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Characterization of individual ice nuclei by single droplet freezing method: A case study in the
Asian dust outflow region
Ayumi Iwata1and Atsushi Matsuki2
1
Graduate school of Natural Science and Technology, Kanazawa University, Kakuma, Kanazawa,
Ishikawa, 920-1192, Japan
2
Institute of Nature and Environmental Technology, Kanazawa University, Japan.
Keywords: Ice nuclei, Mixed phase cloud, Mixing state.
INTRODUCTION
Ice nucleation in clouds substantially affects the climate by having a significant impact on the radiation
balance and precipitation process in the Earth’s atmosphere. The super-cooled water droplets in the actual
atmosphere generally form ice crystals at higher temperatures by the aid of aerosol particles that have the
ability to nucleate ice than pure water. The physical and chemical properties of aerosols, which act as the
ice nuclei, play an essential role in the formation of ice crystals. However, a considerable uncertainty still
exists as to the response of IN processes to the changes in the hosting aerosol properties, due to the lack of
fundamental understanding of the interaction of aerosol particles with the ice crystal formation.
Many previous IN experiments were performed under laboratory conditions and provided valuable
knowledge on IN properties of pure component particles and artificially generated aerosol mixtures
(Murray et al., 2012). However, the situation is even more complex in the ambient atmosphere, where
particles are often present as a complex mixture of different compounds. Therefore, the necessity
remained to conduct experiments that reflect the particle mixing state in the actual atmosphere. In
particular, the reaction processes, coating and aging states at the surface of the particles can dramatically
change from their original properties though internal mixing (Trochikin et al., 2003). It is often the case
that internal mixing states can vary from particle to particle, therefore, individual particle analysis is
necessary for establishing complete understanding of the ice nucleation by ambient aerosol particles.
This study is designed to investigate how morphology, chemical composition, and mixing state of
ambient aerosol particles influence their ice nucleating activities under conditions relevant for the mixed
phase clouds. We further demonstrate that we can also keep track on the individual ice nucleating particle
by continuously controlling the ambient conditions during the ice nucleating experiment.
The individual droplet freezing method (IDFM) is an experimental method used in this study, with
which ice crystal formation on each particles under controlled condition can be monitored while
maintaining individual particles distinguishable. We exposed the sampled particles to the conditions
relevant for mixed phase cloud formation, thus simulating the ice nucleation at immersion freezing mode.
By drying and evaporating the particles that formed ice crystals and/or droplets, further, one can keep
track on their exact location as ice and/or droplet residues. This method enables detailed post sampling
analysis on both ice forming and non-ice forming particles on individual particle basis. Here, we applied
the method on ambient aerosols that were under the influence Asian dust outflow.
METHODS
The sample particles were deposited onto Si wafer substrate with hydrophobic coating. After particles
were observed under an optical microscope (Fig.1a), the particles on the substrate were exposed to the
water super saturation condition that initiates droplet formation by adjusting both the dew point of the air
flow (0.5 l/min) introduced into cold stage cell (THMSG600, Linkam Scientific Instruments, UK) and the
temperature of the sample on the cold stage (Fig. 1b). Then, the temperature of the stage was cooled down
to -30 ˚C. The formation of ice crystals on individual droplets can be visually identified by the rapidly
increasing size and their irregular shapes (Fig. 1c). After the substrate reached -30˚C, the temperature of
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the stage was increased up to -10˚C. After reaching -10˚C, the dry air flow (0.5 l/min) was introduced into
the cell to expose the formed ice crystals and droplets to the sub-saturation condition for ice. As a result of
evaporation and/or sublimation of water the nuclei particles were left visible on the substrate (Fig. 1d).
a
b
c
d
20µm
20µm
20µm
20µm
Fig. 1. Optical images of sampled particles on substrate before the ice experiment (a), exposed to the water
super saturation condition at -9 ˚C (b), cooled substrate at -30 ˚C (c), and after the ice experiment (d).
