Divergent ice nucleation (IN) efficiencies of quartz, an important component
of atmospheric mineral dust, have been reported in previous studies. We show
here that quartz particles obtain their IN activity from milling and that
quartz aged in water loses most of its IN efficiency relative to freshly
milled quartz. Since most studies so far reported IN activities of commercial
quartz dusts that were milled already by the manufacturer, IN active samples
prevailed. Also, the quartz surface – much in contrast to that of feldspars
– is not prone to ammonia-induced IN enhancement. In detail we investigate
the influence of solutes on the IN efficiency of various silica
(SiO2) particles (crystalline and amorphous) with special focus on
quartz. We performed immersion freezing experiments and relate the observed
variability in IN activity to the influence of milling, the aging time and to
the exposure conditions since milling. Immersion freezing with silica
particles suspended in pure water or aqueous solutions of NH3,
(NH4)2SO4, NH4HSO4, Na2SO4 and
NaOH, with solute concentrations corresponding to water activities
aw=0.9–1.0, were investigated in emulsified droplets by means
of differential scanning calorimetry (DSC) and analyzed in terms of the onset
temperature of the heterogeneous freezing signal Thet and the
heterogeneously frozen water volume fraction Fhet. Quartz
particles, which originate from milling coarse samples, show a strong
heterogeneous freezing peak in pure water with Thet equal to
247–251 K. This IN
activity disappears almost completely after aging for 7 months in pure water
in a glass vial. During this time quartz slowly grew by incorporating silicic
acid leached from the glass vial. Conversely, the synthesized amorphous
silica samples show no discernable heterogeneous freezing signal unless they
were milled. This implies that defects provide IN activity to silica
surfaces, whereas the IN activity of a natural quartz surface is negligible,
when it grew under near-equilibrium conditions. For suspensions containing
milled quartz and the solutes (NH4)2SO4, NH4HSO4
or Na2SO4, Thet approximately follows
ThetΔawhet(aw), the
heterogeneous freezing onset temperatures that obey Δawhet criterion, i.e., ThetΔawhet(aw)=Tmelt(aw+Δawhet) with Δawhet being
a constant offset with respect to the ice melting point curve, similar to
homogeneous IN. This water-activity-based description is expected to hold
when the mineral surface is not altered by the presence of the solutes. On
the other hand, we observe a slight enhancement in Fhet in the
presence of these solutes, implying that the compliance with the Δawhet criterion does not necessarily imply constant
Fhet. In contrast to the sulfates, dilute solutions of
NH3 or NaOH (molality ≥5×10-4 mol kg-1) reveal
Thet by 3–8 K lower than ThetΔawhet(aw), indicating a significant impact
on the mineral surface. The lowering of Thet of quartz suspended
in dilute NH3 solutions is opposite to the distinct increase in
Thet that we found in emulsion freezing experiments with
aluminosilicates, namely feldspars, kaolinite, gibbsite and micas. We ascribe
this decrease in IN activity to the increased dissolution of quartz under
alkaline conditions. The defects that constitute the active sites appear to
be more susceptible to dissolution and therefore disappear first on a
dissolving surface.
Introduction
The influence of cirrus and mixed-phase clouds on Earth's radiative budget is
well recognized, yet not fully understood (Baker, 1997; DeMott et al., 2010;
Storelvmo et al., 2011). Ice formation in clouds may be initiated via
homogeneous ice nucleation (IN) below 237 K, whereas it requires an ice
nucleating particle (INP) to occur heterogeneously at higher temperatures
between 237 and 273 K (Pruppacher and Klett, 1994; Vali et al., 2015).
Mineral dusts are a well-established class of aerosol particles, consisting
of various minerals, such as feldspars, clay minerals, micas, calcite and
quartz, which exhibit widely varying IN abilities (Murray et al., 2011;
Atkinson et al., 2013; Kaufmann et al., 2016). The atmospheric relevance of
these different minerals as INPs depends on both their abundance in airborne
dusts and their IN activity, which in turn may depend on their production
process and atmospheric aging.
For a long time, clay minerals have been considered the dominating IN active
species amongst mineral dust particles. This is because of their
well-documented IN ability together with their high abundance in the fine
particle fraction, which facilitates long-range and high-altitude transport
(Usher et al., 2003; Matsuki et al., 2005; Murray et al., 2012; Pinti et al.,
2012). However, they are IN active only at temperatures too low to explain
many observed instances of cloud glaciation (Atkinson et al., 2013). More
recently, feldspars, and more specifically potassium-containing feldspars
(K-feldspars) have been suggested as the determinant species for the IN
activity of airborne desert dusts (Atkinson et al., 2013). Yet, follow-up
studies have shown that not all K-feldspars exhibit the same high IN activity
(Harrison et al., 2016; Kaufmann et al., 2016; Peckhaus et al., 2016), and
that microcline, the K-feldspar with the highest freezing temperatures,
constitutes only a minor fraction of collected desert dusts (Boose et al.,
2016b; Kaufmann et al., 2016).
Quartz, the dominant dust component collected near source regions, is a
crystalline mineral composed of silicon and oxygen atoms in a continuous
framework of SiO4 tetrahedra, with each oxygen atom being shared by
two tetrahedra. Therefore, quartz has an overall chemical formula of silicon
dioxide (SiO2), also called silica (Götze and Möckel,
2014). Quartz is a potentially relevant mineral dust for heterogeneous IN in
the atmosphere (Field et al., 2006; Murray et al., 2012; Boose et al.,
2016b). Moreover, it is found in high proportions in atmospherically
transported Saharan dust samples (Avila et al., 1997; Caquineau et al., 1998;
2002; Alastuey et al., 2005; Kandler et al., 2009). Boose et al. (2016b)
found a correlation of IN activity with the quartz concentration in dust
samples, which they collected either after being airborne and transported or
directly at the surface from deserts worldwide, suggesting that quartz
particles have the potential to be relevant INPs for cloud glaciation in the
atmosphere. Indeed, quartz particles showed IN activity in laboratory
studies, albeit with very different IN efficiencies (Pruppacher and
Sänger, 1955; Isono and Ikebe, 1960; Zimmermann et al., 2008; Atkinson et
al., 2013; Zolles et al., 2015; Kaufmann et al., 2016). Some early studies
including Pruppacher and Sänger (1955) and Isono and Ikebe (1960) found
quartz to be IN active in their experiments. For supermicrcon quartz
particles immersed in pure water, Zimmermann et al. (2008) reported an
activated fraction of 1 % at 261 K
(RHw > 100 %). In droplet freezing
experiments, Atkinson et al. (2013) and Zolles et al. (2015) found 50 %
of droplets frozen as high as 249 K and as low as 235 K, depending on
sample origin and pretreatment (e.g., milling). These examples show that
although quartz has a simple chemical composition, the quartz surface seems
to show large variations with respect to its surface properties resulting in
highly variable IN activities.
Mineral surfaces may undergo changes due to interaction with atmospheric
chemical species while being transported over long distances (Prospero, 1999;
Schepanski et al., 2009; Uno et al., 2009). These changes can potentially
alter their IN ability (Salam et al., 2007, 2008; Kulkarni et al., 2012;
Augustin-Bauditz et al., 2014). Several studies reported an indifferent
behavior of mineral surfaces to dissolved species so that immersion freezing
in solutions can be simply described as a freezing point depression due to
the solute (Zuberi et al., 2002; Zobrist et al., 2008; Rigg et al., 2013),
with a constant offset in water activity (Δaw= const.),
similar to the water-activity-based description of homogeneous IN by Koop et
al. (2000). This description was further elaborated as the activity-based
immersion freezing model (ABIFM) by Knopf and Alpert (2013). In contrast,
other studies have shown that IN efficiencies in immersion mode deteriorated
due to irreversible surface destruction (e.g., in the presence of acids)
(Niedermeier et al., 2011; Augustin-Bauditz et al., 2014; Wex et al., 2014;
Burkert-Kohn et al., 2017) as well as in deposition mode (Sullivan et al.,
2010b; Reitz et al., 2011). However, Sullivan et al. (2010a) showed that
size-selected Arizona Test Dust (ATD) exposed to nitric acid resulted in
hampering of IN efficiency deposition mode but had no impact on freezing
above water saturation. On the other hand, Kanji et al. (2019) reported no
effect of secondary organic aerosol (SOA) coating on the IN efficiency of
Saharan and Asian dust samples in immersion freezing mode.
This is the second part of three companion papers on IN activity of silicates
and aluminosilicates. In Part 1 (Kumar et al., 2018a) we have shown that
immersion freezing onset temperatures of microcline in aqueous solutions
strongly deviate from a constant Δaw. The observed
deviations were both to higher and lower IN temperatures, depending on
solute type and concentration. This finding is in accordance with Whale et
al. (2018), who found an increase in IN activity for the K-feldspars
microcline and sanidine in dilute (NH4)2SO4 solutions, but a
decrease in the presence of dilute NaCl.
In this paper and Part 3 (Kumar et al., 2019) of the companion papers, we
relate IN activities of mineral surfaces more closely with the mineral
surface properties by investigating the differences in IN activity of
structurally similar minerals in pure water and aqueous solutions. In Part 3
we investigate the differences in IN activities of aluminosilicates and
whether the enhancement observed for microcline in dilute
NH3/NH4+-containing solutions (shown in Part 1) is a
more general property of aluminosilicates. In the current study, we present
immersion freezing experiments of silicas (crystalline quartz and amorphous
silicas) in pure water and in solution droplets in order to investigate their
IN activity and how it is influenced by the presence of NH3 and
several inorganic salts, namely (NH4)2SO4,
NH4HSO4, Na2SO4 and NaOH. To elucidate which
surface structures provide IN activity, we compare the IN activity of quartz
with other silica particles, assuming that IN does not occur on the whole
particle surface with a uniform probability but that the surface exhibits
active sites, i.e., preferred locations for IN with areas of 10–50 nm2
based on estimates using classical nucleation theory (Vali, 2014; Vali et
al., 2015; Kaufmann et al., 2017).
Quartz is one of the most abundant minerals in the Earth's crust. Since
quartz is of relevance to different scientific fields such as material
sciences, geochemistry and chemical engineering, quartz surface properties
and processes have been the subject of many scientific studies. The
dissolution and crystallization of quartz has gained great attention because
it influences geochemical processes, such as the formation of mineralized
deposits or the silica concentration in natural and industrial waters
(Crundwell, 2017). Dry applications of quartz powders is of concern
because of the pathogenicity of the ground particles (Fubini et
al., 1989). We make use of the detailed characterization resulting from such
studies to relate the IN activity of quartz to its surface structure.
MethodologyMineralogy, size distribution, milling and BET surface area
measurements
Mineralogy. Silica (three-dimensional polymeric network of
SiO2) can exist in many different forms that can be crystalline or amorphous. Amorphous silica shows only short-range order and lacks a
crystalline structure as shown via X-ray diffraction (XRD) measurements
(Poulsen et al., 1995). Quartz, the most common form of crystalline silica,
has a continuous regular framework of tetrahedral SiO4 units with
Si in the center and oxygen atoms at the tetrahedral corners. Each oxygen
atom is shared between two tetrahedra. It is a hard mineral (Mohs hardness 7)
with no preferred cleavage plane (owing to the roughly equal bond strengths
throughout the crystal structure) and typically breaks with conchoidal
fracture. Quartz is the last mineral to crystallize from a magma, i.e., it
crystallizes at lower temperatures compared to other minerals, and therefore
it grows to fill the spaces remaining between the other crystals in the form of a
common impurity (Bowen, 1922, 1928).
Size distribution. Quartz from Sigma-Aldrich (∼99 %) was
the primary sample used in this study (see Sect. 4.4.5 for a discussion of
the 1 % of impurities). We will refer to this sample in the following as
SA quartz. As per manufacturer, SA quartz is a naturally occurring
microcrystalline silica that has been finely ground resulting in a particle
size range of 0.5–10 µm (approx. 80 % between
1–5 µm). In addition, we determined the number size distribution
with a TSI 3080 scanning mobility particle sizer (SMPS) and a TSI 3321
aerodynamic particle sizer (APS). Two lognormal distributions were fitted to
the bimodal size distribution yielding mode diameters of 482 nm and
1.52 µm (see the Supplement). Scanning electron microscopy (SEM)
measurements were also performed on the quartz sample at the ScopeM facility
at ETH Zurich. The quartz samples were prepared by placing them on a graphite
plate and coated with Pt/Pd alloy by sputtering before taking the
images (see the Supplement). SEM measurements mostly concur to the size range
provided by the manufacturer with the exception of the presence of very few
larger particles (> 10 µm in diameter), which are not
measured by APS since they are not well aerosolized owing to their higher
mass. We also determined the mineralogical composition of the SA quartz
sample by means of XRD in order to assess the mineralogical purity of the
mineral. Rietveld refinement using Profex software (Döbelin and Kleeberg,
2015) was performed for a quantitative analysis. Based on the X-ray
diffractogram, the sample of SA quartz consists of 98.9 % (±0.29 %) quartz, mixed with kaolinite (0.32%±0.2%) and
topaz (0.76%±0.2%). The amorphous content is estimated as 4.5±0.5 %. The Brunauer–Emmett–Teller (BET) nitrogen adsorption method
was used to determine the specific surface area of this quartz sample as
4.91 m2 g-1. In addition, thermogravimetric analysis (TGA) was
performed on the quartz sample to assess the presence of volatile species.
The 0.30 % loss in weight was observed in TGA up to 350 ∘C (see
the Supplement).
Additional milling. To assess the effect of further milling on the
IN efficiency of the SA quartz sample, we milled a portion of the sample with
a tungsten carbide disc mill for 40 s (with 1 min gap after the first 20 s
of milling) before running emulsion freezing experiments.
Other samples. For comparison, we also used four other silica dusts
besides SA quartz:
The quartz sample (BET value 3.67 m2 g-1) that showed little
IN activity in Kaufmann et al. (2016) (termed Kaufmann quartz) (see
Appendix A for details and explanation for its previously reported low IN
activity). The 0.17 % loss in weight was observed in TGA up to
350 ∘C (see the Supplement). The amorphous content is estimated as 6.4±0.5 %.
A crystalline quartz sample procured from the Technical University of Vienna
(termed TU Vienna quartz), which is “quartz I” of Zolles et al. (2015).
The 0.20 % loss in weight was observed in TGA up to 350 ∘C (see the
Supplement).
