Aluminosilicates and quartz constitute the majority of airborne mineral dust.
Despite similarities in structures and surfaces they differ greatly in terms
of their ice nucleation (IN) efficiency. Here, we show that determining
factors for their IN activity include surface ion exchange, NH3 or
NH4+ adsorption, and surface degradation due to the slow dissolution
of the minerals. We performed immersion freezing experiments with the
(Na-Ca)-feldspar andesine, the K-feldspar sanidine, the clay mineral
kaolinite, the micas muscovite and biotite, and gibbsite and compare their IN
efficiencies with those of the previously characterized K-feldspar microcline
and quartz. Samples were suspended in pure water as well as in aqueous
solutions of NH3, (NH4)2SO4, NH4Cl and
Na2SO4, with solute concentrations corresponding to water activities
aw equal to 0.88–1.0. Using differential scanning calorimetry
(DSC) on emulsified micron-sized droplets, we derived onset temperatures of
heterogeneous (Thet) and homogeneous (Thom) freezing as
well as heterogeneously frozen water volume fractions (Fhet).
Suspensions in pure water of andesine, sanidine and kaolinite yield
Thet equal to 242.8, 241.2 and 240.3 K, respectively, while no
discernable heterogeneous freezing signal is present in the case of the micas
or gibbsite (i.e., Thet≈Thom≈237.0 K). The
presence of NH3 and/or NH4+ salts as solutes has distinct
effects on the IN efficiency of most of the investigated minerals. When
feldspars and kaolinite are suspended in very dilute solutions of NH3
or NH4+ salts, Thet shifts to higher temperatures (by
2.6–7.0 K compared to the pure water suspension). Even micas and gibbsite
develop weak heterogeneous freezing activities in ammonia solutions.
Conversely, suspensions containing Na2SO4 cause the Thet
of feldspars to clearly fall below the water-activity-based immersion
freezing description (Δaw= const.) even in very dilute
Na2SO4 solutions, while Thet of kaolinite follows the
Δaw= constant curve. The water activity determines how the
freezing temperature is affected by solute concentration alone, i.e., if the
surface properties of the ice nucleating particles are not affected by the
solute. Therefore, the complex behavior of the IN activities can only be
explained in terms of solute-surface-specific processes. We suggest that the
immediate exchange of the native cations (K+, Na+,
Ca2+) with protons, when feldspars are immersed in water, is a
prerequisite for their high IN efficiency. On the other hand, excess cations
from dissolved alkali salts prevent surface protonation, thus explaining the
decreased IN activity in such solutions. In kaolinite, the lack of
exchangeable cations in the crystal lattice explains why the IN activity is
insensitive to the presence of alkali salts (Δaw= const.).
We hypothesize that adsorption of NH3 and NH4+ on the
feldspar surface rather than ion exchange is the main reason for the
anomalous increased Thet in dilute solutions of NH3 or
NH4+ salts. This is supported by the response of kaolinite to
NH3 or NH4+, despite lacking exchangeable ions. Finally, the
dissolution of feldspars in water or solutions leads to depletion of Al and
formation of an amorphous layer enriched in Si. This hampers the IN activity
of andesine the most, followed by sanidine, then eventually microcline, the
least soluble feldspar.
Introduction
Clouds interacting with incoming solar radiation and outgoing longwave
radiation influence the Earth's radiation budget (IPCC, 2013). Ice crystals
are found in cold cirrus clouds as well as mixed-phase clouds, modulating
their radiative properties (Sun and Shine, 1994; Sassen and Benson, 2001).
Moreover, mixed-phase clouds are responsible for precipitation formation via
the Wegener–Bergeron–Findeisen process (Rogers and Yau, 1989; Korolev and
Field, 2008). Ice can either form homogeneously from liquid droplets
supercooled to temperatures below about 237 K or heterogeneously at higher
temperatures with the help of foreign particles, called ice nucleating
particles (INPs), which lower the free energy required to form a critical ice
embryo growing into crystalline ice (Vali et al., 2015). Heterogeneous ice
nucleation (IN) can proceed via various pathways, namely (1) immersion
freezing, when ice forms on an INP suspended inside a supercooled droplet;
(2) condensation freezing, when IN is concurrent with the activation of an
aerosol particle to a cloud droplet and (3) contact nucleation, when ice forms
in a supercooled droplet upon collision with an INP (Pruppacher and Klett,
1994; Vali et al., 2015). These freezing mechanisms all occur under
participation of a liquid phase. Ice may also form at conditions
supersaturated with respect to ice but subsaturated with respect to liquid
water, a process termed deposition nucleation (Vali et al., 2015).