By using the IDFM method, we identified the ice nucleating particles from the ambient aerosol
particles. Five types of standard samples were also tested with the same method for comparison (Quartz,
K-feldspar, Na-feldspar, Arizona test dust; ATD, Asian dust source particle; ADS). The ambient particles
were sampled by using an impactor having a 50% cutoff diameter at 1.1µm at a flow rate of 1.0 L/min at
Kanazawa University campus (36.54˚N, 136.70˚E, 149 m. a. s. l.), Japan on 28 February 2016 and 10
April 2016. On both sampling days, arrivals of Asian dust plumes were reportd.
Both IN active and non-active atmospheric particles identified by the IN experiments of the
atmospheric particles were analyzed on individual particle basis using an atomic force microscope (AFM,
CombiScopeTM 1000, AIST-NT, Inc.) and a micro-Raman spectroscopy (Nanofinder®HE, Tokyo
Insturuments, Inc.), for the characterization of 3 dimensional morphology and detection of surface
chemical compounds under ambient conditions, respectively. Furthermore, the exact same particles were
analyzed by a scanning electron microscope (SEM, S-3000N, HITACHI) coupled with an energy
dispersive X-ray spectroscopy (EDX, EMAX-500, HORIBA), in order to obtain their elemental
composition.
RESULTS
Fig. 2. Summary of the detection frequencies for the assigned components in the non-active, the IN active,
and the Asian dust source (ADS) particles by micro-Raman analysis.
The heterogeneous IN by all standard samples tested in this study was always observed at higher
temperatures than the homogeneous freezing temperature. The standard samples of single mineral
components included pure water, K-feldspar, Na-feldspar and quartz, and their freezing onset
temperatures were -36.5 ˚C, -20.3 ˚C, -20.7 ˚C and -25.7 ˚C, respectively. Therefore, the IN activity of Kfeldspar was highest and that of quartz was lowest among these samples. The freezing temperatures of
ATD and ADS particle were also found to be at -22.5˚C and -26.6˚C, respectively. Meanwhile, most of the
IN active particles collected from the actual atmosphere formed ice below -28 ˚C.
The morphological images and the maximum height based on the cross sectional shape of 22 IN active
particles and 67 non-active particles were obtained by AFM observation on ambient samples. The results
suggested that the IN active particles were predominantly irregular particles.
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Chemical species contained in the 42 IN active particles and 131 non-active particles were identified
by the Raman spectra (Fig. 2). The significantly larger fraction (76%) of the IN active particles showed
fluorescence in the Raman spectra. In addition to the fraction of fluorescent particles, BBC such as humic
like substances or black carbon, CaSO4, and quartz were detected in the IN active particles with obviously
higher frequencies than the non-active particles.
Elemental compositions of 37 IN active particles and 114 non-active particles were analyzed by using
SEM-EDX. As shown in Fig. 3, the relative abundance of particle groups were clearly different between
the IN active and non-active particles. The mineral dust particle groups (mineral dust and mineral dust +
inclusions) accounted for 55% of the IN active particles and were the most dominant types. On the other
hand, the majority (62%) of the non-active particles was dominated by the fresh and the aged sea salt
particles. In the Ca-rich and sulfate groups, their pure component groups were relatively larger as
compared to their internal mixtures (+ inclusion) among the non-active particles.
Fig. 3. Frequencies of the identified particle groups by SEM-EDX for the non-active and the IN active
particles during April and February sampling.
DISCUSSIONS
Na
Based on the results of SEM-EDX and micro-Raman analysis,
Non-active
IN active
most of sea salt particles can be considered as the aged sea salts
particles that were internally mixed with nitrates (in particular
MgNO3), sulfates or organics. As a result of the current freezing
experiment, it was clearly demonstrated that these aged sea salt
particles are not efficient ice nuclei in the mixed phase cloud
formation. We show ternary diagram of Na-(Al+Mg+Fe)-(Ca+S)
for all analyzed particle in (Fig. 4). The diagram clearly indicates
that particles having Na larger than 35% were predominant in the
Ca+S
Al+Mg+Fe
non-active particles. That is an indication that internal mixing
Fig.4. Ternary diagram of Nawith sea salt particle may potentially act as an important
(Ca+S)-(Al+Mg+Fe) for all analyzed
inhibiting factor for the IN within the mixed phase clouds.
particles by SEM-EDX analysis.