Amorphous silica particles procured from Alfa Aesar (particle size: 0.4–0.6 µm (characterization by manufacturer),
BET: 5.72 m2 g-1). In order to assess the effect of milling on
the IN efficiency of amorphous silica, we milled a portion of this sample
with a tungsten carbide disc mill for 40 s (with 1 min gap after the first
20 s of milling).
Nonporous, amorphous silica particles procured from Zurich University of
Applied Sciences (named Stöber particles in this article, mean particle
diameter: 0.34±0.02µm, BET: 11 m2 g-1, see the
Supplement for synthesis procedure).
Please note that both Alfa Aesar and Stöber silica particles are
synthetically grown (using tetraethyl orthosilicate, TEOS, in alkaline
conditions) samples used in this study and have not been milled. Only when
indicated we milled the Alfa Aesar silica particles on purpose.
Emulsion freezing of quartz freshly suspended in pure water or
solutions
Immersion freezing experiments were carried out with the differential
scanning calorimeter (DSC) setup (Q10 from TA Instruments). The 5 wt % SA
quartz suspensions in water (molecular biology reagent water from Sigma-Aldrich) were prepared in borosilicate glass vials with varying solute
concentrations (0 wt %–20 wt %) viz. (NH4)2SO4
(Sigma-Aldrich, ≥99 %), NH4HSO4 (Sigma-Aldrich, ≥99.5 %), Na2SO4 (Sigma-Aldrich, ≥99 %), NaOH
(Fluka Chemical, ≥99 %), NH3 solution (Merck, 25 %).
For comparison, 5 wt % TU Vienna quartz and a concentrated
(8 wt %–9 wt %) Kaufmann quartz sample in water, as well as
10 wt % Stöber silica and Alfa Aesar silica suspensions in water
containing NH3 and (NH4)2SO4, were also prepared for
freezing experiments (See Appendix A for freezing experiment details for the
Kaufmann quartz sample).
To avoid particle aggregation, the suspensions prepared in pure water or
solutions were sonicated for 5 min before preparing the emulsions. The
aqueous suspension and an oil/surfactant mixture (95 wt % mineral oil,
Sigma-Aldrich, and 5 wt % lanolin, Fluka Chemical) were mixed in a
ratio of 1:4 and emulsified with a rotor–stator homogenizer (Polytron
PT 1300D with a PT-DA 1307/2EC dispersing aggregate) for 40 s at 7000 rpm.
This procedure leads to droplet size distributions peaking at about
2–3 µm in number and a broad distribution in volume with highest
values between 4 and 12 µm similar to the size distributions shown
in Fig. 1 of Marcolli et al. (2007), Pinti et al. (2012) and Kaufmann et
al. (2016). Regular inspection under the microscope did not reveal any effect
of dust particles or solutes on the droplet size distribution. We placed
4–8 mg of this emulsion in an aluminum pan, which was hermetically closed
and subjected to three freezing cycles in the DSC following the method
developed and described by Marcolli et al. (2007). The first and the third
freezing cycles were executed at a cooling rate of 10 K min-1 to
control the stability of the emulsion. The second freezing cycle was run at
1 K min-1 cooling rate and used for evaluation (Zobrist et al., 2008;
Pinti et al., 2012; Kaufmann et al., 2016; Kumar et al., 2018a).
DSC thermograms of various quartz and amorphous silica particles
suspended in pure water. St: Stöber silica (10 wt %); AA: Alfa Aesar
silica (10 wt %); AA milled: milled Alfa Aesar silica (10 wt %);
KQ: quartz sample from Kaufmann et al. (2016) (8 wt %–9 wt %);
TUQ: TU Vienna quartz (5 wt %); SAQ: Sigma-Aldrich quartz (5 wt %,
milled by Sigma-Aldrich); SAQ milled: additionally milled Sigma-Aldrich
quartz (5 wt %). All curves are normalized such that the total areas
under the heterogeneous plus homogeneous freezing curves sum up to the same
value.
The DSC registers the heat release when emulsion droplets freeze. When
emulsions are prepared from an aqueous suspension of INPs, the larger
droplets are expected to freeze heterogeneously because they contain many
particles while the smaller ones rather freeze homogeneously because they
contain only few or no particles. Typical DSC thermograms therefore contain a
freezing peak below about 237 K due to homogeneous IN, while freezing above
this temperature is due to heterogeneous IN. The onset temperatures of the
heterogeneous freezing peak (Thet) and the homogeneous freezing
peak (Thom) were determined as the intersection of the tangent
drawn at the point of greatest slope at the leading edge of the peak with the
extrapolated baseline, whereas the melting temperature (Tmelt)
was determined as the maximum of the ice melting peak (see Fig. 1 of Kumar et
al., 2018a). The heat release is approximately proportional to the volume of
water that froze heterogeneously or homogeneously and is represented by the
integral of the peak over time. Note that this proportionality is only
approximate because the enthalpy of freezing exhibits a temperature
dependence (Speedy, 1987; Johari et al., 1994). We quantified the
heterogeneously frozen fraction, Fhet, as the ratio of the
heterogeneous freezing signal to the total freezing signal of the thermogram
in the time domain (see Kumar et al., 2018a for details). The evaluation of
Fhet does not include the spikes that occur before the appearance
of the heterogeneous freezing signal. These spikes originate from single
droplets in the tail of the droplet size distribution (mostly between
100–300 µm with some up to 500 µm in diameter) being
orders of magnitude larger in volume than the average droplets and not
representative for the sample. Freezing experiments were performed on
emulsions prepared from at least two separate quartz suspensions for each
solute concentration and means are reported. Average precisions in
Thet are ±0.2 K with maximum deviations not exceeding
0.9 K (i.e., uncertainties slightly higher than in Kumar et al., 2018a).
Thom and Tmelt are precise within ±0.1 K.
Absolute uncertainties in Fhet are on average ±0.02 and do
not exceed ±0.12. However, Fhet carries larger uncertainties
(up to ±0.19) when heterogeneous freezing signals are weak or overlap
(forming a shoulder) with the homogeneous freezing signal (as was the case
for amorphous silica in various solutions and quartz aged in NaOH
solution).
Aging and reversibility of quartz suspended in water or
solutions
SA quartz (5 wt %) suspended in pure water, NH3 solution
(0.005 molal; pH 7.9; aw≈0.999),
(NH4)2SO4 solution (10 wt %; pH 5.5; aw≈0.963), NH4HSO4 solution (2 wt %; pH 1.1;
aw≈0.988), Na2SO4 solution (5 wt %;
pH 6.8; aw≈0.986) or NaOH solution (5×10-3 molal; pH 9.5; aw≈1 and 5×10-6 molal; pH 7.1; aw≈1) were aged in borosilicate
glass vials and tested over a period of 5 days. Immersion freezing
experiments were carried out with the DSC setup with emulsions prepared from
at least two separate aging experiments for each solute concentration.
Measurements were done on the day of preparation (fresh) and on the
subsequent 5 days in order to assess the evolution and long-term effect of
these solutes on the IN efficiency of quartz. Aging experiments conducted in
pure water, NH3 solution (0.005 molal) and NaOH solution (5×10-3 molal) were repeated in polypropylene Falcon tubes to assess the
influence of leached contaminants from the borosilicate glass vials on the IN
activity of quartz.
After aging for 5 days the suspensions were centrifuged for 2 min at
600 rpm, the supernatant solution was removed and the settled particles were
washed with pure water. This process was repeated five times and the washed
particles were resuspended in pure water and the IN efficiency of emulsions
prepared from these suspensions were tested with the DSC setup.
In order to assess the leaching of contaminants from quartz and the vial
walls, suspensions of quartz were prepared in pure water in both borosilicate
glass vials and polypropylene Falcon tubes and aged for 72 h. The freshly
prepared and aged suspensions were centrifuged to remove the particles. The
supernatant liquid in each case was collected and tested for the
concentration of leached elements via inductively coupled plasma mass
spectroscopy (ICP-MS). The results of ICP-MS measurements are summarized in
the Supplement (Sect. S10).
ResultsIce nucleation activity of quartz and amorphous silica in pure
water
Figure 1 shows the DSC thermograms of suspensions of quartz and amorphous
silica particles in pure water prepared as emulsion droplets. Fhet and Thet are listed in Table 1. As can be easily seen, there are
large differences in IN activities between the samples, from barely to highly
IN active.
Comparison of IN efficiency (in pure water) of various quartz and
amorphous silica particles based on emulsion freezing experiments.
a Mean Thom of pure water emulsions is
reported here since the onset of the homogeneous freezing signal cannot be
separated from the presumed heterogeneous freezing signal.
b Onsets of heterogeneous and homogeneous freezing signals are
nearly indistinguishable. The observed onset lies within the precision range
of Thom (237.0 K, taken as the point dividing heterogeneous and
homogeneous freezing signal to evaluate Fhet).
c Onsets of the two shoulders exhibited by these samples (see
Fig. 1). Fhet is based on the whole heterogeneous freezing
signal. d Milled quartz as obtained from
Sigma-Aldrich.e Additionally milled Sigma-Aldrich quartz.
Quartz
In the emulsion freezing experiments, all investigated quartz samples clearly
show IN activity. TU Vienna quartz is slightly more IN active (with respect
to both Thet and Fhet) than SA and Kaufmann quartz
at similar suspension concentrations. Thet of TU Vienna quartz
(250.9 K) is slightly lower than the freezing onset reported by Zolles et
al. (2015) (∼252 K). We ascribe this difference to the higher quartz
surface area present in the Zolles et al. (2015) droplet freezing setup
compared to our emulsion freezing experiments. In accordance with Zolles et
al. (2015), we observe an enhancement in IN efficiency of quartz due to
milling (Table 1).
The quartz sample from Kaufmann et al. (2016) is clearly IN active,
exhibiting a distinct heterogeneous freezing signal with Thet≈242 K and a shoulder extending to higher temperature with
Thet≈250 K. Kaufmann et al. (2016) reported a
heterogeneous freezing onset temperature of ∼247 K for the same
sample, yet with a very weak heterogeneous freezing signal corresponding to
an IN active particle fraction of only 0.01. We explain in Appendix A that
this was due to an underestimation of the coarse particle fraction because of
the presence of very large particles (> 20 µm), which
were not accounted for in the particle size distribution determined by
SMPS/APS leading to a bias resulting in a too low estimate of the IN active
fraction of quartz particles.
Amorphous silica particles
DSC thermograms of both amorphous silica samples (Fig. 1) show only a single
freezing peak with onsets of 237.2 and 237.1 K for Stöber and Alfa Aesar
silica, respectively, listed under Thet in Table 1. This freezing
temperature is slightly higher but still within the uncertainty range of
Thom of pure water emulsions (i.e., 237.0 K). Note that due to
the volume dependence of homogeneous IN rates, Thom of the quartz
samples is slightly lower than Thom of the pure water emulsions.
Only the smallest droplets of the emulsified quartz suspensions are empty and
therefore freeze homogeneously at a lower temperature than the larger
droplets that give rise to the onset of the homogeneous freezing peak in
pure water emulsions. The calculation of the heterogeneously frozen fraction
for both amorphous silica samples assumes that the heat signal at
T > 237.0 K is due to heterogeneous freezing, leading to
Fhet below the uncertainty limits (see Table 1). Since both
samples consist of submicron particles that should be well distributed
between emulsion droplets (droplet size needed to incorporate on average one
silica particle is ∼1.1µm in diameter), a prevalence of
empty droplets cannot be the reason of the absence of detectable IN activity.
Therefore, based on our emulsion freezing experiments we consider the
amorphous silica particles as inactive or barely IN inactive in water.
When the Alfa Aesar sample is milled, a clearly visible heterogeneous
freezing signal develops consisting of two shoulders on the warmer end of the
homogeneous freezing peak with onsets of 246.8 and 239.7 K, again
demonstrating the fundamental importance of the milling process.
Dependence of the heterogeneous freezing temperatures on the presence of
solutes
For freezing experiments in the presence of solutes we concentrate on the SA
quartz. The mean heterogeneous freezing onset (Thet), homogeneous
freezing onset (Thom) and ice melting temperatures
(Tmelt) for 5 wt % SA quartz suspensions in water and
aqueous solution droplets are shown in Fig. 2a as a function of the solution
water activity (aw). The aw is obtained from the
evaluation of the melting point depression measured during the heating cycle
using the Koop et al. (2000) parameterization. Hence all melting temperatures
lie exactly on the melting curve, except in the case of Na2SO4 where
above the eutectic concentration of 4.6 wt % a hydrate of
Na2SO4 crystallizes together with ice and aw had to
be calculated based on the solute concentration using the AIOMFAC
thermodynamic model at 298 K (Zuend et al., 2008, 2011). The measured
Thom follows a similar aw dependency as
Tmelt and has been parameterized by Koop et al. (2000). We
construct this line by a constant shift of the melting curve by ΔawhomT=0.30 (dotted black line)
derived using the averaged Thom of all experiments of this study
(which is in good agreement with ΔawhomT=0.305 reported by Koop et al., 2000). Similarly, we apply
a constant offset Δawhet=0.221 to shift
the ice melting curve to the heterogeneous freezing temperature of pure
water, yielding the solid black line, which for simplicity will be referred
to as ThetΔawhetaw from here onwards (see Kumar et al., 2018a for more
details).
(a) Onset freezing temperatures of emulsion freezing
experiments performed with 5 wt % SA quartz suspended in aqueous
solutions of inorganic solutes (for symbols and colors see insert).
Heterogeneous freezing onset temperatures, Thet (filled solid
symbols connected by thin lines); homogeneous freezing onset temperatures,
Thom (open symbols at T<237 K), and ice melting
temperatures, Tmelt (open symbols at T>262 K) are
given as functions of the solution's water activity, aw.
Dash-dotted black line: ice melting curve. Dotted black line: homogeneous ice
freezing curve for supercooled aqueous solutions obtained by horizontally
shifting the ice melting curve by a constant offset Δawhom(T)=0.30. Solid black line: horizontally
shifted from the ice melting curve by Δawhet(T)=0.221 derived from the heterogeneous
freezing temperature of the suspension of quartz in pure water (filled black
square at aw=1). Symbols are the mean of at least two emulsion
freezing experiments (using at least two separate suspensions). Two symbols
carry error bars to show representative experimental variations (min to max)
in Thet and aw. (b) Heterogeneously frozen
fraction Fhet as a function of the solution's water activity
(aw). Five symbols carry error bars showing representative
experimental variations (min to max) in Fhet and aw.