Traditionally, it has been ascribed to deposition of water molecules directly
from the vapor phase onto a solid INP without involvement of a liquid phase.
Though, it has been questioned whether this process actually occurs, or
whether ice forms by pore condensation and freezing (PCF), after stabilizing
water in pores by an inverse Kelvin effect (Marcolli, 2014). The most
important IN mechanisms in mixed-phase clouds are viewed to be immersion and
condensation freezing (Hoose et al., 2008; Ansmann et al., 2009; Twohy et
al., 2010; de Boer et al., 2011).
Various studies have shown that mineral dust, typically composed of
feldspars, clay minerals, quartz, micas, calcite and metal oxides,
constitutes an important class of INPs (Pruppacher and Klett, 1994; Murray et
al., 2011; Hoose and Möhler, 2012; Atkinson et al., 2013; Cziczo et al.,
2013; Kanji et al., 2017). The impact of mineral dusts on cloud properties
has been shown in several observational and modeling studies (Lohmann and
Diehl, 2006; Hoose et al., 2010; Seifert et al., 2010). Feldspars have been
reported to be the most IN active minerals although the individual members of
the feldspar group exhibit very different IN efficiencies in immersion
freezing mode (Atkinson et al., 2013; Zolles et al., 2015; Harrison et al.,
2016; Kaufmann et al., 2016). During long-range transport in the atmosphere,
mineral dust particles can acquire organic and inorganic coatings (Usher et
al., 2003; Sullivan et al., 2007), which may change their IN efficiencies
(Zuberi et al., 2002; Zobrist et al., 2008; Eastwood et al., 2009;
Augustin-Bauditz et al., 2014, 2016; Kumar et al., 2018a; Kanji et al.,
2019). When the INPs are fully covered by a water-soluble coating, IN may
occur by an immersion freezing mechanism, typically after the aerosol
particles experience increasing humidity (Augustin-Bauditz et al., 2014).
Indeed, studies conducted with size-selected Arizona Test Dust (ATD, 300 nm)
coated with sulfuric acid have shown irreversible impairment of its IN
efficiency in both water-subsaturation and supersaturation regimes (Sullivan
et al., 2010b; Niedermeier et al., 2011; Reitz et al., 2011). On the other
hand, Sullivan et al. (2010a) also reported decreased IN efficiency of
size-selected ATD particles (200 nm) coated with nitric acid at
RHw<97 %, which recovered at conditions with RHw≥100 %. In addition, Sihvonen et al. (2014) discussed the hampering
of IN efficiency of clay minerals (kaolinite and montmorillonite) treated
with aqueous acids at conditions below water saturation on the basis of the
reaction products and surface physicochemical changes generated on the
mineral surface due to acid–mineral interaction. Although, Kanji et
al. (2019) reported no effect of secondary organic aerosol (SOA) coating on
the IN efficiency of two types of natural dust samples (Saharan and Asian) in
immersion freezing mode.
Koop et al. (2000) provided observational evidence and theoretical
underpinning that the homogeneous freezing temperatures of aqueous solution
droplets can be described as a function of the water activity of solution
(aw) alone, namely by shifting the ice melting curve by a constant
Δaw to higher aw values. A similar approach has
been used, with a suitably reduced Δaw, to describe the
immersion freezing temperatures of various types of INPs as a function of
aw. While the water-activity-based description of homogeneous IN is
well established, for heterogeneous IN this description assumes implicitly
that there is no interaction between particle surface and solute (Zobrist et
al., 2008). Several studies suggest that the IN efficiency of various INPs
can indeed be approximated by such a water-activity-based description
(Archuleta et al., 2005; Zobrist et al., 2006, 2008; Koop and Zobrist, 2009;
Knopf et al., 2011; Knopf and Forrester, 2011; Knopf and Alpert, 2013; Rigg
et al., 2013). However, recent studies by Whale et al. (2018) and Kumar et
al. (2018a) showed independently that the IN temperatures of some minerals –
including the K-feldspars microcline and sanidine – deviate significantly
from a freezing point line with Δaw= const., with a shift
to higher temperatures in the presence of dilute NH3- and
NH4+-containing solutions and a shift to lower temperatures in the
presence of alkali salts.