The fluorescent particles identified by micro-Raman can
be associated with mineral dust (especially those enriched in
clay minerals) from the comparison with the SEM-EDX analysis. Therefore, both the SEM-EDX and
micro-Raman analyses suggested that mineral dust particles act as efficient ice nuclei under conditions
relevant for the mixed phase cloud formation in the atmosphere.
Additionally, In order to verify the importance of mineralogy (e.g. whether K-feldspar is dominant
within the ice nucleating particles), the particle group identified as mineral dust were further classified
based on the ratio of the detected elements. It turns out that, K-feldspar dominant particles were hardly
observed in the atmospheric mineral dust particles analyzed in this study. Also, the peaks of feldspar and
quartz were hardly identified in the Raman spectra of the analyzed mineral dust particles. Further, the
standard K-feldspar and quartz samples were indeed much more efficient IN than atmospheric IN active
particles. The above results suggested that ice nucleating mineral dust particles as efficient as pure
component K-feldspar or quartz are extremely rare in the actual atmosphere. On the other hand, it was
demonstrated that most of the ice nucleating particles above -30 ˚C in the atmosphere were dominated by
mineral dust particles composed mainly of clay, with or without minor mixing of other mineral
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components, that involves fluorescence most likely as a result of the defects and/or impurities such as
humic organics in their clay minerals (Gaft et al., 2005; Sovanska et al., 2014).
In terms of Ca-rich particles classified by SEM-EDX, those particles with small S content (S/Ca<0.2)
that can be regarded as predominantly calcite were more common in the non-active particles. The 50% of
the analyzed those Ca-rich particles were detected with the peak of carbonates in the Raman spectra,
confirming the presence of calcite in the non-active particles. In the case of sulfate particles, the nonactive sulfate particles showed peak of (NH4)2SO4. In contrast, the particles identified as CaSO4 and
fluorescence peak were clearly showed relatively higher aboundance within the IN active sulfate particles.
With regard to the Ca-rich and sulfates particles studied, in summary, our results show evidence that the
(NH4)2SO4 or intact calcite particles in the actual atmosphere were suggested to be inactive IN under
mixed phase cloud formation. Meanwhile, CaSO4 or the particles internally mixied with minor fraction of
clay mineral or sulfates particles may have higher chance of nucleating ice than the (NH4)2SO4 or intact
calcite particles.
With all above results considered, it is suggested that, in addition to the original composition and related
IN activities, the aging process in the atmosphere must also be taken into account for precisely predicting
the IN activity of the ambient aerosols.
CONCLUSIONS
The IN experiments on both the standard mineral samples and the ambient aerosol particles were
performed on individual particle basis by the IDFM. In addition, morphology and composition of both the
IN active and non-active particles were directly measured by three different individual particles analysis.
Among the ambient aerosol particles, alumino-silicate mineral dust and internal mixtures particles in
sulfates or Ca-rich particles were identified as ice nucleation active particle types. The mineral dust were
suggested to be clay mineral or mixtures of several mineral components rather than single mineral species
that were previously associated with high ice nucleation activity (e.g. K-feldspar). Our result suggests that
the freezing temperatures of individual IN in the actual atmosphere do not show large variation and fall in
the relatively narrow range that can be represented by ice nucleation activity of clay minerals.
On the other hand, the aged sea salts, pure calcite, and pure sulfates were found less active as IN.
Especially, the mixing with sea salt particles during transport is suggested as an important factor inhibiting
the IN activity of the mixing counterparts (e.g. mineral dust). Although pure calcite and sulfate particles
were identified as inert particle groups, interestingly, their internal mixture showed relatively higher IN
activity. This may have an important implication such that the atmospheric aging (including cloud
processing) could potentially enhance the originally inert IN activity of calcite or (NH4)2SO4 particles.
This study successfully related the immersion-mode IN activity of atmospheric particles and their
morphology, composition and mixing states on individual particle basis. This was made possible by the
direct and comprehensive particle analysis on the individual ice residue particles. We believe the method
can be used to verify the aerosol IN theories previously proposed mostly based on experiments using
single component and/or bulk samples.
REFERENCES
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and Shen Z. (2003). Mineral aerosol particles collected in Dunhuang, China, and their comparison with
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