Absolute uncertainties in Fhet do not exceed ±0.12.
Analogous to the parameterization for ΔawhomT based on the thermodynamic
homogeneous IN description of Koop et al. (2000), ThetΔawhetaw assumes that the
water activity dependence of Thet is determined by solute-driven
changes in the structure of the water alone, while interactions of the solute
with the INP surface are excluded. From Kumar et al. (2018a, 2019) we know that
the assumption of Δawhet fails when there
are specific interactions between the solute and the mineral surface. As can
be seen from Fig. 2a the measured heterogeneous freezing onset temperatures,
Thet, follow ThetΔawhetaw within measurement
uncertainties for quartz suspensions in (NH4)2SO4,
NH4HSO4 and Na2SO4, but fall below this line in the
presence of the bases NH3 and NaOH. In these alkaline solutions,
Thet for quartz emulsions strongly falls below
ThetΔawhetaw at dilute solute concentrations (aw≥0.99) and stay
almost parallel to ThetΔawhetaw to higher solute
concentrations. This decrease in Thet is less pronounced in the
presence of NH3 than for suspensions in NaOH.
DSC thermograms of 5 wt % SA quartz particles suspended in
ammonium sulfate (AS) solution droplets of increasing concentrations
(0 wt %–20 wt % AS). All curves are normalized such that the areas
under the heterogeneous and homogeneous freezing curves sum up to the same
value. The dotted brown line connects the heterogeneous freezing onset
temperatures (Thet) of the emulsions. With increasing AS
concentration Thet decreases monotonically while the intensity of
the heterogeneous freezing signal increases initially in dilute AS solution
and remains high up to high solute concentrations.
Dependence of the heterogeneously frozen fraction on the presence of
solutes
While the addition of neutral or acidic solutes does not influence the
freezing temperature beyond the expected freezing point depression described
by ThetΔawhetaw, it does affect the heterogeneously frozen fraction. Figure 3 shows
the DSC thermograms for emulsion freezing of 5 wt % SA quartz suspended
in increasingly concentrated (NH4)2SO4 solutions. The dotted
brown line connecting the onsets of the heterogeneous freezing signals
depicts the continuous decrease in Thet as the
(NH4)2SO4 concentration increases. An increase in
heterogeneous-to-homogeneous freezing ratio with increasing solute
concentration is apparent. Fhet increases up to aw=0.998 (0.5 wt %) and stays around this increased value when the solute
concentration is further increased. Figure 2b shows the evaluation of the
freezing signals in terms of the heterogeneously frozen fraction,
Fhet, as a function of aw for all investigated
solutes. For solutions containing the salts (NH4)2SO4,
NH4HSO4 and Na2SO4, Fhet shows a
constant increase compared with the pure water case, albeit hardly exceeding
the maximum uncertainty limit. Despite the decrease in Thet,
there seems to be a slight increase in Fhet for quartz suspended
in aqueous solutions with higher NH3 (aw≥0.98)
concentrations. On the other hand, even very low concentrations of NaOH
(aw≤0.99) strongly decrease Fhet. The
heterogeneously frozen fraction slightly recovers at higher concentrations of
NaOH (aw=0.99–0.96) yet remains significantly below the pure
water case.
DSC thermograms of 10 wt % Alfa Aesar (a) and
10 wt % Stöber (b) silica particles suspended in pure
water, ammonia (NH3; 0.05 and 0.5 molal) and ammonium sulfate (AS;
0.05 wt % and 1 wt %) solution droplets of varying concentrations
(corresponding to aw range of 1–0.987). All curves are
normalized such that the areas under the heterogeneous and homogeneous
freezing curves sum up to the same value. DSC thermograms of both amorphous
silica samples show only one clear freezing signal that is indistinguishable
from the homogeneous freezing signal. Thom of the corresponding
emulsion freezing experiments with the pure solutions (in the absence of the
silica particles) has been taken as the dividing temperature of heterogeneous
and homogeneous freezing to evaluate Fhet (Table 2; see
Sect. 3.3).
Furthermore, in Part 3 (Kumar et al., 2019) we show that micas (muscovite
and biotite) and gibbsite, which reveal no IN activity in pure water emulsion
freezing experiments, develop a heterogeneous freezing signal in the presence
of NH3 and (NH4)2SO4. We therefore tested the
amorphous silica samples for a similar effect. DSC thermograms of both
amorphous silica samples suspended in NH3 (0.05 and 0.5 molal) and
(NH4)2SO4 (0.05 wt % and 1 wt %) solutions,
corresponding to an aw range of 1–0.987 (Fig. 4), show only one
clear freezing signal. We report the onset of this freezing peak as
Thet in Table 2, while under Thom we list the onset
freezing temperature of the reference solutions (prepared with the solute
only). Neither the Alfa Aesar nor the Stöber silica particles
(10 wt % suspensions) show a significant increase in the freezing onset
compared with the reference measurements. We evaluate the heterogeneously
frozen fraction by attributing the freezing signal at temperatures above the
reported Thom of the reference measurement to heterogeneous
freezing, yielding Fhet within the uncertainty range (see
Table 2). We therefore conclude that NH3 and
(NH4)2SO4 do not lead to a discernable enhancement of the IN
activity of amorphous silica.
Summary of the freezing experiments with emulsified aqueous solution
droplets containing Alfa Aesar and Stöber amorphous silica particles
(10 wt %). Note that the absolute uncertainty in Fhet may be
up to ±0.12.
am= molality; b mean
Thom of pure water/solution emulsions (taken as the point
dividing heterogeneous and homogeneous freezing signal to evaluate
Fhet) is reported here since the onset of the homogeneous
freezing signal cannot be separated from the presumed heterogeneous freezing
signal.
Aging and recovery experiments of quartz in water and aqueous
solutions
In order to assess the long-term effect of solutes on the IN efficiency of
quartz, aging experiments were performed over a period of 5 days with
5 wt % SA quartz suspensions in pure water (prepared in borosilicate
glass vials and polypropylene Falcon tubes) and various inorganic solutes.
Every day, aliquots were taken from the suspension and tested in emulsion
freezing experiments. For these experiments, Thet and
Fhet are given in the upper and lower panels of Figs. 5 and 6,
respectively. Figure 5 shows the results of experiments performed in glass
vials while Fig. 6 compares Thet and Fhet of
experiments performed in glass vials and polyprolylene tubes. After aging,
the quartz suspensions were decanted, washed and resuspended in pure water in
order to assess the reversibility of any surface modification occurring
during the aging period. Figures 5 and 6 also show the change in
Thet and Fhet when aged quartz is resuspended in pure
water. (Note that Thet and Fhet in the experiments
with fresh dusts can be lower than in the reversibility tests, because the
former were performed in solutions and the latter in pure water.)
Development of Thet(a) and Fhet(b) for 5 wt % SA quartz suspended in water,
(NH4)2SO4 solution (10 wt %), NH4HSO4
solution (2 wt %), Na2SO4 solution (5 wt %) and NaOH
solution (5×10-6 molal) over a period of 5 days. All
suspensions were prepared and aged in borosilicate glass vials. After 5 days of aging the reversibility was tested: the suspensions were centrifuged,
the supernatant decanted, the aged particles washed several times with pure
water, resuspended in pure water, and subjected to an emulsion freezing
experiment. All data points are means of at least two separately aged
suspensions. The error bars show representative experimental variations
(min to max).
Same as Fig. 5, except that it shows a comparison of IN efficiency
of SA quartz (5 wt %) suspended in water/aqueous solutions
(NH3 0.005 molal with pH 7.9 and NaOH 5×10-3 molal
with pH 9.5) prepared in borosilicate glass vials (open symbols) and
polypropylene Falcon tubes (solid symbols). After 5 days aging the
reversibility was tested as explained in Fig. 5.
Aging of quartz suspensions in pure water in glass vials decreases the IN
activity in terms of both Thet and Fhet. Strong
decreases in IN efficiency occur during aging in the presence of
Na2SO4, NaOH and NH3. On the other hand,
Thet and Fhet remain constant when quartz is
suspended in 2 wt % NH4HSO4 solution. Repetition of the
aging experiments reveals considerable variability in the decrease in IN
efficiency over time, as indicated by the large min-to-max bars after
2–5 days. The reversibility tests show complete or almost complete recovery of
IN efficiency after washing in the case of aging in pure water and in
solutions containing Na2SO4, NaOH (except at high pH 9.5) and
(NH4)2SO4. Interestingly, instead of recovering, the IN
efficiency after aging in a dilute NH3 solution decreases even
further when the particles were washed and resuspended in pure water.
To elucidate whether leached material from the surface of the glass vial had
any impact on the decrease in IN activity during aging of quartz in glass
vials, experiments exhibiting a
strong decrease in IN efficiency were repeated in polypropylene Falcon tubes.
Indeed, in contrast to aging in glass vials, quartz aged in pure water in
polypropylene tubes exhibited a stable Thet and a slight
enhancement in Fhet (yet within the uncertainty range) (Fig. 6).
When quartz is suspended in a dilute NH3 (pH 7.9) solution in
polypropylene tubes, similar trends were observed as in glass vials, with
constant Thet after an initial decrease, followed by a further
decrease after washing with pure water. Interestingly, quartz freshly
suspended in NaOH (pH 9.5) in polypropylene tubes shows a stronger initial
decrease in IN efficiency (Thet and Fhet) but stays
higher from the second day onwards than in the corresponding experiments
performed in a glass vial until the end of the experiment.
Another set of SA quartz suspensions (5 wt %) was prepared in pure water
in glass vials and aged for 7 months to investigate the aging effect over
very long timescales. Figure 7 shows that the aged particles almost
completely lost their IN efficiency and barely any of it was recovered after
the aged particles were washed and resuspended in pure water. In Sect. 4.4,
we relate the results of the aging and reversibility experiments to surface
processes occurring in the different solutions.
DiscussionIN efficiency of quartz and amorphous silica particles in pure
water
As shown in Fig. 1 and Table 1, the IN activity of quartz is superior to the
one of amorphous silica particles. The freezing onset temperatures in pure
water for the investigated quartz samples range from 247 to 251 K (see
Table 1), covering a similar temperature range as the quartz samples
investigated by Atkinson et al. (2013) and Zolles et al. (2015). On the other
hand, the IN activity of the silica particles (Stöber and Alfa Aesar) is
negligible. Hardly any IN activity of amorphous silica particles is in
agreement with Zobrist et al. (2008) who observed freezing at around 255 K
in bulk freezing experiments with 3 µL droplets containing
109×1010 particles (total mean
particle surface area 0.25–2.5 cm2). This freezing temperature is
close to the one of pure water droplets of this size (252–253 K). Whale et
al. (2018) found IN active site densities of silica particles from
Sigma-Aldrich (silica gel) of ns≈10 cm-2 at 251 K
and ns≈0.1 cm-2 at 261 K, corresponding to a
slightly higher IN activity of these silica particles compared with those
synthesized by Zobrist et al. (2008).
Zolles et al. (2015) investigated the density of IN active sites
of three quartz samples and found a very high variability from hardly IN
active to an activity similar to that of microcline. Their quartz sample
from Sigma-Aldrich corresponding to the TU Vienna quartz sample in this
study was the most IN active and a natural quartz sample the least IN
active. Moreover, the IN activity of the natural quartz sample increased
considerably upon milling. Our comparison of emulsion freezing experiments
with SA quartz additionally milled and original SA quartz (used as obtained
from the manufacturer) corroborate an increased IN efficiency in terms of
Thet and Fhet for the additionally milled sample as shown in Fig. 1
and Table 1. Moreover, milling of the amorphous silica sample from Alfa
Aesar also had a very positive effect on its IN activity.
Heterogeneous IN of quartz in aqueous solutions
The water-activity-based description predicts heterogeneous IN temperatures
(ThetΔawhetaw) as a function of aw by shifting the ice melting curve
by a constant offset in aw. It is expected to be valid in the
absence of specific interactions between the solute and the ice-nucleating
surface so that the only effect of the solute is a freezing point depression.
Such a description has been suggested by several studies in the recent past
(Zuberi et al., 2002; Archuleta et al., 2005; Cantrell and Robinson, 2006;
Zobrist et al., 2006, 2008; Alpert et al., 2011a, b; Knopf and Forrester,
2011; Rigg et al., 2013).
In Part 1 of this series, we showed that heterogeneous freezing onsets of
microcline exhibit strong deviations from the water-activity-based
description. Higher Thet compared to predicted values were
observed for microcline suspended in very dilute NH3/NH4+-containing solutions, while a substantial decrease in IN efficiency was
observed in more concentrated solutions of inorganic salts including
NH4+-containing salts and NH3 (Kumar et al., 2018a).
In Part 3 (Kumar et al., 2019) we extended this investigation to other
aluminosilicates and found that an increase in Thet in the
presence of NH4+-containing solutes is a
general feature of feldspars, clay minerals and micas, while the decrease in
IN efficiency at higher solute concentration is a more specific
characteristic of K-feldspars and most pronounced for microcline.
Conversely, SA quartz follows quite well the water-activity-based prediction
(black line in Fig. 2a) of Thet in the case of pH neutral
suspensions. Fhet shows an initial increase in very dilute
solutions of (NH4)2SO4, NH4HSO4 and
Na2SO4, which is preserved to higher concentrations (Fig. 2b).
Whale et al. (2018) compared the IN activity of quartz in dilute NaCl and
(NH4)2SO4 with the one in pure water and observed no change
of the freezing onset temperatures in the presence of the solutes, but an
increase in the density of IN active sites towards lower temperatures in a
dilute (NH4)2SO4 solution and a decrease in a dilute NaCl
solution.