In the companion paper Part 1 (Kumar et al., 2018a) we discussed the
interactions of inorganic solutes with the microcline surface and the effects
on its IN efficiency. In this and the companion paper Part 2 (Kumar et al.,
2019) we relate IN on mineral surfaces with the mineral surface properties by
investigating the differences in IN activity of chemically and/or
structurally similar minerals in pure water and aqueous solutions. The
analysis of our previous freezing experiments suggests that heterogeneous IN
does not occur on the whole surface of INPs with a uniform probability.
Rather, there are preferred locations, so-called active sites, which are
responsible for the IN activity of a surface (Fletcher, 1969; Marcolli et
al., 2007; Vali, 2014; Vali et al., 2015; Kaufmann et al., 2017). Estimates
based on classical nucleation theory suggests that minimum surface areas of
these sites need to cover about 10–50 nm2 (Kaufmann et al., 2017). In
the present paper, for the discussion of IN activity of aluminosilicates we
will assume that IN active surface structures need to be of such size. We
will further assume that IN active sites exhibit surface functional groups
that are characteristic of the mineral, but either occurring in a special 2-D
arrangement (e.g., higher density of certain end groups on a flat surface) or
with a special 3-D feature (e.g., a step, crack or wedge), making them IN
active.
In our companion papers (Kumar et al., 2018a, 2019) we discuss the specific
aluminosilicate microcline and various aluminum-free silicas (both amorphous
and crystalline forms) with a special focus on quartz (a crystalline form of
silica). The present study investigates the differences in IN activities of a
number of other aluminosilicates, and offers an overall summary. The
aluminosilicates investigated by immersion freezing include sanidine
(K-feldspar), andesine (Na-Ca-feldspar), kaolinite, micas
(muscovite and biotite) and gibbsite, dispersed in solution droplets
containing ammonia or the inorganic salts ammonium sulfate, ammonium chloride
and sodium sulfate.
MethodologySources of samples, mineralogical composition and particle size
distribution
Feldspar (sanidine and andesine) and mica (muscovite and biotite) samples
were obtained from the Institute of Geochemistry and Petrology of ETH Zurich
and milled with a tungsten carbide disc mill. Particle number size
distributions were obtained with a TSI 3080 scanning mobility particle sizer
(SMPS) and a TSI 3321 aerodynamic particle sizer (APS). The dry particles
were dispersed using a fluidized bed. The detailed size distribution and
mineralogical composition (using X-ray diffraction, XRD) of the feldspars,
kaolinite and muscovite are given in Kaufmann et al. (2016). Rietveld
refinement using Profex software (Döbelin and Kleeberg, 2015) was
performed for a quantitative analysis. The microcline sample
(Si : Al ≈ 3.1; Al : K ≈ 1.4) consists of
86.33 % (±1.71 %) microcline, mixed with orthoclase
(6.18 % ± 0.72 %) and albite (7.49 % ± 0.48 %)
(Kumar et al., 2018a). The sanidine sample proved to be pure sanidine with
Si : Al ≈ 3.1 and Al : K ≈ 1.6 while andesine
proved to be pure andesine with Si : Al ≈ 1.7 and
Ca / (Ca + Na) = 64 %. Note that andesine would be a
labradorite based on the Ca / (Ca + Na) ratio, but
crystallographically it fits best with an andesine. Also note that the
andesine sample is termed as “plagioclase” in Kaufmann et al. (2016).
Biotite shows a bimodal particle size distribution with mode diameters of
241 nm and 1.7 µm (see Supplement). XRD analysis showed no traces of
tungsten carbide in any of the milled samples. The kaolinite used in this
study was KGa-1b from the Clay Mineral Society. Gibbsite
(Al(OH)3⋅xH2O, < 45 µm) was obtained from
Sigma-Aldrich.
