-feldspar microcline, which is highly IN active. Freezing of emulsified droplets with microcline suspended in aqueous solutions of NH3, (NH4)2SO4, NH4HSO4, NH4NO3, NH4Cl, Na2SO4, H2SO4, K2SO4 and KCl, with solute concentrations corresponding to water activities aw = 0.9–1.0, were investigated by means of a differential scanning calorimeter (DSC). The measured heterogeneous IN onset temperatures, Thet(aw), deviate strongly from ThetΔawhet(aw), the values calculated from the water-activity-based approach (where ThetΔawhet(aw)=Tmelt(aw+Δawhet) with a constant offset Δawhet with respect to the ice melting point curve). Surprisingly, for very dilute solutions of NH3 and NH4+ salts (molalities ≲1 mol kg−1 corresponding to aw ≳ 0.96), we find IN temperatures raised by up to 4.5 K above the onset freezing temperature of microcline in pure water (Thet(aw=1)) and 5.5 K above ThetΔawhet(aw), revealing NH3 and NH4+ to significantly enhance the IN of the microcline surface. Conversely, more concentrated NH3 and NH4+ solutions show a depression of the onset temperature below ThetΔawhet(aw) by as much as 13.5 K caused by a decline in IN ability accompanied with a reduction in the volume fraction of water frozen heterogeneously. All salt solutions not containing NH4+ as cation exhibit nucleation temperatures Thet(aw)<ThetΔawhet(aw) even at very small solute concentrations. In all these cases, the heterogeneous freezing peak displays a decrease as solute concentration increases. This deviation from Δawhet = const. indicates specific chemical interactions between particular solutes and the microcline surface not captured by the water-activity-based approach. One such interaction is the exchange of K+ available on the microcline surface with externally added cations (e.g., NH4+). However, the presence of a similar increase in IN efficiency in dilute ammonia solutions indicates that the cation exchange cannot explain the increase in IN temperatures. Instead, we hypothesize that NH3 molecules hydrogen bonded on the microcline surface form an ice-like overlayer, which provides hydrogen bonding favorable for ice to nucleate on, thus enhancing both the freezing temperatures and the heterogeneously frozen fraction in dilute NH3 and NH4+ solutions. Moreover, we show that aging of microcline in concentrated solutions over several days does not impair IN efficiency permanently in case of near-neutral solutions since most of it recovers when aged particles are resuspended in pure water. In contrast, exposure to severe acidity (pH ≲1.2) or alkalinity (pH ≳11.7) damages the microcline surface, hampering or even destroying the IN efficiency irreversibly. Implications for IN in airborne dust containing microcline might be multifold, ranging from a reduction of immersion freezing when exposed to dry, cold and acidic conditions to a 5 K enhancement during condensation freezing when microcline particles experience high humidity (aw≳0.96) at warm (252–257 K) and NH3/NH4+-rich conditions.

scholarly journals Ice nucleation efficiency of clay minerals in the immersion mode

2012 ◽  
Vol 12 (1) ◽  
pp. 3213-3261 ◽  
Author(s):  
V. Pinti ◽  
C. Marcolli ◽  
B. Zobrist ◽  
C. R. Hoyle ◽  
T. Peter
Keyword(s):  
Clay Minerals ◽  
Clay Mineral ◽  
Acid Treatment ◽  
Pure Water ◽  
Source Region ◽  
Freezing Temperature ◽  
Nucleation Site ◽  
Ice Nucleation ◽  
Nucleation Sites ◽  
Mineral Society