Opposite to the effect of NH3 on aluminosilicates, there is a
decrease in Thet for SA quartz in NH3 solutions. In the
past, several infrared spectroscopy studies have investigated the adsorption
of NH3 molecules on various types of mineral oxides (Mapes and
Eischens, 1954; Eischens and Pliskin, 1958; Peri and Hannan, 1960). The
quartz surface provides several centers for interaction with NH3
molecules viz. (i) hydrogen bonding via one of its hydrogen atoms with a
surface oxygen atom of a silanol group; (ii) hydrogen bonding via its nitrogen atom with the hydrogen of a
surface hydroxyl group and (iii) coordination to an electron-deficient Si
(Lewis acid site) (Folman, 1961; Cant and Little, 1965; Blomfield and Little,
1973; Tsyganenko et al., 1975; Morrow and Cody, 1976; Morrow et al., 1976;
Fubini et al., 1992; Li and Nelson, 1996; Wright and Walsh, 2012). In
addition to reversible coordination to silanols via hydrogen bonding,
NH3 may interact irreversibly with strained siloxane bridges by
disrupting them into Si-NH2 and Si-OH groups (Folman, 1961;
Peri, 1966), although, water displays more affinity than NH3 for
this reaction (Blomfield and Little, 1973; Morrow and Cody, 1976; Fubini et
al., 1992). Figure 2 shows that the sum of these interactions seems to affect
the IN activity of the quartz surface by decreasing Thet but
slightly increasing Fhet at higher NH3 concentrations.
Using sum frequency generation spectroscopy, Wei et al. (2002) have found an
enhancement of the hydrogen bonded OH peak in the presence of ammonia. They
proposed that NH3 molecules may form strong hydrogen bonds with the
silanol groups on the silica surface with the nitrogen atoms facing silica,
resulting in an excessive number of protons (NH bonds) pointing into ice.
However, these interactions of NH3 with the quartz surface do not
seem to lead to an enhanced IN activity compared with the pure water case.
Wright and Walsh (2012) found no strong and stable hydrogen bonding of
NH4+ to hydroxylated quartz in their
first principles molecular dynamics simulations. The slight decrease in
Thet in the presence of (NH4)2SO4 together with
the slight increase in Fhet indicate that the presence of
ammonium in the solution influences the IN activity, although only slightly.
The decrease in IN activity in NaOH-containing suspensions as shown in Fig. 2 can be ascribed to the detrimental effect of alkaline conditions (pH
between 7.1 and 13.6) on the stability of the quartz surface (House and
Orr, 1992; Crundwell, 2017) and is discussed in the next section.
Aging effect and reversibility of surface modifications
The aging experiments performed with quartz suspended in different solutions
point to surface processes that influence the IN activity of quartz over
time. The IN activity was maintained over the whole aging period (5 days) in
the case of NH4HSO4 (2 wt %, pH 1.1). In contrast, in pure
water, (NH4)2SO4 (10 wt %, pH 5.5), Na2SO4
(5 wt %, pH 6.8) and very dilute NaOH (5×10-6 molal;
pH 7.1), the IN activity decreased over time but was completely or almost
completely restored after washing in water (Fig. 5). In more alkaline
solutions, namely NH3 (0.005 molal, pH 7.9) and more concentrated
NaOH (5×10-3 molal, pH 9.5), IN activity was permanently lost
(Fig. 6). Also, when quartz was aged for 7 months in pure water in glass
vials, the IN activity was almost completely destroyed (Fig. 7). In contrast,
the IN efficiency in pure water was barely affected during aging in
polypropylene tubes over 5 days. Even after aging for 7 months the quartz
sample proved to remain IN active. Yet, it showed a permanent decrease in IN
efficiency (Thet=243.4±1.6 K, Fhet=0.59±0.1) that did not recover after washing in water, rather there was a slight
further decrease to Thet=242.4±2.5 K and Fhet=0.40±0.1.
DSC thermograms of SA quartz (5 wt %) suspended in pure water
in borosilicate glass vials and measured right after preparation (marked as
“Fresh”). The same suspension was aged for 7 months and remeasured (marked
as “Aged”). The aged suspension was centrifuged, the supernatant decanted,
the aged particles washed several times with pure water, resuspended in pure
water and measured again to examine the recovery of IN efficiency after aging
(marked as “Recovery”). This procedure was done with three different
suspensions and their mean Thet and Fhet are reported
next to each curve. All curves are normalized such that the areas under the
heterogeneous and homogeneous freezing curves sum up to the same value.
A decrease in IN activity due to aging for 72 h in glass vials in pure water
was also observed for Kaufmann quartz (Fig. A2) and TU Vienna quartz
(Fig. A3). In the following, we relate the results of the aging and
reversibility experiments to surface processes occurring in the different
solutions.
Dissolution and growth of quartz in pure water
Several studies have discussed the effect of solution pH on dissolution rates
of quartz (Henderson et al., 1970; Kline and Fogler, 1981; Schwartzentruber
et al., 1987; Bennett et al., 1988; Knauss and Wolery, 1988). The dissolution
rate of quartz at 25 ∘C is 10-13 to 10-12 moles Si
m-2 s-1 at low to neutral pH (0.5–7) and increases roughly
linearly with increasing pH, reaching a value of 10-10 moles Si
m-2 s-1 at pH 12 (House and Orr, 1992; Crundwell, 2017). The
dissolution of quartz is considered to occur on deprotonated silanols, i.e.,
Si-O- (Brady and Walther, 1989, 1990). Deprotonation of the silanol
weakens the remaining siloxane bridges, facilitating the attack by water
molecules and ultimately releasing the Si in the form of silicic acid
(H4SiO4). Moreover, dissolution of quartz increases with
increasing ionic strength of the solution, i.e., increasing salt
concentration (Brady and Walther, 1990). At low pH, different dissolution
mechanisms may be involved, such as H2O hydrolysis of Si centers or
adsorption of H+ onto siloxane bridging oxygen (Xiao and Lasaga, 1994;
Criscenti et al., 2006; Bickmore et al., 2008), but with a low efficiency.
The SA quartz as provided by the manufacturer contains a minor share of
amorphous material (4.5±0.5 %), produced most likely by grinding
(Fubini et al., 1989). Because of the higher dissolution rate of amorphous
silica compared to quartz, silicic acid should be released at a higher rate
from the amorphous material (Crundwell, 2017). Since also the solubility of
amorphous silica is higher (∼50 ppm Si at 25 ∘C) than the one
of quartz (1–3 ppm Si at 25 ∘C) (Walther and Helgeson, 1977),
quartz should grow at the expense of amorphous silica over time when kept
together in a closed vessel. After aging for 72 h in water (in a glass
vessel), the amorphous fraction of the SA quartz sample indeed showed a
slight decrease, but still within experimental error (4.0±0.5 %
for the aged sample compared with 4.2±0.5 % for the sample exposed
to water for ∼15 min). This is in agreement with the fact that the
conversion of amorphous silica to quartz is a slow process.
ICP-MS measurements (see Tables S1 and S2 in the Supplement) performed with
the supernatant of a 0.5 wt % SA quartz suspension that was aged in pure
water for 72 h in a borosilicate glass vial shows a concentration of
11.9 ppm Si, which is well above the solubility of quartz in pure water and
well below that of amorphous silica. The Si concentration after aging a
0.7 wt % SA quartz suspension for 72 h in polypropylene Falcon tubes
reaches only 4.9 ppm, indicating that a considerable fraction of the
dissolved Si stems from the glass vial in the aged sample. Indeed, the glass
vial continuously leaches Si to the water (Bunker, 1994). The ICP-MS
measurement of pure water, which was in contact with the glass vial for less
than an hour, shows a concentration of only 0.2 ppm Si but reached 4.5 ppm
Si after 72 h. This slower increase in Si concentration compared to the
immediate increase in the case of the quartz samples (1.3–6 ppm after less
than an hour in water) is simply due to the smaller surface area of the glass
vial (∼2 orders of magnitude) exposed to water compared with the one of
the SA quartz sample. In addition, also the release of Si from the quartz
sample is expected to stem mostly from its amorphous
share. Subsequently, silicic acid
in water may form dimers, trimers and cyclic species due to
autopolycondensation that sets in when the silicic acid concentration
approaches the solubility limit (Perry, 1989, 2003; Belton et al., 2012).
These oligomerization reactions are reversible (Tamahrajah and Brehm, 2016).
We assume that at high Si concentration, silicic acid and its oligomers
adsorb on the quartz surface, covering large parts of the crystalline surface
or at least a relevant fraction of the IN active sites, thus hampering the IN
activity of the quartz samples aged in glass vials. Indeed, monolayer
adsorption of silicic acid on quartz has been observed under conditions
supersaturated with respect to crystalline quartz (Berger et al., 1994).
Since leaching of Si from the glass vials is slow owing to the comparatively
small exposed surface area, the freshly prepared suspensions of SA quartz in
glass vials are not affected by an adsorbed layer, which is in accordance
with our experiments (see Figs. 5 and 6). The Si concentration in the
polypropylene tubes remains too low to give rise to an adsorbed layer on the
quartz surface even during aging and the IN activity is not hampered (see
Fig. 6).
With time, the Si supersaturation with respect to quartz should lead to
crystalline quartz growth. We assume that covalent bonds form between the
adsorbed siliceous layer and the quartz surface leading to a grown, intact
quartz surface with few defects. Since the emulsion freezing experiments of
the SA quartz sample aged for 7 months in a glass vial show hardly any IN
activity even after washing with pure water (see Fig. 7), we conclude that
slowly grown quartz surfaces are indeed not amorphous, but have a regular
crystalline structure. These are barely IN active and only milling provides
the quartz surface with IN active sites.
Dissolution and growth of quartz in solutions
Under alkaline conditions both quartz dissolution and growth rates are
increased. Indeed, centimeter-sized synthetic quartz crystals are grown from
amorphous silica on seed crystals at high temperatures and pressures in
highly alkaline conditions (Baughman, 1991). Such rigorous conditions are
applied to accelerate the crystal growth. In the 5×10-3 molal
NaOH solution (pH 9.5), the solubility of amorphous silica and quartz are
higher than at near-neutral conditions and dissolution and growth of quartz
are enhanced. We therefore ascribe the strong and immediate loss of IN
activity of SA quartz suspended in 5×10-3 molal NaOH solution in
polypropylene tubes to the dissolution of quartz, which levels off when the
equilibrium condition is approached. Therefore, during the following days of
aging, there is no further decrease in IN activity, rather Thet
and Fhet show a slight increase (but within the uncertainty
limits). After washing with water, Fhet recovers to the initial
value in 5×10-3 molal NaOH but remains clearly below the value
measured in pure water. When the experiment is carried out in glass vials (in
5×10-3 molal NaOH), the initial loss of IN activity is less
pronounced, probably because the additional leaching of Si from the glass
vial leads to a quick increase in the Si concentration above the saturation
level with respect to quartz. As a consequence, the quartz sample is in
growth conditions for most of the aging time and we ascribe the irreversible
loss of IN activity to growth of intact quartz layers, which is faster than at
neutral conditions because of the higher Si solubility under alkaline
conditions.
Interestingly, during aging in NH3, the main decrease in Thet is
observed after one day while Fhet is preserved during the 5 days of
aging but is reduced when NH3 is removed from the suspension. This
implies that the presence of NH3 can temporarily stabilize the surface
followed by a strong decrease when it is removed.
Which factors determine the IN activity of quartz?Crystallinity and substrate–ice lattice match
Crystallinity and lattice match between substrate and ice are often
considered to provide IN activity to the substrate. While ice and silica
exhibit structural similarities in the form of tetrahedral building units, the
most common hexagonal (Ih) and cubic (Ic) ice phases show structural
analogies to the crystalline silica tridymite and cristobalite,
respectively, but not to quartz (Tribello et al., 2010).
Indeed, when the quartz surface is not produced by milling but by crystal
growth, quartz is similarly inactive as amorphous silica. Conversely, when
hardly active amorphous silica particles are milled, they become IN active.
This suggests that the regular quartz surface is not able to template ice
growth and that the crystallinity of quartz is not a prerequisite for its IN
activity.
Milling and radical site formation
Micrometer- and nanometer-sized quartz particles are usually obtained by
milling. Due to the covalent nature of the quartz crystal lattice,
considerable force needs to be exerted to obtain small particles by milling.
The shear and compression applied to quartz leads to a disturbed amorphous
zone of 10–30 nm thickness with dangling Si-O⚫ and
Si⚫ radical sites that can be detected by electron
paramagnetic resonance (EPR) (Fubini et al., 1987, 1989; Makoto and Motoji,
1996). When quartz is ground in the normal atmosphere, the resulting
Si-O⚫ and Si⚫ radical sites react with
atmospheric species (Fubini et al., 1987). Reactions with O2 lead
to the formation of Si-O-O⚫, Si-O-O-Si or
Si-O-O-O-Si, which react in the presence of water (vapor) to
Si-OH and Si-O-OH. In addition, hydroxylation of strained
Si-O-Si groups (Fubini et al., 1987) results in silanol groups
(≡Si-OH) rendering the surface highly hydroxylated. Depending on
their arrangement on the silica surface, silanols are isolated
((Si-O)3Si-OH), germinal (=Si(OH)2) or vicinal
(=Si(OH)-O-Si(OH)=). The relatively non-polar siloxane bridges
(≡Si-O-Si≡) may be strained and easily hydrolyzed to
silanols or regular and hardly reactive (Morrow and Cody, 1976; Morrow et
al., 1976; Brinker et al., 1986, 1988; Zhdanov et al., 1987; Bolis et al.,
1991; Fubini et al., 1992).
The IN activity observed for milled Alfa Aesar silica suggests that shear and
compression applied to amorphous silica leads to similar radical sites as in
the case of quartz, providing amorphous silica with IN activity. XRD analysis
of Kaufmann quartz and SA quartz support the presence of amorphous shares of
about 6.4 wt % and 4.5 wt % in the samples, respectively. A part of
this amorphous material may agglomerate as separate particles or in specific
regions of the quartz surface. In the Supplement, we show a collection of
representative SEM images of SA and Kaufmann quartz that show agglomerates on
top of the quartz surface, which might be amorphous. Nevertheless, an
amorphous surface layer on top of the quartz particles cannot be excluded.
Because of the strong correlation of milling and IN activity of silica
surfaces, we propose that the surface functionalization of silica particles
arising from breaking covalent Si-O bonds during milling gives rise to the
IN activity of these materials.
Interestingly, the conjecture that surface functionalization resulting from
the cleavage of covalent Si-O bonds rather than the ordered crystalline
structure determines the surface properties of silica is confirmed by
findings from a completely different research field. Ground quartz particles
may induce silicosis, lung cancer and autoimmune diseases (Donaldson and
Borm, 1998; Fubini, 1998). This pathogenicity is totally absent in chemically
prepared amorphous silica or synthesized (grown) quartz particles. Since
exposure to ground amorphous (vitreous) silica also leads to adverse health
effects (Turci et al., 2016), the pathogenicity is considered to arise rather
from the mechanical cleavage of the covalent Si-O bonds than from the
crystallinity (Fubini et al., 1987, 1989; Turci et al., 2016).