Emulsion freezing experiments with mineral dusts freshly suspended
in pure water or solutions
We described the general setup of immersion freezing experiments in Part 1 of
this series of papers (Kumar et al., 2018a). Here we repeat essential aspects
for convenience. The experiments were carried out with the differential
scanning calorimetry (DSC; TA Instruments, Q10) setup (Zobrist et al., 2008).
Sanidine and andesine suspensions (2 wt %), kaolinite suspensions
(5 wt %), mica (muscovite and biotite; 5 wt % and 10 wt %)
suspensions and gibbsite (10 wt %) suspensions in water (molecular
biology reagent water from Sigma-Aldrich) were prepared with varying
concentrations (0–20 wt %) of (NH4)2SO4 (Sigma-Aldrich, ≥ 99 %), NH4Cl (Sigma-Aldrich, ≥ 99.5 %),
Na2SO4 (Sigma-Aldrich, ≥ 99 %) and diluted NH3
solutions (Merck, 25 %). To avoid particle aggregation, the suspensions
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) taken in a ratio of
1 : 4 were mixed using a rotor–stator homogenizer (Polytron PT 1300D with
a PT-DA 1307/2EC dispersing aggregate) for emulsification (40 s at
7000 rpm); 4–10 mg of this emulsion was placed in an aluminum pan, which
was hermetically closed; and then, following the method developed and
described by Marcolli et al. (2007), three freezing cycles in the DSC were
performed. 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). Emulsions prepared by this procedure exhibit droplet
size distributions peaking at diameters of about 2–3 µm in number
and a broad distribution in volume with highest contributions from particles
with diameters between 4 and 12 µm similar to the ones shown in
Fig. 1 of Marcolli et al. (2007), Pinti et al. (2012) and Kaufmann et
al. (2016). For a clear heterogeneous signal, dust particles need to be of
similar size or smaller than the droplets. Large particles
(> 10 µm) present in dust samples contribute significantly to
the dust mass but hardly to the heterogeneous freezing signal.
Typically, a DSC thermogram of the cooling cycle performed with an emulsion
containing INPs features two freezing signals, as depicted in Kumar et
al. (2018a). The first peak occurring at a warmer temperature displays the
heat release accompanied by heterogeneous freezing and the second peak
occurring at a colder temperature is due to homogeneous freezing. The
freezing temperatures (Thet and Thom) are determined as
the onset of the freezing peaks (i.e., intersection of the tangent drawn at
the point of greatest slope at the leading edge of the thermal peak with the
extrapolated baseline). Droplets with diameters of about 12 µm are
considered to be relevant for the freezing onset. The loading of these
droplets with particles depends on the particle size distribution and the
suspension concentration. For 2 wt %–5 wt % suspensions,
12 µm droplets contain about 100–1000 particles (Kumar et al.,
2018a). The melting temperature (Tmelt) was determined as the
maximum of the ice melting peak. For the investigated samples, average
precision in Thet was ±0.1 K with maximum deviations not
exceeding 0.5 K. Precisions of Thom and Tmelt are
precise within ±0.1 K.
The heat release is related to the frozen water volume and is given by the
integral of the heat signal over time. Since the enthalpy of freezing is
temperature dependent, this evaluation is only approximate (Speedy, 1987;
Johari et al., 1994). Fhet is defined as the ratio of the
heterogeneous freezing signal to the total freezing signal (heterogeneous and
homogeneous). More details about the evaluation of Thet and
Fhet can be found in Kumar et al. (2018a). Absolute uncertainties
in Fhet are on average ±0.02 and do not exceed ±0.1 in
cases where the heterogeneous freezing signal is clearly distinguishable from
the homogeneous freezing signal. It is important to highlight that
Fhet carries larger uncertainties (>±0.1) in cases where
heterogeneous freezing signals are weak and overlap (forming a flattened
shoulder; see the Supplement) with the homogeneous freezing signal (e.g., in
case of strong hampering of IN ability of feldspars in alkali solutes; see
Sect. 3.1.1). Spikes occurring before the appearance of the heterogeneous
freezing signal are excluded from the evaluation as they originate from
single droplets (mostly between 100 and 300 µm with some up to
500 µm in diameter) in the tail of the droplet size distribution,
which are orders of magnitude larger in volume than the average droplets, and
not representative for the sample.