Abstract. Emulsion and bulk freezing experiments were performed to investigate immersion ice nucleation on clay minerals in pure water, using various kaolinites, montmorillonites, illites as well as natural dust from the Hoggar Mountains in the Saharan region. DSC (differential scanning calorimeter) measurements were performed on the kaolinites KGa-1b and KGa-2 from the Clay Mineral Society and kaolinite from Sigma-Aldrich; the montmorillonites SWy-2 and STx-1b from the Clay Mineral Society and the acid treated montmorillonites KSF and K-10 from Sigma Aldrich; the illites NX and SE from Arginotec. The emulsion experiments provide information on the average freezing behaviour characterized by the average nucleation sites. These experiments revealed one to two distinct heterogeneous freezing peaks, which suggest the presence of a low number of qualitatively distinct average nucleation site classes. We refer to the peak at the lowest temperature as "standard peak" and to the one at higher temperatures as "special peak". Conversely, freezing in bulk samples is not initiated by the average nucleation sites, but by a very low number of "best sites". The kaolinites showed quite narrow standard peaks with onset temperatures 239 K < Tonstd < 242 K and best sites with averaged median freezing temperature Tmedbest = 257 K. Only the kaolinite from Sigma Aldrich featured a special peak with freezing onset at 248 K. The illites showed broad standard peaks with freezing onsets at 244 K < Tonstd < 246 K and best sites with averaged median freezing temperature Tmedbest = 262 K. Montmorillonites had standard peaks with onsets 238 K < Tonstd < 240 K and best sites with Tmedbest=257 K. SWy-2, M K10, and KSF featured special peaks with onsets at Tonspcl=247, 240, and 242 K, respectively. M K10 and KSF both from Sigma Aldrich had less intense standard peaks compared to the ones from the Clay Mineral Society suggesting that a fraction of the standard sites are lost by the acid treatment. The acid treatment had however, no evident effect on best sites. Our investigations demonstrate that immersion freezing temperatures of clay minerals strongly depend on the amount of clay mineral present per droplet and on the exact type (location of collection and pre-treatment) of the clay mineral. We suggest that apparently contradictory results obtained by different groups with different setups can indeed be brought into good agreement when only clay minerals of the same type and amount per droplet are compared. The natural sample from the Hoggar Mountains, a region whose dusts have been shown to be composed mainly of illite, showed very similar freezing characteristics to the illites with freezing peak onsets 247 K < Tonstd < 248 K for the average and Tmedbest = 261 K for the best sites. Relating the concentration of best IN to the dust concentration in the atmosphere suggested that the best IN in the Hoggar sample would be common enough downwind of their source region to account for ambient IN number densities in the temperature range of 250–260 K at least during dust events.

scholarly journals Not all feldspar is equal: a survey of ice nucleating properties across the feldspar group of minerals

2016 ◽  
Author(s):  
Alexander D. Harrison ◽  
Thomas F. Whale ◽  
Michael A . Carpenter ◽  
Mark A. Holden ◽  
Lesley Neve ◽  
...  
Keyword(s):  
Crystal Structures ◽  
Recent Work ◽  
Mineral Dust ◽  
Freezing Temperature ◽  
Ice Nucleation ◽  
Dust Particles ◽  
Chemical Compositions ◽  
The Mean ◽  
Mixed Phase Clouds ◽  
Alkali Feldspars

Abstract. Mineral dust particles from wind-blown soils are known to act as effective ice nucleating particles in the atmosphere and are thought to play an important role in the glaciation of mixed phase clouds. Recent work suggests that feldspars are the most efficient nucleators of the minerals commonly present in atmospheric mineral dust. However, the feldspar group of minerals is complex, encompassing a range of chemical compositions and crystal structures. To further investigate the ice-nucleating properties of the feldspar group we measured the ice nucleation activities of 15 well-characterised feldspar samples. We show that alkali feldspars, in particular the potassium feldspars, generally nucleate ice more efficiently than feldspars containing significant amounts of calcium in the plagioclase series. We also find that there is variability in ice nucleating ability within these groups. While five out of six potassium-rich feldspars have a similar ice nucleating ability, one potassium rich feldspar sample and one sodium-rich feldspar sample were significantly more active. The hyper-active Na-feldspar was found to lose activity with time suspended in water with a decrease in mean freezing temperature of about 16 °C over 16 months; the mean freezing temperature of the hyper-active K-feldspar decreased by 2 °C over 16 months, whereas the ‘standard’ K-feldspar did not change activity within the uncertainty of the experiment. These results, in combination with a review of the available literature data, are consistent with the previous findings that potassium feldspars are important components of soil dusts for ice nucleation. However, we also show that there is the possibility that some alkali feldspars can have enhanced ice nucleating abilities, which could have implications for prediction of ice nucleating particle concentrations in the atmosphere.