Surface OH groups
While siloxanes and silanols dominate the surfaces of amorphous silica and
quartz, the relative surface densities of these groups show large variation.
Fully hydroxylated quartz surfaces may carry up to 9.5 OH nm-2 on the
(001) surface and still 5.8 OH nm-2 on the (011) and (101) surfaces
(Musso et al., 2009). On the other hand, fully hydroxylated amorphous silica
surfaces carry typically only 4.6–4.9 OH nm-2. Highly hydroxylated
surfaces are dominated by vicinal and geminal silanols, with few isolated
silanol and siloxane groups (Zhuravlev, 2000; Muster et al., 2001). Due to
their more ordered structure, silanols on quartz surfaces tend to form
networks of chains of hydrogen bonds, whereas amorphous silica surfaces
rather exhibit patches of hydrogen bonded silanols, even if their average
silanol densities are the same (Musso et al., 2011).
When quartz and silica samples are heated, surface hydroxylation decreases
due to the replacement of silanols by siloxanes. The more severe the heating
conditions (temperature, vacuum, duration), the more dehydroxylized the
surface becomes. Heating (calcination) to 670 K in vacuum removes all
vicinal silanols of amorphous silica while isolated silanols are still
present but become continuously scarcer by further heating (Zhuravlev, 2000).
Quartz, on the other hand, is less easily dehydroxylated (Bolis et al.,
1985). Dehydroxylated surfaces slowly rehydroxylate when they are exposed to
humidity or in contact with liquid water. The Stöber particles shown in
Fig. 1 have been heated to 823 K, which strongly decreased the number of
vicinal silanols and subsequently hydrolyzed so that vicinal silanols should
be restored to a full hydroxylation level of amorphous silica (i.e.,
4.6–4.9 OH nm-2).
Musso et al. (2012) showed in an ab initio molecular dynamics study that the
silanol surface of quartz (100) induces an ice-like structure of water in the
proximity of the surface, which is more pronounced when the silanol density
at the surface is higher. While water molecules spontaneously form hydrogen
bonds to isolated silanols, hydrogen bonding to vicinal silanols involves
breaking the surface hydrogen bond network between them. This is an activated
process with an activation energy that increases with increasing length of
the interconnected silanol chains (Musso et al., 2011). Water was only able
to disrupt the weak internal hydrogen bonds between surface silanols with
H ⋯ O > 2 Å but not the stronger ones with
H ⋯ O < 2 Å (Musso et al., 2012). Milling decreases
the long-range order of silanols and generates hydrophilic and hydrophobic
patches (Turci et al., 2016). The generated defects may disrupt the chain of
interconnected silanols and free them to participate in hydrogen bonding with
water molecules. This could explain the enhanced IN activity of freshly
milled quartz as well as freshly milled amorphous silica.
Recently, it had been suggested that the OH density and the substrate–water
interaction strength are useful descriptors of a material's IN ability
(Pedevilla et al., 2017). The example of quartz shows that OH density alone
is indeed insufficient as a predictor for IN ability when strong hydrogen
bonding amongst surface OH groups prevail over substrate–water interactions.
Protonation or deprotonation of surface OH groups and surface
charge
Depending on the solution pH, the silanol groups protonate or deprotonate,
thus changing the surface charge. The quartz surface is at the point of zero
charge (PZC) around pH 2 and becomes more negative with increasing pH
(Vidyadhar and Hanumantha Rao, 2007; Turci et al., 2016). At PZC, Si-OH
groups prevail, and the number of Si-OH2+ equals the number of
Si-O- groups. The ordering of water at the quartz surface was
shown to be pH dependent. At pH 1.5 when the surface is slightly positively
charged and at pH 12.3 when the surface is negatively charged, water
molecules are ordered but the orientation is reversed from low to high pH. At
intermediate pH, there is more disorder (Du et al., 1994; Richmond, 2002).
Zeta potential measurements show that the milled quartz surface is slightly
less negatively charged than grown quartz surfaces at neutral conditions.
However, milling increases the heterogeneity of the silanols and creates more
acidic sites as indicated by a shallower increase in surface charge with
decreasing pH (Turci et al., 2016).
Besides surface functionalization, surface charge is a factor that has been
shown to influence IN activity (Marcolli et al., 2016; Abdelmonem et al.,
2017; Kumar et al., 2019). Abdelmonem et al. (2017) found that the freezing
temperature of water at the α-Al2O3
(0001) surface is highest when the
surface is close to the PZC and the water molecules at the surface are least
ordered. In contrast, Kumar et al. (2019) shows that hydroxylated surfaces
that had PZCs at low pH such as feldspars showed higher IN activity in pure
water than surfaces with PZC shifted to neutral or alkaline conditions.
Impurities on mineral surfaces
Often IN activity of mineral surfaces has been related to the presence of
impurities introducing special sites on the mineral surface (O'Sullivan
et al., 2014; Zolles et al., 2015). Milling can accumulate impurities from
within the crystal lattice (DeMott et al., 2003b; Boose et al., 2016a) on
the surface because regions rich in impurities provide preferred cleavage
planes (Whale et al., 2017). If such impurities are
surface active and remain adsorbed on quartz when the particles are immersed
in water, they can either block active sites and decrease the IN activity or
generate new active sites. Milling of quartz leads to high energy surface
sites that may attract semivolatile impurities to reduce the surface energy.
Indeed, organic semivolatile material has been considered to provide IN
activity to mineral surfaces (O'Sullivan et al., 2014; Tobo et al.,
2014).
We therefore determined the presence of semivolatile material on the Kaufmann
and SA quartz samples by performing TGA. Since only a mere 0.17 % and
0.30 % loss in weight was observed in TGA up to 350 ∘C
(the Supplement) in Kaufmann and SA quartz samples, respectively, the influence
of semivolatile material on the IN efficiency is unlikely. To investigate the
possibility that nonvolatile but water-soluble impurities provide IN activity
to the quartz surface, we performed ICP-MS tests on the supernatant liquid of
quartz suspensions (0.7 wt %) freshly prepared in water in polypropylene
Falcon tubes. Si had a contribution of more than 80 % of the leached
elements in both quartz cases (see the Supplement; Table S2 for details). The
most abundant impurities were Na, Al, K, Ca, Ba, Fe and Co with a combined
contribution of only 12 % and 18 % to the total leached elements for
SA and Kaufmann quartz, respectively. Depletion of these elements from the
quartz surface cannot explain the loss of IN activity during aging because
the IN activity can be restored by washing the quartz particles with pure
water. We therefore regard the abundance of Si-OH groups and their
arrangement on the quartz surface relevant for IN activity in emulsion
freezing experiments rather than the presence of foreign components. However,
we do not exclude that IN activity observed at higher temperatures in bulk
freezing experiments might be due to impurities (O'Sullivan et al., 2014).
Indeed, the notion that different types of sites are relevant at higher than
at lower freezing temperatures is corroborated by Kaufmann et al. (2016) who
showed in their Fig. 6 that only a weak correlation exists between freezing
temperatures of bulk and emulsion freezing experiments of different mineral
dusts.
From the above discussion, we conclude that milling is a requirement for
quartz to be IN active and that IN occurs on specific active sites
introduced by milling, which are more reactive than the grown quartz surface.
Despite the high degree of hydroxylation, the regular quartz surface does
not give rise to IN activity, likely because most silanols are tightly
interconnected by hydrogen bonds in a network that is too strong to be
disrupted by water molecules.
Atmospheric implications
Arid and semiarid regions are the main sources of mineral dust (e.g., Saharan
and Gobi deserts) (Laurent et al., 2006, 2008), which can have atmospheric
lifetimes of several days (Huneeus et al., 2011). The dust, while being
transported, can interact with a variety of trace gases (Usher et al., 2003;
Kolb et al., 2010), which can lead to changes in the surface physicochemical
properties. Not only ground-collected and/or near-source but also transported dusts
have been reported to be rich in quartz (Avila et al., 1997; Alastuey et al.,
2005; Field et al., 2006; Boose et al., 2016b; Kaufmann et al., 2016). Quartz
can be found in high proportions in atmospherically transported Saharan dust
samples (Avila et al., 1997; Caquineau et al., 1998; Caquineau et al., 2002;
Alastuey et al., 2005; Kandler et al., 2009). However, barely any information
is available about the alteration of the IN efficiency of quartz due to cloud
processing and atmospheric chemical species.
This work and Zolles et al. (2015) have shown a large variability in the IN
activity of quartz particles. Milled quartz samples showed high IN activity,
while quartz layers grown over 7 months were almost IN inactive. This is in
agreement with the low IN activity of a natural quartz sample investigated by
Zolles et al. (2015) with freezing onset < 238 K. The question
therefore arises whether naturally eroded quartz particles from
atmospherically relevant dust source regions would have significant IN
activity.
Kaufmann et al. (2016) performed emulsion freezing
experiments with natural dust samples ground-collected in different deserts
worldwide and correlated the IN activity with the mineralogical composition.
A sample collected in Oman showed low IN activity despite its considerable
quartz content. Conversely, heterogeneous freezing on quartz surfaces may
indeed account for the freezing signal observed for higher suspension
concentrations of desert dust samples collected in Israel. For the
Antarctica sample with 24 % quartz content,
Kaufmann et al. (2016) performed emulsion freezing
experiments using the sieved fraction and after the sieved fraction was
milled. Indeed, the heterogeneously frozen fraction increased due to
milling.
Boose et al. (2016b) found a positive correlation between the quartz content
and the freezing between 238 and 245 K of dust particles sampled from
deserts worldwide. However, this positive correlation strongly relies on two
milled samples with the highest quartz content (∼64 % and ∼93 % in samples from Australia and Morocco), which exhibit the highest IN
activity. In addition, two more samples were sieved and milled as well. In
the case of the Atacama sample, with quartz content of ∼17 % after
sieving and 10 % after milling, milling increased the IN activity. In
the case of the Israel sample with quartz contents of ∼7 % after
sieving and ∼6 % after milling, the IN activity was decreased after
milling. This shows that the IN activity of collected mineral dusts depends
in a complex way on the preprocessing of the samples. More samples need to
be collected from desert regions and tested for IN activity without prior
milling to assess the correlation between IN activity and quartz content.
Conclusions and outlook
The analysis of emulsion freezing experiments of quartz and amorphous silica
particles allowed us to attribute the IN activity of quartz surfaces to
specific surface properties.
Surface hydroxylation seems to be a necessary but not sufficient condition
for the IN activity of quartz and amorphous silica. Silanols may form
hydrogen bonds with water molecules that are able to direct them into an
ice-like arrangement. However, as it seems, most of the silanols on
synthesized amorphous silica and grown quartz surfaces are engaged in
hydrogen bonds amongst each other with an insufficient number of OH groups
left for hydrogen bonding to water molecules.
Milling leads to the cleavage of Si-O-Si bridges resulting in
Si-O⚫ and Si⚫ radical sites that react in the
presence of water vapor to finally form Si-OH and Si-O-OH on the surface.
Defects seem to disrupt the interconnected chains of silanols on the surfaces
of milled silica particles, thus increasing the number of silanols that are
available for hydrogen bonding to water molecules. These defects indeed seem
to be a prerequisite for the IN activity of amorphous silica and quartz.
Usually, amorphous silica particles are synthesized and quartz particles are
milled, therefore the crystalline surface of quartz could be expected to
match ice. Our emulsion freezing
experiments with milled amorphous silica and grown quartz surfaces show that
crystallinity is not relevant for the IN activity of silica. Rather a highly
defective surface is required.
Both barely IN active grown quartz and highly IN active milled quartz
particles carry a negative surface charge at neutral pH conditions. This
indicates that surface charge alone is unreliable as a predictor for IN
activity.
The onset freezing temperatures of quartz suspensions freshly prepared in
neutral and acidic salt solutions containing (NH4)2SO4,
NH4HSO4 and Na2SO4 follow approximately the prediction of
the water-activity-based description, while the heterogeneously frozen
fraction increases slightly in the presence of these salts.
The IN activity of quartz is decreased in alkaline solutions. The
interaction with NH3 suppresses the IN activity of the quartz surfaces.
This is in contrast to the increased IN activity of gibbsite and
aluminosilicates, i.e., feldspars, mica and kaolinite in the presence of
NH3. We ascribe this decrease in IN activity to the increased
dissolution of quartz under alkaline conditions. The defects that constitute
the active sites seem to be more susceptible and therefore disappear first
on a dissolving surface.
Suspending quartz particles over days in aqueous solutions at
near-neutral pH conditions in a glass vial decreases the IN activity
considerably. This decrease is reversible and the original IN activity in
terms of Fhet is almost restored when the quartz particles are
washed and resuspended in pure water. We assume that a part of the silicic
acid leached from the glass vial forms an oligomerized silicic acid layer on
top of the quartz surface and blocks the active sites. This layer dissolves
when the particles are rinsed in pure water. The formation of this layer
might be the first step to quartz growth.
The sensitivity of the IN activity of quartz surfaces to environmental
conditions makes it difficult to come to general conclusions regarding the
relevance of quartz particles for cloud glaciation. Dry erosion may fracture
quartz particles and introduce active sites while wet erosion may destroy
active sites. To assess the IN activity of airborne quartz particles, a
correlation between the quartz content and the IN activity must be
established for samples that did not undergo milling before they were tested
for freezing.
Data availability
The data for freshly prepared quartz suspensions in water
or aqueous solutions (Fig. 2) and aging tests (Figs. 5 and 6) presented in
this publication are available at 10.3929/ethz-b-000286931 (Kumar et
al., 2018b).
Ice nucleation efficiency of quartz from
Kaufmann et al. (2016)
A recent study from our group, Kaufmann et al. (2016), reported a very low IN
efficiency of a quartz sample suspended in pure water using the same
experimental equipment and procedure as in the present study. In emulsion
freezing experiments, a heterogeneous freezing signal was observed up to
∼247 K, yet with a very low IN active particle fraction (0.01). The
quartz sample used in Kaufmann et al. (2016) was procured as a stone from the
Institute of Geochemistry and Petrology of ETH Zurich and milled to a fine
powder in a tungsten carbide ball mill. Approximately 0.4 wt % of
tungsten carbide was introduced as an impurity in the quartz sample due to
milling.