For feldspars, freezing experiments were at least performed in duplicates
with separate emulsions, prepared from a single suspension for each
concentration and the means are reported. For kaolinite and micas, freezing
experiments were performed with emulsions prepared from at least two separate
suspensions for each solute concentration and means are reported.
Representative DSC thermograms of all experiments are shown in the
Supplement.
Aging experiments with kaolinite suspended in pure
water or solutions
Similar to our experiments with microcline (Kumar et al., 2018a), we let
kaolinite (5 wt % suspension) age in pure water, ammonia solution
(0.005 molal), and ammonium sulfate solutions (0.1 wt % and
10 wt %) over a period of 5 days and its IN activity was tested during
this period in emulsion freezing experiments with the DSC setup. For each
solute concentration two separate suspensions were prepared and aged. Small
portions were taken from the suspension and emulsified for freezing
experiments (as described in the previous section) on the day of suspension
preparation (fresh) and the subsequent 5 days in order to assess the
long-term effect of ammonia- and ammonium-containing solutes on the IN
efficiency of kaolinite.
Results and discussionFeldspars
Feldspars are crystalline aluminosilicates, primarily of igneous origin, with
the general formula XAl1-2Si3-2O8, often written as
XT4O8 with T= (Al, Si) in tetrahedral
coordination with oxygen. X represents an alkali or alkaline earth
metal, acting as a charge-compensating cation. The tetrahedra with Al at the
center carry single negative charges that are compensated by K+/Na+
(for one Al atom) or Ca2+ (for two Al atoms). Microcline, sanidine
and orthoclase (all K-rich feldspars) are polymorphs and form the potassium
feldspar group (Hofmeister and Rossman, 1983). Ideally, all of them exhibit a
Si : Al ratio of 3 : 1, while our microcline and sanidine samples exhibit
a Si : Al ratio of 3.1. In sanidine, which forms at high temperatures, Al
and Si are fully disordered with Al taking random positions in the
aluminosilicate framework leading to monoclinic symmetry. In microcline, Al
is fully ordered leading to triclinic symmetry while orthoclase takes an
intermediate position with Al and Si being partially disordered (Brown and
Parsons, 1989). Despite the difference in charge, Na+ and
Ca2+ are sufficiently similar in ionic radius so that there is a
complete solid solution series, the plagioclase series, between albite
(NaAlSi3O8) and anorthite (CaAl2Si2O8), with intermediate
members divided arbitrarily on the basis of anorthite contribution (see
Fig. 1). In this series, the Ca : Na and Si : Al ratios are linked with
the Al content that changes according to the Ca content. Andesine takes an
intermediate position of (Na-Ca)-feldspars with a Ca / (Ca + Na) = 30 %–50 %
(Greenwood and Earnshaw, 1998). Our andesine (Si : Al ≈ 1.7)
crystallographically fits best with an andesine though it exhibits
Ca / (Ca + Na) = 64 %.
Feldspar classification diagram. Numbers are contributions of the
end-member minerals in wt % (adapted from Greenwood and Earnshaw, 1998).
Dependence of the heterogeneous freezing temperatures and
heterogeneously frozen fractions on water activity
Figure 2a and b show mean heterogeneous (Thet) and homogeneous
freezing onsets (Thom) and ice melting temperatures
(Tmelt) of the feldspars sanidine and andesine for all investigated
solutes (NH4Cl, (NH4)2SO4, Na2SO4) as a function
of aw (water activity of solution). The water activity is defined
as the ratio of the equilibrium vapor pressure of water over the flat surface
of the solution and the saturation vapor pressure over the flat surface of
pure water at the same temperature. We derive aw from the melting
point depression measured by DSC during the heating cycle (thus, the measured
melting points, Tmelt, lie by definition exactly on the melting
curve). This procedure was not applicable to Na2SO4, because above
the eutectic concentration of 4.6 wt % a hydrate of Na2SO4
crystallizes together with ice (Negi and Anand, 1985). Therefore, water
activities for Na2SO4 solutions have been calculated based on the
solute concentration using the AIOMFAC thermodynamic model at 298 K (Zuend
et al., 2008, 2011). The homogeneous freezing curve (dotted black line) is
obtained by a constant shift of the melting curve by Δawhom(T)=0.294, calculated to best fit the current
dataset (see Kumar et al., 2018a, for more details of the derivation). This
offset is in good agreement with Δawhom(T)=0.305 found by Koop et al. (2000). Following Koop et al. (2000), we assume
aw to be temperature independent between Thom and
Tmelt.