scholarly journals Not all feldspars are equal: a survey of ice nucleating properties across the feldspar group of minerals

2016 ◽  
Vol 16 (17) ◽  
pp. 10927-10940 ◽  
Author(s):  
Alexander D. Harrison ◽  
Thomas F. Whale ◽  
Michael A. Carpenter ◽  
Mark A. Holden ◽  
Lesley Neve ◽  
...  
Keyword(s):  
Crystal Structures ◽  
Recent Work ◽  
Mineral Dust ◽  
Freezing Temperature ◽  
Ice Nucleation ◽  
Dust Particles ◽  
Chemical Compositions ◽  
The Mean ◽  
Mixed Phase Clouds ◽  
Alkali Feldspars

Abstract. Mineral dust particles from wind-blown soils are known to act as effective ice nucleating particles in the atmosphere and are thought to play an important role in the glaciation of mixed phase clouds. Recent work suggests that feldspars are the most efficient nucleators of the minerals commonly present in atmospheric mineral dust. However, the feldspar group of minerals is complex, encompassing a range of chemical compositions and crystal structures. To further investigate the ice-nucleating properties of the feldspar group we measured the ice nucleation activities of 15 characterized feldspar samples. We show that alkali feldspars, in particular the potassium feldspars, generally nucleate ice more efficiently than feldspars in the plagioclase series which contain significant amounts of calcium. We also find that there is variability in ice nucleating ability within these groups. While five out of six potassium-rich feldspars have a similar ice nucleating ability, one potassium rich feldspar sample and one sodium-rich feldspar sample were significantly more active. The hyper-active Na-feldspar was found to lose activity with time suspended in water with a decrease in mean freezing temperature of about 16 °C over 16 months; the mean freezing temperature of the hyper-active K-feldspar decreased by 2 °C over 16 months, whereas the "standard" K-feldspar did not change activity within the uncertainty of the experiment. These results, in combination with a review of the available literature data, are consistent with the previous findings that potassium feldspars are important components of arid or fertile soil dusts for ice nucleation. However, we also show that there is the possibility that some alkali feldspars may have enhanced ice nucleating abilities, which could have implications for prediction of ice nucleating particle concentrations in the atmosphere.

scholarly journals Sensitivity of cirrus and mixed-phase clouds to the ice nuclei spectra in McRAS-AC: single column model simulations

2012 ◽  
Vol 12 (22) ◽  
pp. 10679-10692 ◽  
Author(s):  
R. Morales Betancourt ◽  
D. Lee ◽  
L. Oreopoulos ◽  
Y. C. Sud ◽  
D. Barahona ◽  
...  
Keyword(s):  
Cloud Microphysics ◽  
Ice Nucleation ◽  
Mixed Phase ◽  
Water Path ◽  
Ice Water Path ◽  
Single Column ◽  
Effective Transfer ◽  
Mixed Phase Clouds ◽  
Ice Water ◽  
The Impact

Abstract. The salient features of mixed-phase and ice clouds in a GCM cloud scheme are examined using the ice nucleation parameterizations of Liu and Penner (LP) and Barahona and Nenes (BN). The performance of both parameterizations was assessed in the GEOS-5 AGCM using the McRAS-AC cloud microphysics framework in single column mode. Four dimensional assimilated data from the intensive observation period of ARM TWP-ICE campaign was used to drive the fluxes and lateral forcing. Simulation experiments were established to test the impact of each parameterization in the resulting cloud fields. Three commonly used IN spectra were utilized in the BN parameterization to describe the availability of IN for heterogeneous ice nucleation. The results showed large similarities in the cirrus cloud regime between all the schemes tested, in which ice crystal concentrations were within a factor of 10 regardless of the parameterization used. In mixed-phase clouds there were some persistent differences in cloud particle number concentration and size, as well as in cloud fraction, ice water mixing ratio, and ice water path. Contact freezing in the simulated mixed-phase clouds contributed to the effective transfer of liquid to ice, so that on average, the clouds were fully glaciated at T 260 K, irrespective of the ice nucleation parameterization used. Comparison of simulated ice water path to available satellite derived observations were also performed, finding that all the schemes tested with the BN parameterization predicted average values of IWP within ±15% of the observations.

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.

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