To explain the low IN efficiency of the Kaufmann quartz compared with SA
quartz, we further characterized these two samples. Scanning electron
microscopy (SEM) revealed the presence of giant particles (diameter
> 20 µm) in the milled quartz sample from Kaufmann et
al. (2016) as shown in Fig. A1, which were absent in SA quartz. SA quartz is
dominated by particles ranging from 0.5 to 10 µm as specified by
Sigma-Aldrich. As a hard mineral, quartz is difficult to mill down to very
fine particles without partial amorphization. The amorphous fraction of the
Kaufmann quartz was estimated as 6.4±0.5 % based on the background
signal observed in the XRD diffractogram compared to 4.5±0.5 % for
the SA quartz.
The size distribution measurements performed with SMPS/APS in
Kaufmann et al. (2016) did not capture the coarse
particle fraction due to inefficient aerosolization of large particles in
the fluidized bed. Even when aerosolized, coarse particles can sediment in
the tubing and connections during transport to the SMPS/APS. When
calculating the fraction of droplets filled with particles, the number of
particles per sample mass was highly overestimated because the coarse
particles did not appear in the size distribution determined by SMPS/APS.
Therefore, in Kaufmann et al. (2016), the homogeneous
freezing signal observed in the emulsion freezing experiments was wrongly
assigned to droplets filled with IN inactive quartz particles although it
was mostly due to empty droplets.
To remove the coarse particles present in the Kaufmann quartz, we suspended a
concentrated suspension of this sample for an hour in pure water so that
particles with average diameters ≥3µm should settle. After this
time period, the supernatant was collected and immediately used for an
emulsion freezing experiment in order to avoid any further aging of the
particles. The concentration of the supernatant was determined as ∼8 wt %–9 wt % by evaporating the water and weighing the dried
residual.
In addition, emulsion freezing experiments were carried out on the same
supernatant suspension aged for 2, 24 and 72 h. The DSC thermograms of these
measurements are shown in Fig. A2. The heterogeneous freezing onset
temperature of the fresh supernatant was 250.2 K, which is even higher than
the Thet of SA quartz, while the heterogeneously frozen fraction
is almost the same for these two quartz samples. This confirms that the low
IN efficiency of quartz reported in Kaufmann et al. (2016) was biased by the
presence of giant quartz particles not captured in the SMPS/APS measurements.
SEM images of two different quartz samples at 500×
magnification. (a) Quartz sample from Kaufmann et al. (2016);
(b) quartz sample (from Sigma-Aldrich) used in this study.
DSC thermograms of Kaufmann quartz taken from the supernatant of a
suspension of the same quartz sample as used in Kaufmann et al. (2016) that
had settled for 1 h. Settling resulted in particle sizes with diameter ≤3µm and a suspension concentration of 8 wt %–9 wt % in the
supernatant. In order to assess the effect of aging, emulsion freezing
experiments were performed on the supernatant right after extraction as well
as after 2, 24 and 72 h after extraction and the corresponding
Thet and Fhet is reported next to each curve. All
curves are normalized such that the areas under the heterogeneous and
homogeneous freezing curves sum up to the same value. The * symbol represents
the onset temperatures of the two shoulders of the heterogeneous freezing
signal and the corresponding Fhet is calculated based on the
complete freezing signal.
DSC thermograms of 5 wt % TU Vienna quartz suspension in pure
water, obtained via emulsion freezing experiments performed on the day of
suspension preparation (fresh) and the subsequent 3 days and the
corresponding Thet and Fhet are reported next to each
curve. Suspensions were prepared in borosilicate glass vials. All curves are
normalized such that the areas under the heterogeneous and homogeneous
freezing curves sum up to the same value.
Aging tests on TU Vienna quartz suspended in pure water
Suspensions of TU Vienna quartz (5 wt %) in pure water were prepared in
glass vials and tested over a period of 3 days. Immersion freezing
experiments were carried out with the DSC setup with emulsions prepared from
at least two separate suspensions. Measurements were done on the day of
preparation (fresh) and on the subsequent 3 days in order to assess the
long-term effect of aging on the IN efficiency of the quartz sample. Figure
A3 shows the DSC thermograms from freezing experiments during this aging
period. Like SA and Kaufmann quartz, TU Vienna quartz also loses its IN
efficiency drastically over the measured time period. This shows that the
decrease in IN efficiency, during aging in conditions with supersaturated Si
concentration, is a common feature of all quartz samples.
The supplement related to this article is available online at: https://doi.org/10.5194/acp-19-6035-2019-supplement.
Author contributions
AK and CM planned the experiments. AK conducted the experiments and
prepared the initial paper draft. AK, CM, and TP contributed to the
interpretation of the results and the discussion.
Competing interests
The authors declare that they have no conflict of
interest.
Acknowledgements
This work was supported by the Swiss National Foundation, project number
200020_156251. We thank the following colleagues from ETH Zürich:
Annette Röthlisberger and Marion Rothaupt for help in carrying out BET
and XRD measurements and Michael Plötze for carrying out detailed XRD
analysis; Fabian Mahrt for providing the SMPS and the APS for size
distribution measurements; Nadine Borduas and Julie Tolu for their continuous
help and support during ICP-MS measurements; Eszter Barthazy for carrying out
SEM analysis. Jonas Fahrni and Dominik Brühwiler from Institute of
Chemistry and Biotechnology (Zürich University of Applied Sciences,
Wädenswil, ZHAW) for the synthesis and characterization of Stöber
particles and functionalization of Alfa Aesar particles. We would also like to
thank Alexei Kiselev from the Institute of Meteorology and Climate Research
(Karlsruhe Institute of Technology) for providing valuable feedback on this
paper.
Review statement
This paper was edited by Ryan Sullivan and reviewed by two anonymous referees.
ReferencesAbdelmonem, A., Backus, E. H. G., Hoffmann, N., Sánchez, M. A., Cyran, J.
D., Kiselev, A., and Bonn, M.: Surface-charge-induced orientation of
interfacial water suppresses heterogeneous ice nucleation on α-alumina (0001), Atmos. Chem. Phys., 17, 7827–7837,
10.5194/acp-17-7827-2017, 2017.Alastuey, A., Querol, X., Castillo, S., Escudero, M., Avila, A., Cuevas, E.,
Torres, C., Romero, P.-M., Exposito, F., García, O., Pedro Diaz, J.,
Dingenen, R. V., and Putaud, J. P.: Characterisation of TSP and PM2.5 at
Izaña and sta, Cruz de Tenerife (Canary Islands, Spain) during a Saharan
dust episode (July 2002), Atmos. Environ., 39, 4715–4728,
10.1016/j.atmosenv.2005.04.018, 2005.Alpert, P. A., Aller, J. Y., and Knopf, D. A.: Ice nucleation from aqueous
NaCl droplets with and without marine diatoms, Atmos. Chem. Phys., 11,
5539–5555, 10.5194/acp-11-5539-2011, 2011a.Alpert, P. A., Aller, J. Y., and Knopf, D. A.: Initiation of the ice phase by
marine biogenic surfaces in supersaturated gas and supercooled aqueous
phases, Phys. Chem. Chem. Phys., 13, 19882–19894, 10.1039/C1CP21844A,
2011b.Archuleta, C. M., DeMott, P. J., and Kreidenweis, S. M.: Ice nucleation by
surrogates for atmospheric mineral dust and mineral dust/sulfate particles at
cirrus temperatures, Atmos. Chem. Phys., 5, 2617–2634,
10.5194/acp-5-2617-2005, 2005.Atkinson, J. D., Murray, B. J., Woodhouse, M. T., Whale, T. F., Baustian, K.
J., Carslaw, K. S., Dobbie, S., O'Sullivan, D., and Malkin, T. L.: The
importance of feldspar for ice nucleation by mineral dust in mixed-phase
clouds, Nature, 498, 355–358, 10.1038/nature12278, 2013.Augustin-Bauditz, S., Wex, H., Kanter, S., Ebert, M., Niedermeier, D., Stolz,
F., Prager, A., and Stratmann, F.: The immersion mode ice nucleation behavior
of mineral dusts: A comparison of different pure and surface modified dusts,
Geophys. Res. Lett., 41, 7375–7382, 10.1002/2014GL061317, 2014.Avila, A., Queralt-Mitjans, I., and Alarcón, M.: Mineralogical
composition of African dust delivered by red rains over northeastern Spain,
J. Geophys. Res.-Atmos., 102, 21977–21996, 10.1029/97JD00485, 1997.Baker, M. B.: Cloud microphysics and climate, Science, 276, 1072–1078,
10.1126/science.276.5315.1072, 1997.Baughman, R. J.: Quartz crystal growth, J. Cryst. Growth, 112, 753–757,
10.1016/0022-0248(91)90132-O, 1991.Belton, D. J., Deschaume, O., and Perry, C. C.: An overview of the
fundamentals of the chemistry of silica with relevance to biosilicification
and technological advances, FEBS J., 279, 1710–1720,
10.1111/j.1742-4658.2012.08531.x, 2012.Bennett, P. C., Melcer, M. E., Siegel, D. I., and Hassett, J. P.: The
dissolution of quartz in dilute aqueous solutions of organic acids at
25 ∘C, Geochim. Cosmochim. Ac., 52, 1521–1530,
10.1016/0016-7037(88)90222-0, 1988.Berger, G., Cadore, E., Schott, J., and Dove, P. M.: Dissolution rate of
quartz in lead and sodium electrolyte solutions between 25 and
300 ∘C: Effect of the nature of surface complexes and reaction
affinity, Geochim. Cosmochim. Ac., 58, 541–551,
10.1016/0016-7037(94)90487-1, 1994.Bickmore, B. R., Wheeler, J. C., Bates, B., Nagy, K. L., and Eggett, D. L.:
Reaction pathways for quartz dissolution determined by statistical and
graphical analysis of macroscopic experimental data, Geochim. Cosmochim. Ac.,
72, 4521–4536, 10.1016/j.gca.2008.07.002, 2008.Blomfield, G. A. and Little, L. H.: Chemisorption of ammonia on silica, Can.
J. Chem., 51, 1771–1781, 10.1139/v73-265, 1973.Bolis, V., Fubini, B., Coluccia, S., and Mostacci, E.: Surface hydration of
crystalline and amorphous silicas, J. Therm. Anal. Calorim., 30, 1283–1292,
10.1007/bf01914297, 1985.Bolis, V., Fubini, B., Marchese, L., Martra, G., and Costa, D.: Hydrophilic
and hydrophobic sites on dehydrated crystalline and amorphous silicas, J.
Chem. Soc., 87, 497–505, 10.1039/FT9918700497, 1991.Boose, Y., Sierau, B., García, M. I., Rodríguez, S., Alastuey, A.,
Linke, C., Schnaiter, M., Kupiszewski, P., Kanji, Z. A., and Lohmann, U.: Ice
nucleating particles in the Saharan air layer, Atmos. Chem. Phys., 16,
9067–9087, 10.5194/acp-16-9067-2016, 2016a.Boose, Y., Welti, A., Atkinson, J., Ramelli, F., Danielczok, A., Bingemer, H.
G., Plötze, M., Sierau, B., Kanji, Z. A., and Lohmann, U.: Heterogeneous
ice nucleation on dust particles sourced from nine deserts worldwide – Part
1: Immersion freezing, Atmos. Chem. Phys., 16, 15075–15095,
10.5194/acp-16-15075-2016, 2016b.
Bowen, N. L.: The reaction principle in petrogenesis, J. Geol., 30, 177–198,
1922.
Bowen, N. L.: The evolution of igneous rocks, Princeton University Press,
Princeton, 1928.Brady, P. V. and Walther, J. V.: Controls on silicate dissolution rates in
neutral and basic pH solutions at 25 ∘C, Geochim. Cosmochim. Ac.,
53, 2823–2830, 10.1016/0016-7037(89)90160-9, 1989.Brady, P. V. and Walther, J. V.: Kinetics of quartz dissolution at low
temperatures, Chem. Geol., 82, 253–264, 10.1016/0009-2541(90)90084-K,
1990.Brinker, C. J., Tallant, D. R., Roth, E. P., and Ashley, C. S.: Sol-gel
transition in simple silicates: III. Structural studies during densification,
J. Non-Cryst. Solids, 82, 117–126, 10.1016/0022-3093(86)90119-5, 1986.Brinker, C. J., Kirkpatrick, R. J., Tallant, D. R., Bunker, B. C., and
Montez, B.: NMR confirmation of strained “defects” in amorphous silica, J.
Non-Cryst. Sol., 99, 418–428, 10.1016/0022-3093(88)90448-6, 1988.Bunker, B. C.: Molecular mechanisms for corrosion of silica and silicate
glasses, J. Non-Cryst. Solids, 179, 300–308,
10.1016/0022-3093(94)90708-0, 1994.Burkert-Kohn, M., Wex, H., Welti, A., Hartmann, S., Grawe, S., Hellner, L.,
Herenz, P., Atkinson, J. D., Stratmann, F., and Kanji, Z. A.: Leipzig ice
nucleation chamber comparison (LINC): Intercomparison of four online ice
nucleation counters, Atmos. Chem. Phys., 17, 11683–11705,
10.5194/acp-17-11683-2017, 2017.Cant, N. W. and Little, L. H.: The infrared spectrum of ammonia adsorbed on
cabosil silica powder, Can. J. Chem., 43, 1252–1254, 10.1139/v65-170,
1965.Cantrell, W. and Robinson, C.: Heterogeneous freezing of ammonium sulfate and
sodium chloride solutions by long chain alcohols, Geophys. Res. Lett., 33,
L07802, 10.1029/2005GL024945, 2006.Caquineau, S., Gaudichet, A., Gomes, L., Magonthier, M.-C., and Chatenet, B.:
Saharan dust: Clay ratio as a relevant tracer to assess the origin of
soil-derived aerosols, Geophys. Res. Lett., 25, 983–986,
10.1029/98GL00569, 1998.Caquineau, S., Gaudichet, A., Gomes, L., and Legrand, M.: Mineralogy of
Saharan dust transported over northwestern tropical Atlantic ocean in
relation to source regions, J. Geophys. Res., 107, 4251,
10.1029/2000JD000247, 2002.Criscenti, L. J., Kubicki, J. D., and Brantley, S. L.: Silicate glass and
mineral dissolution:? Calculated reaction paths and activation energies for
hydrolysis of a Q3 Si by H3O+ using ab initio methods, J.