Measured freezing temperatures (a, b) and heterogeneously
frozen fractions (c, d) of emulsion freezing experiments with
2 wt % feldspar, namely sanidine (a, c) and
andesine (b, d), in various solutions (color coded).
(a, b) Heterogeneous freezing onset temperatures, Thet
(filled solid symbols connected by thin lines to guide the eye); homogeneous
freezing onset temperatures, Thom (open symbols at T<237 K);
and ice melting temperatures, Tmelt (open symbols at T>265 K)
as functions of water activity of solutions, aw, for various
solutes (symbols and colors see insert). Dash-dotted black line: ice melting
point 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)=0.294. Solid black line: horizontally shifted from the ice melting curve by
Δawhet(T)=0.264 and 0.254 for
sanidine (a) and andesine (b), respectively, with offsets
derived from the heterogeneous freezing temperature of the suspension of the
mineral in pure water (filled black square at aw=1). Symbols are
the mean of at least two separate emulsion freezing experiments. Difference
between the two measurements plus instrumental uncertainty in Thet
and aw are smaller than the symbol size (see Sect. 2.2).
(c, d) Heterogeneously frozen fraction, Fhet, as a function
of aw, for sanidine (c) and andesine (d).
Measurement difference and uncertainty in Fhet do not exceed
±0.1.
A constant offset Δawhet is also applied to the
heterogeneous freezing temperatures. Here, the offset in aw is
chosen so that the heterogeneous freezing line passes through the freezing
temperature of the pure water case. This yields the solid black lines with
Δawhet=0.264 for sanidine and Δawhet=0.254 for andesine, which will be referred to as
ThetΔaw,san(aw) and
ThetΔaw,and(aw), respectively. The
heterogeneous freezing behavior would be expected to follow these curves if
specific chemical interactions between the solute and the ice-nucleating
surface were absent, so that the only effect of the solute is a freezing
point depression. However, as can be seen from Fig. 2a and b, the measured
heterogeneous freezing onset temperatures, Thet, deviate from the
ThetΔaw(aw) curves for both feldspars. For
both NH4+ solute cases ((NH4)2SO4 and NH4Cl),
sanidine shows a strong increase in Thet (by up to ≈ 7 K
with respect to the pure water case) at low solute concentrations
(aw>0.99). This increase is followed by a decrease at higher
concentrations back to ThetΔaw,san(aw)
in the case of NH4Cl solutions (aw≈0.925) and even
slightly below ThetΔaw,san(aw) in the case
of (NH4)2SO4 solutions (aw≤0.98). Andesine also shows
an increase in Thet, though smaller in magnitude (≈ 2.6 K with respect to the pure water case), but this enhancement persists
to higher NH4+ concentrations. The observed enhancements with
respect to ThetΔaw,and(aw) are
∼ 5.1 and ∼ 4.0 K for NH4Cl and (NH4)2SO4,
respectively.
In contrast to NH3/NH4+ solutions, freezing experiments in the
presence of Na2SO4 as a non-NH4+ solute show a strong
decrease in Thet below ThetΔaw(aw) by ∼ 1.7 K and ∼ 2.4 K for sanidine
and andesine, respectively, at aw≈0.99. No discernible
heterogeneous freezing signal was observed for higher Na2SO4
concentrations (aw<0.99).
Figure 2c and d show the heterogeneously frozen fraction Fhet (the
ratio of the heterogeneous freezing signal to the total freezing signal) as a
function of aw for sanidine and andesine. In dilute
NH4+-containing solutions, sanidine shows an enhancement in
Fhet up to ≈ 0.75 at aw≈0.99 compared
to the suspension in pure water (Fhet≈0.42 at aw=1). With decreasing water activities, this enhancement reverses into a
decline (for aw<0.99 in case of (NH4)2SO4 and for
aw<0.98 in case of NH4Cl). In contrast, Fhet of
andesine is less influenced by the presence of NH4+ solutes since
there is no significant enhancement at low concentrations and much less of a
decrease at higher concentrations.
For both feldspars, the addition of Na2SO4 leads to a decrease of
Fhet for the lowest solute concentration (aw≥0.99)
and even to an almost complete inhibition of the IN activity for aw<0.99.
Comparison with previous IN studies performed with feldspars
Thet and Fhet trends for sanidine and andesine are
similar to the ones of the K-feldspar microcline reported in our previous
study, Kumar et al. (2018a), but with significant variations. All
investigated feldspars show an increase in Thet at low
concentrations of NH4+ solutes (aw≥0.99). This
increase is highest for sanidine and lowest for andesine. On the other hand,
the decrease of Thet at higher solute concentration is most
pronounced for microcline with values falling below the prediction from the
water activity-based description. In case of non-NH4+ solutes,
Thet decreases below the prediction from the water activity-based
approach for all investigated feldspars even at low solute concentrations.
Similar trends among the feldspars are also observed for Fhet.
These findings are also in general agreement with droplet freezing
experiments by Whale et al. (2018) who observed an increase of active-site
densities for microcline and sanidine in the presence of NH4+
solutes and a decrease in alkali halide solutions.
The IN activity of feldspars has been investigated in numerous studies. All
studies agree that the IN activity of K-feldspars is high among mineral
dusts (Atkinson et al., 2013; Harrison et al., 2016; Kaufmann et al., 2016;
Peckhaus et al., 2016; Welti et al., 2019). Kaufmann et al. (2016) and Welti
et al. (2019) found that the IN activity of microcline is superior compared
with other K-feldspars. Several studies have been dedicated to microcline.
Niedermeier et al. (2015) and Burkert-Kohn et al. (2017) found frozen
fractions of 0.5 at 244–245.5 K for condensation freezing on 300 nm
microcline particles that were strongly reduced when the samples were aged
under acidic conditions. A strong reduction of IN activity when feldspars
were coated with sulfuric acid was also observed by Kulkarni et al. (2014).
The IN activity of microcline after aging in pure water seems to depend on
the specific sample. Samples with the highest onset freezing temperatures
also show the strongest reduction in IN activity after aging, indicating that
the best active sites might also be the most labile ones (Harrison et al.,
2016; Peckhaus et al., 2016). Harrison et al. (2016) found that one sample of
Na-feldspar (albite) was similarly active as microcline when freshly
suspended in water, but lost its high IN activity after having been suspended
for months in water, indicating that its IN activity may be related to sites
with high solubility that are lost over time by dissolution. Kiselev et
al. (2017) found that high-energy (100) surface patches of K-rich feldspars
are best suited for deposition growth of aligned ice crystals below water
saturation. However, it is not clear how resistant these high-energy sites
are when immersed in water. Whale et al. (2017) related the exceptionally
high IN ability of K-feldspars to microtextures, giving rise to topographic
features with high IN temperatures. The IN site densities that they
investigated with their setup were in the range from 10-1 to
103 cm-2 with IN temperatures up to 271 K. With the DSC emulsion
freezing experiments, we investigate the properties of sites that are common
to submicrometer particles, i.e., with surface densities in the range from
1010 to 105 cm-2 nucleating ice up to 252 K. While the sites
investigated by Whale et al. (2017) may be well correlated with perthitic
structures, it is unlikely that such features are common enough in
submicrometer particles to account for the IN activity observed in our
emulsion freezing experiments. Welti et al. (2019) found a clear decrease in
IN activity for all investigated feldspars (microclines, orthoclases,
sanidine, pericline and labradorites) with decreasing particle size from
800–50 nm, indicating the better active sites are rarer.