Phys. Chem. A, 110, 198–206, 10.1021/jp044360a, 2006.Crundwell, F. K.: On the mechanism of the dissolution of quartz and silica in
aqueous solutions, ACS Omega, 2, 1116–1127, 10.1021/acsomega.7b00019,
2017.DeMott, P. J., Cziczo, D. J., Prenni, A. J., Murphy, D. M., Kreidenweis, S.
M., Thomson, D. S., Borys, R., and Rogers, D. C.: Measurements of the
concentration and composition of nuclei for cirrus formation, P. Natl. Acad.
Sci. USA, 100, 14655, 10.1073/pnas.2532677100, 2003b.DeMott, P. J., Prenni, A. J., Liu, X., Kreidenweis, S. M., Petters, M. D.,
Twohy, C. H., Richardson, M. S., Eidhammer, T., and Rogers, D. C.: Predicting
global atmospheric ice nuclei distributions and their impacts on climate, P.
Natl. Acad. Sci. USA, 107, 11217–11222, 10.1073/pnas.0910818107, 2010.Döbelin, N. and Kleeberg, R.: Profex: A graphical user interface for the
rietveld refinement program BGMN, J. Appl. Crystallogr., 48, 1573–1580,
10.1107/S1600576715014685, 2015.Donaldson, K. E. N., and Borm, P. J. A.: The quartz hazard: A variable
entity, Ann. Occup. Hyg., 42, 287–294, 10.1093/annhyg/42.5.287, 1998.
Du, Q., Freysz, E., and Shen, Y. R.: Vibrational spectra of water molecules
at quartz/water interfaces, Phys. Rev. Lett., 72, 238–241, 1994.
Eischens, R. P. and Pliskin, W. A.: The infrared spectra of adsorbed
molecules, in: Advances in catalysis, edited by: Eley, D. D., Frankenburg, W.
G., Komarewsky, V. I., and Weisz, P. B., Academic Press, 1–56, 1958.Field, P. R., Möhler, O., Connolly, P., Krämer, M., Cotton, R.,
Heymsfield, A. J., Saathoff, H., and Schnaiter, M.: Some ice nucleation
characteristics of Asian and Saharan desert dust, Atmos. Chem. Phys., 6,
2991–3006, 10.5194/acp-6-2991-2006, 2006.Folman, M.: Infra-red studies of NH3 adsorption on chlorinated
porous vycor glass, Trans. Faraday Soc., 57, 2000–2006,
10.1039/TF9615702000, 1961.Fubini, B.: Surface chemistery and quartz hazard, Ann. Occup. Hyg., 42,
521–530, 10.1093/annhyg/42.8.521, 1998.Fubini, B., Bolis, V., and Giamello, E.: The surface chemistry of crushed
quartz dust in relation to its pathogenicity, Inorg. Chim. Acta, 138,
193–197, 10.1016/S0020-1693(00)81222-0, 1987.Fubini, B., Giamello, E., Pugliese, L., and Volante, M.: Mechanically induced
defects in quartz and their impact on pathogenicity, Solid State Ionics,
32/33, 334–343, 10.1016/0167-2738(89)90238-5, 1989.Fubini, B., Bolis, V., Cavenago, A., and Ugliengo, P.: Ammonia and water as
probes for the surface reactivity of covalent solids: Cristobalite and
silicon carbide, J. Chem. Soc., 88, 277–289, 10.1039/FT9928800277, 1992.
Götze, J. and Möckel, R.: Quartz: Deposits, mineralogy and analytics,
Springer, Berlin, Heidelberg, 2014.Harrison, A. D., Whale, T. F., Carpenter, M. A., Holden, M. A., Neve, L.,
O'Sullivan, D., Vergara Temprado, J., and Murray, B. J.: Not all feldspars
are equal: A survey of ice nucleating properties across the feldspar group of
minerals, Atmos. Chem. Phys., 16, 10927–10940,
10.5194/acp-16-10927-2016, 2016.Henderson, J. H., Syers, J. K., and Jackson, M. L.: Quartz dissolution as
influenced by pH and the presence of a disturbed surface layer, Isr. J.
Chem., 8, 357–372, 10.1002/ijch.197000042, 1970.House, W. A. and Orr, D. R.: Investigation of the pH dependence of the
kinetics of quartz dissolution at 25 degree C, J. Chem. Soc., 88, 233–241,
10.1039/FT9928800233, 1992.Huneeus, N., Schulz, M., Balkanski, Y., Griesfeller, J., Prospero, J., Kinne,
S., Bauer, S., Boucher, O., Chin, M., Dentener, F., Diehl, T., Easter, R.,
Fillmore, D., Ghan, S., Ginoux, P., Grini, A., Horowitz, L., Koch, D., Krol,
M. C., Landing, W., Liu, X., Mahowald, N., Miller, R., Morcrette, J. J.,
Myhre, G., Penner, J., Perlwitz, J., Stier, P., Takemura, T., and Zender, C.
S.: Global dust model intercomparison in aerocom phase I, Atmos. Chem. Phys.,
11, 7781–7816, 10.5194/acp-11-7781-2011, 2011.Isono, K. and Ikebe, Y.: On the ice-nucleating ability of rock-forming
minerals and soil particles, J. Meteorol. Soc. Japn. Ser. II, 38, 213–230,
10.2151/jmsj1923.38.5_213, 1960.Johari, G. P., Fleissner, G., Hallbrucker, A., and Mayer, E.: Thermodynamic
continuity between glassy and normal water, J. Phys. Chem., 98, 4719–4725,
10.1021/j100068a038, 1994.
Kandler, K., Schutz, L., Deutscher, C., Ebert, M., Hofmann, H., Jackel, S.,
Jaenicke, R., Knippertz, P., Lieke, K., and Massling, A.: Size distribution,
mass concentration, chemical and mineralogical composition and derived
optical parameters of the boundary layer aerosol at Tinfou, Morocco, during
SAMUM 2006, Tellus B, 61, 32–50, 2009.Kanji, Z. A., Sullivan, R. C., Niemand, M., DeMott, P. J., Prenni, A. J.,
Chou, C., Saathoff, H., and Möhler, O.: Heterogeneous ice nucleation
properties of natural desert dust particles coated with a surrogate of
secondary organic aerosol, Atmos. Chem. Phys., 19, 5091–5110,
10.5194/acp-19-5091-2019, 2019.Kaufmann, L., Marcolli, C., Hofer, J., Pinti, V., Hoyle, C. R., and Peter,
T.: Ice nucleation efficiency of natural dust samples in the immersion mode,
Atmos. Chem. Phys., 16, 11177–11206,
10.5194/acp-16-11177-2016, 2016.Kaufmann, L., Marcolli, C., Luo, B., and Peter, T.: Refreeze experiments with
water droplets containing different types of ice nuclei interpreted by
classical nucleation theory, Atmos. Chem. Phys., 17, 3525–3552,
10.5194/acp-17-3525-2017, 2017.Kline, W. E. and Fogler, H. S.: Dissolution kinetics: Catalysis by strong
acids, J. Colloid Interface Sci., 82, 93–102,
10.1016/0021-9797(81)90127-2, 1981.Knauss, K. G., and Wolery, T. J.: The dissolution kinetics of quartz as a
function of pH and time at 70 ∘C, Geochim. Cosmochim. Ac., 52,
43–53, 10.1016/0016-7037(88)90055-5, 1988.Knopf, D. A. and Forrester, S. M.: Freezing of water and aqueous NaCl
droplets coated by organic monolayers as a function of surfactant properties
and water activity, J. Phys. Chem. A, 115, 5579–5591, 10.1021/jp2014644,
2011.Knopf, D. A. and Alpert, P. A.: A water activity based model of heterogeneous
ice nucleation kinetics for freezing of water and aqueous solution droplets,
Farad. Discussions, 165, 513–534, 10.1039/C3FD00035D, 2013.Kolb, C. E., Cox, R. A., Abbatt, J. P. D., Ammann, M., Davis, E. J.,
Donaldson, D. J., Garrett, B. C., George, C., Griffiths, P. T., Hanson, D.
R., Kulmala, M., McFiggans, G., Pöschl, U., Riipinen, I., Rossi, M. J.,
Rudich, Y., Wagner, P. E., Winkler, P. M., Worsnop, D. R., and O' Dowd, C.
D.: An overview of current issues in the uptake of atmospheric trace gases by
aerosols and clouds, Atmos. Chem. Phys., 10, 10561–10605,
10.5194/acp-10-10561-2010, 2010.
Koop, T., Luo, B., Tsias, A., and Peter, T.: Water activity as the
determinant for homogeneous ice nucleation in aqueous solutions, Nature, 406,
611–614, 2000.Kulkarni, G., Fan, J., Comstock, J. M., Liu, X., and Ovchinnikov, M.:
Laboratory measurements and model sensitivity studies of dust deposition ice
nucleation, Atmos. Chem. Phys., 12, 7295–7308,
10.5194/acp-12-7295-2012, 2012.Kumar, A., Marcolli, C., Luo, B., and Peter, T.: Ice nucleation activity of
silicates and aluminosilicates in pure water and aqueous solutions – Part 1:
The K-feldspar microcline, Atmos. Chem. Phys., 18, 7057–7079,
10.5194/acp-18-7057-2018, 2018a.Kumar, A., Marcolli, C., and Peter, T.: Research Data supporting “Ice
nucleation activity of silicates and aluminosilicates in pure water and
aqueous solutions, Part 2 – Quartz and amorphous silica”,
10.3929/ethz-b-000286931, last access: 1 November 2018b.Kumar, A., Marcolli, C., and Peter, T.: Ice nucleation activity of silicates and aluminosilicates in pure water
and aqueous solutions – Part 3: Aluminosilicates, Atmos. Chem. Phys., 19, 6059–6084,
10.5194/acp-19-6059-2019, 2019.Laurent, B., Marticorena, B., Bergametti, G., and Mei, F.: Modeling mineral
dust emissions from chinese and mongolian deserts, Glob. Planet. Change, 52,
121–141, 10.1016/j.gloplacha.2006.02.012, 2006.Laurent, B., Marticorena, B., Bergametti, G., Léon, J. F., and Mahowald,
N. M.: Modeling mineral dust emissions from the sahara desert using new
surface properties and soil database, J. Geophys. Res.-Atmos., 113, D14218,
10.1029/2007JD009484, 2008.Li, C.-Z. and Nelson, P. F.: Interactions of quartz, zircon sand and
stainless steel with ammonia: Implications for the measurement of ammonia at
high temperatures, Fuel, 75, 525–526, 10.1016/0016-2361(95)00256-1,
1996.Makoto, H. and Motoji, I.: Effects of UV exposure on e' centers formed in
crushed quartz grains, Jpn. J. Appl. Phys., 35, 4463–4467,
10.1143/JJAP.35.4463, 1996.Mapes, J. E. and Eischens, R. P.: The infrared spectra of ammonia chemisorbed
on cracking catalysts, J. Phys. Chem., 58, 1059–1062,
10.1021/j150522a002, 1954.Marcolli, C., Gedamke, S., Peter, T., and Zobrist, B.: Efficiency of
immersion mode ice nucleation on surrogates of mineral dust, Atmos. Chem.
Phys., 7, 5081–5091, 10.5194/acp-7-5081-2007, 2007.Marcolli, C., Nagare, B., Welti, A., and Lohmann, U.: Ice nucleation
efficiency of AgI: Review and new insights, Atmos. Chem. Phys., 16,
8915–8937, 10.5194/acp-16-8915-2016, 2016.Matsuki, A., Iwasaka, Y., Shi, G. Y., Zhang, D. Z., Trochkine, D., Yamada,
M., Kim, Y. S., Chen, B., Nagatani, T., and Miyazawa, T.: Morphological and
chemical modification of mineral dust: Observational insight into the
heterogeneous uptake of acidic gases, Geophys. Res. Lett., 32, L22806,
10.1029/2005GL024176, 2005.Morrow, B. A. and Cody, I. A.: Infrared studies of reactions on oxide
surfaces, 5. Lewis acid sites on dehydroxylated silica, J. Phys. Chem., 80,
1995–1998, 10.1021/j100559a009, 1976.Morrow, B. A., Cody, I. A., and Lee, L. S. M.: Infrared studies of reactions
on oxide surfaces, 7. Mechanism of the adsorption of water and ammonia on
dehydroxylated silica, J. Phys. Chem., 80, 2761–2767,
10.1021/j100566a009, 1976.Murray, B. J., Broadley, S. L., Wilson, T. W., Atkinson, J. D., and Wills, R.
H.: Heterogeneous freezing of water droplets containing kaolinite particles,
Atmos. Chem. Phys., 11, 4191–4207, 10.5194/acp-11-4191-2011,
2011.Murray, B. J., O'Sullivan, D., Atkinson, J. D., and Webb, M. E.: Ice
nucleation by particles immersed in supercooled cloud droplets, Chem. Soc.
Rev., 41, 6519–6554, 10.1039/C2CS35200A, 2012.Musso, F., Sodupe, M., Corno, M., and Ugliengo, P.: H-Bond features of fully
hydroxylated surfaces of crystalline silica polymorphs: A periodic B3LYP
study, J. Phys. Chem. C, 113, 17876–17884, 10.1021/jp905325m, 2009.Musso, F., Ugliengo, P., and Sodupe, M.: Do H-Bond features of silica
surfaces affect the H2O and NH3 adsorption? Insights from
periodic B3LYP calculations, J. Phys. Chem. A, 115, 11221–11228,
10.1021/jp203988j, 2011.Musso, F., Mignon, P., Ugliengo, P., and Sodupe, M.: Cooperative effects at
water-crystalline silica interfaces strengthen surface silanol hydrogen
bonding. An ab initio molecular dynamics study, Phys. Chem. Chem. Phys., 14,
10507–10514, 10.1039/C2CP40756F, 2012.Muster, T. H., Prestidge, C. A., and Hayes, R. A.: Water adsorption kinetics
and contact angles of silica particles, Colloids Surf. Physicochem. Eng.
Aspects, 176, 253–266, 10.1016/S0927-7757(00)00600-2, 2001.Niedermeier, D., Hartmann, S., Clauss, T., Wex, H., Kiselev, A., Sullivan, R.
C., DeMott, P. J., Petters, M. D., Reitz, P., Schneider, J., Mikhailov, E.,
Sierau, B., Stetzer, O., Reimann, B., Bundke, U., Shaw, R. A., Buchholz, A.,
Mentel, T. F., and Stratmann, F.: Experimental study of the role of
physicochemical surface processing on the in ability of mineral dust
particles, Atmos. Chem. Phys., 11, 11131–11144,
10.5194/acp-11-11131-2011, 2011.O'Sullivan, D., Murray, B. J., Malkin, T. L., Whale, T. F., Umo, N. S.,
Atkinson, J. D., Price, H. C., Baustian, K. J., Browse, J., and Webb, M. E.:
Ice nucleation by fertile soil dusts: relative importance of mineral and
biogenic components, Atmos. Chem. Phys., 14, 1853–1867,
10.5194/acp-14-1853-2014, 2014.Peckhaus, A., Kiselev, A., Hiron, T., Ebert, M., and Leisner, T.: A
comparative study of K-rich and Na / Ca-rich feldspar ice-nucleating
particles in a nanoliter droplet freezing assay, Atmos. Chem. Phys., 16,
11477–11496, 10.5194/acp-16-11477-2016, 2016.Pedevilla, P., Fitzner, M., and Michaelides, A.: What makes a good
descriptor for heterogeneous ice nucleation on OH-patterned surfaces, Phys.
Rev. B, 96, 115441, 10.1103/PhysRevB.96.115441, 2017.Peri, J. B.: Infrared study of OH and NH2 groups on the surface of a
dry silica aerogel, J. Phys. Chem., 70, 2937–2945,
10.1021/j100881a037, 1966.Peri, J. B. and Hannan, R. B.: Surface hydroxyl groups on γ-alumina1,
J. Phys. Chem., 64, 1526–1530, 10.1021/j100839a044, 1960.
Perry, C. C.: Chemical studies of biogenic silica, in: Biomineralisation,
chemical and biological perspectives, edited by: Mann, S., Webb, J., and
Williams, R. J. P., VCH, Weinheim, 233–256, 1989.Perry, C. C.: Silicification: The processes by which organisms capture and
mineralize silica, Rev. Mineral. Geochem., 54, 291–327, 10.2113/0540291,
2003.Pinti, V., Marcolli, C., Zobrist, B., Hoyle, C. R., and Peter, T.: Ice
nucleation efficiency of clay minerals in the immersion mode, Atmos. Chem.
Phys., 12, 5859–5878, 10.5194/acp-12-5859-2012, 2012.Poulsen, H. F., Neuefeind, J., Neumann, H. B., Schneider, J. R., and Zeidler,
M. D.: Amorphous silica studied by high energy X-ray diffraction, J.
Non-Cryst. Solids, 188, 63–74, 10.1016/0022-3093(95)00095-X, 1995.Prospero, J. M.: Long-range transport of mineral dust in the global
atmosphere: Impact of African dust on the environment of the Southeastern
United States, P. Natl. Acad. Sci. USA, 96, 3396,
10.1073/pnas.96.7.3396, 1999.Pruppacher, H. R. and Sänger, R.: Mechanismus der vereisung
unterkühlter wassertropfen durch disperse keimsubstanzen, Z. Angew. Math.
Phys., 6, 407–416, 10.1007/bf01589767, 1955.
Pruppacher, H. R. and Klett, J. D.: Microphysics of clouds and precipitation,
Kluwer Academic Publishers, Dordrecht, the Netherlands, 1994.Reitz, P., Spindler, C., Mentel, T. F., Poulain, L., Wex, H., Mildenberger,
K., Niedermeier, D., Hartmann, S., Clauss, T., Stratmann, F., Sullivan, R.
C., DeMott, P. J., Petters, M. D., Sierau, B., and Schneider, J.: Surface
modification of mineral dust particles by sulphuric acid processing:
Implications for ice nucleation abilities, Atmos. Chem. Phys., 11,
7839–7858, 10.5194/acp-11-7839-2011, 2011.Richmond, G. L.: Molecular bonding and interactions at aqueous surfaces as
probed by vibrational sum frequency spectroscopy, Chem. Rev., 102,
2693–2724, 10.1021/cr0006876, 2002.Rigg, Y. J., Alpert, P. A., and Knopf, D. A.: Immersion freezing of water and
aqueous ammonium sulfate droplets initiated by humic-like substances as a
function of water activity, Atmos. Chem. Phys., 13, 6603–6622,
10.5194/acp-13-6603-2013, 2013.Salam, A., Lohmann, U., and Lesins, G.: Ice nucleation of ammonia gas exposed
montmorillonite mineral dust particles, Atmos. Chem. Phys., 7, 3923–3931,
10.5194/acp-7-3923-2007, 2007.Salam, A., Lesins, G., and Lohmann, U.: Laboratory study of heterogeneous ice
nucleation in deposition mode of montmorillonite mineral dust particles aged
with ammonia, sulfur dioxide, and ozone at polluted atmospheric
concentrations, Air Qual. Atmos. Health, 1, 135–142,
10.1007/s11869-008-0019-6, 2008.Schepanski, K., Tegen, I., and Macke, A.: Saharan dust transport and
deposition towards the tropical northern Atlantic, Atmos. Chem. Phys., 9,
1173–1189, 10.5194/acp-9-1173-2009, 2009.Schwartzentruber, J., Fürst, W., and Renon, H.: Dissolution of quartz
into dilute alkaline solutions at 90 ∘C: A kinetic study, Geochim.
Cosmochim. Ac., 51, 1867–1874, 10.1016/0016-7037(87)90177-3, 1987.Speedy, R. J.: Thermodynamic properties of supercooled water at 1 atm, J.
Phys. Chem., 91, 3354–3358, 10.1021/j100296a049, 1987.Storelvmo, T., Hoose, C., and Eriksson, P.: Global modeling of mixed-phase
clouds: The albedo and lifetime effects of aerosols, J. Geophys. Res.-Atmos.,
116, D05207, 10.1029/2010JD014724, 2011.Sullivan, R. C., Miñambres, L., DeMott, P. J., Prenni, A. J., Carrico, C.
M., Levin, E. J. T., and Kreidenweis, S. M.: Chemical processing does not
always impair heterogeneous ice nucleation of mineral dust particles,
Geophys. Res. Lett., 37, L24805, 10.1029/2010GL045540, 2010a.Sullivan, R. C., Petters, M. D., DeMott, P. J., Kreidenweis, S. M., Wex, H.,
Niedermeier, D., Hartmann, S., Clauss, T., Stratmann, F., Reitz, P.,
Schneider, J., and Sierau, B.: Irreversible loss of ice nucleation active
sites in mineral dust particles caused by sulphuric acid condensation, Atmos.
Chem. Phys., 10, 11471–11487, 10.5194/acp-10-11471-2010,
2010b.Tamahrajah, J. and Brehm, A.: Preliminary kinetic data of silicic acid
species prior to the formation of exoskeletal structures, Mar. Chem., 181,
18–24, 10.1016/j.marchem.2016.03.001, 2016.Tobo, Y., DeMott, P. J., Hill, T. C. J., Prenni, A. J., Swoboda-Colberg, N.
G., Franc, G. D., and Kreidenweis, S. M.: Organic matter matters for ice
nuclei of agricultural soil origin, Atmos. Chem. Phys., 14, 8521–8531,
10.5194/acp-14-8521-2014, 2014.Tribello, G. A., Slater, B., Zwijnenburg, M. A., and Bell, R. G.: Isomorphism
between ice and silica, Phys. Chem. Chem. Phys., 12, 8597–8606,
10.1039/B916367K, 2010.Tsyganenko, A. A., Pozdnyakov, D. V., and Filimonov, V. N.: Infrared study of
surface species arising from ammonia adsorption on oxide surfaces, J. Mol.
Struct., 29, 299–318, 10.1016/0022-2860(75)85038-1, 1975.Turci, F., Pavan, C., Leinardi, R., Tomatis, M., Pastero, L., Garry, D.,
Anguissola, S., Lison, D., and Fubini, B.: Revisiting the paradigm of silica
pathogenicity with synthetic quartz crystals: The role of crystallinity and
surface disorder, Part. Fibre Toxicol., 13, 10.1186/s12989-016-0136-6,
2016.Uno, I., Eguchi, K., Yumimoto, K., Takemura, T., Shimizu, A., Uematsu, M.,
Liu, Z., Wang, Z., Hara, Y., and Sugimoto, N.: Asian dust transported one
full circuit around the globe, Nat. Geosci., 2, 557–560,
10.1038/ngeo583, 2009.Usher, C. R., Michel, A. E., and Grassian, V. H.: Reactions on mineral dust,
Chem. Rev., 103, 4883–4940, 10.1021/cr020657y, 2003.Vali, G.: Interpretation of freezing nucleation experiments: Singular and
stochastic; sites and surfaces, Atmos. Chem. Phys., 14, 5271–5294,
10.5194/acp-14-5271-2014, 2014.Vali, G., DeMott, P. J., Möhler, O., and Whale, T. F.: Technical note: A
proposal for ice nucleation terminology, Atmos. Chem. Phys., 15,
10263–10270, 10.5194/acp-15-10263-2015, 2015.Vidyadhar, A. and Hanumantha Rao, K.: Adsorption mechanism of mixed
cationic/anionic collectors in feldspar-quartz flotation system, J. Colloid
Interface Sci., 306, 195–204, 10.1016/j.jcis.2006.10.047, 2007.Walther, J. V. and Helgeson, H. C.: Calculation of the thermodynamic
properties of aqueous silica and the solubility of quartz and its polymorphs
at high pressures and temperatures, Am. J. Sci., 277, 1315–1351,
10.2475/ajs.277.10.1315, 1977.Wei, X., Miranda, P. B., Zhang, C., and Shen, Y. R.: Sum-frequency
spectroscopic studies of ice interfaces, Phys. Rev. B, 66, 085401,
10.1103/PhysRevB.66.085401, 2002.Wex, H., DeMott, P. J., Tobo, Y., Hartmann, S., Rösch, M., Clauss, T.,
Tomsche, L., Niedermeier, D., and Stratmann, F.: Kaolinite particles as ice
nuclei: learning from the use of different kaolinite samples and different
coatings, Atmos. Chem. Phys., 14, 5529–5546,
10.5194/acp-14-5529-2014, 2014.Whale, T. F., Holden, M. A., Kulak, A. N., Kim, Y.-Y., Meldrum, F. C.,
Christenson, H. K., and Murray, B. J.: The role of phase separation and
related topography in the exceptional ice-nucleating ability of alkali
feldspars, Phys. Chem. Chem. Phys., 19, 31186–31193, 10.1039/C7CP04898J,
2017.Whale, T. F., Holden, M. A., Wilson, T. W., O'Sullivan, D., and Murray, B.
J.: The enhancement and suppression of immersion mode heterogeneous
ice-nucleation by solutes, Chem. Sci., 9, 4142–4151,
10.1039/C7SC05421A, 2018.Wright, L. B. and Walsh, T. R.: First-principles molecular dynamics
simulations of NH4+ and CH3COO- adsorption at the
aqueous quartz interface, J. Chem. Phys., 137, 224702, 10.1063/1.4769727,
2012.Xiao, Y. and Lasaga, A. C.: Ab initio quantum mechanical studies of the
kinetics and mechanisms of silicate dissolution: H+(H3O+)
catalysis, Geochim. Cosmochim. Ac., 58, 5379–5400,
10.1016/0016-7037(94)90237-2, 1994.Zhdanov, S. P., Kosheleva, L. S., and Titova, T. I.: IR study of hydroxylated
silica, Langmuir, 3, 960–967, 10.1021/la00078a014, 1987.Zhuravlev, L. T.: The surface chemistry of amorphous silica. Zhuravlev model,
Colloids Surf. Physicochem. Eng. Aspects, 173, 1–38,
10.1016/S0927-7757(00)00556-2, 2000.Zimmermann, F., Weinbruch, S., Schutz, L., Hofmann, H., Ebert, M., Kandler,
K., and Worringen, A.: Ice nucleation properties of the most abundant mineral
dust phases, J. Geophys. Res., 113, D23204, 10.1029/2008JD010655, 2008.Zobrist, B., Marcolli, C., Koop, T., Luo, B. P., Murphy, D. M., Lohmann, U.,
Zardini, A. A., Krieger, U. K., Corti, T., Cziczo, D. J., Fueglistaler, S.,
Hudson, P. K., Thomson, D. S., and Peter, T.: Oxalic acid as a heterogeneous
ice nucleus in the upper troposphere and its indirect aerosol effect, Atmos.
Chem. Phys., 6, 3115–3129, 10.5194/acp-6-3115-2006, 2006.Zobrist, B., Marcolli, C., Peter, T., and Koop, T.: Heterogeneous ice
nucleation in aqueous solutions:? The role of water activity, J. Phys. Chem.
A, 112, 3965–3975, 10.1021/jp7112208, 2008.Zolles, T., Burkart, J., Häusler, T., Pummer, B., Hitzenberger, R., and
Grothe, H.: Identification of ice nucleation active sites on feldspar dust
particles, J. Phys. Chem. A, 119, 2692–2700, 10.1021/jp509839x, 2015.Zuberi, B., Bertram, A. K., Cassa, C. A., Molina, L. T., and Molina, M. J.:
Heterogeneous nucleation of ice in (NH4)2SO4-H2O particles
with mineral dust immersions, Geophys. Res. Lett., 29,
10.1029/2001GL014289,
2002.Zuend, A., Marcolli, C., Luo, B. P., and Peter, T.: A thermodynamic model of
mixed organic-inorganic aerosols to predict activity coefficients, Atmos.
Chem. Phys., 8, 4559–4593, 10.5194/acp-8-4559-2008, 2008.Zuend, A., Marcolli, C., Booth, A. M., Lienhard, D. M., Soonsin, V., Krieger,
U. K., Topping, D. O., McFiggans, G., Peter, T., and Seinfeld, J. H.: New and
extended parameterization of the thermodynamic model AIOMFAC: Calculation of
activity coefficients for organic-inorganic mixtures containing carboxyl,
hydroxyl, carbonyl, ether, ester, alkenyl, alkyl, and aromatic functional
groups, Atmos. Chem. Phys., 11, 9155–9206,
10.5194/acp-11-9155-2011, 2011.