Surface ion exchange
In pure water, the native charge-balancing surface cations (K+,
Na+, Ca2+) immediately undergo cation exchange by
H+/H3O+ (Chardon et al., 2006; Lee et al., 2008), with hardly any
simultaneous structural surface damage (Busenberg and Clemency, 1976). The
native cation may also participate in ion exchange with an externally added
cation depending on the size and charge compatibility of the latter with the
crystal structure (Auerbach et al., 2003; Belchinskaya et al., 2013).
Ammonium ions not only have a strong preference for cation exchange with
K-feldspars and (Na-Ca)-feldspars but part of them remain fixed to the surface in nonexchangeable
form (Nash and Marshall, 1957; Barker, 1964). In Kumar et al. (2018a), we
have shown that the ion exchange is unlikely as a reason for the enhanced IN
efficiency of microcline in dilute NH4+-containing solutions.
Rather, hydrogen bonding of ammonia/ammonium with the surface hydroxyl groups
seems to provide better ice-like template sites for the incoming water
molecules (Anim-Danso et al., 2016), leading to an increase in IN efficiency.
Aluminum depletion and surface dissolution
When suspended in water or solutions,
feldspars undergo slow dissolution, followed and accompanied by the
precipitation of more stable phases like kaolinite, gibbsite or halloysite
(Stillings and Brantley, 1995). Depending on the specific feldspar and
solution pH, steady-state Si dissolution rates vary between 10-10 and
10-14 moles m-2 s-1 (Crundwell, 2015) with lowest values at
near-neutral conditions. The solution pH during the emulsion freezing
experiments reported in Fig. 2 are close to neutral with pH values ranging
from 5.5 to 6.7. For the feldspars investigated in this study, steady-state
dissolution rates are lowest for microcline with 4×10-14–8×10-14 moles m-2 s-1 at pH ∼ 6 (Crundwell, 2015)
followed by sanidine with a rate of ∼2×10-13 moles m-2 s-1 at near-neutral conditions (Crundwell,
2015), while dissolution of andesine occurs with a steady-state rate of
10-12–10-11 moles m-2 s-1 at pH ∼ 8
(Gudbrandsson et al., 2014). Hereby, initial dissolution rates of freshly
suspended feldspar may be higher by up to 3 orders of magnitude than the
rates at steady state (Zhu et al., 2016). With these rates, dissolution is in
the range of one monolayer within the timescale of a DSC freezing experiment
(1–1.5 h) for andesine, and below it for sanidine and microcline.
At near-neutral conditions, the dissolution proceeds via protonation of the
oxygen of ≡Al-O-Si≡ bridges with subsequent release of
Al3+ resulting in an incongruent (deviation of ratio of released Si
to Al from stoichiometric ratio) initial dissolution for most feldspars when
they are freshly suspended in water (Oelkers and Schott, 1995; Oelkers et
al., 2009), and leading to a ≡Si-OH rich surface (Oxburgh et
al., 1994; Stillings and Brantley, 1995; Oelkers et al., 2009). Oelkers et
al. (2009) found aluminum surface depletion to occur readily in their 20 min
titration experiments with the Na-feldspar albite. Thus, the feldspar
surfaces in pure water suspensions can be considered at least partly or even
completely depleted in aluminum with the dangling bonds replaced by silanol
groups within the timescales of our experiments. The dissolution incongruence
with respect to Al atoms depends on the Si/Al ordering and the
Si : Al ratio of the feldspar lattice (Yang et al., 2014a, b). Yang et
al. (2014b) investigated the stoichiometry of feldspar dissolution under
acidic conditions (pH 1.8) and found that microcline with Si : Al = 3.0
dissolved with a ratio of Si : Al = 2.1. Their sanidine sample with
Si : Al = 2.87 released Al with Si :Al = 1.36, while their
andesine sample with Si : Al = 1.95 dissolved at a ratio
Si : Al = 0.76 : 1. This dissolution incongruence leads to a layer
leached in Al, with a thickness that depends on the specific feldspar and
increases with decreasing pH. At steady state, this Al-depleted layer is
6.6–8.5 nm thick for microcline at pH 1 (Lee et al., 2008), but only
1–2 nm at pH 3 (Stillings and Brantley, 1995). For andesine, it reaches
60–120 nm at pH 3.5 and still 15–30 nm at pH 5.7 (Muir et al., 1990).
Since Al dissolution at steady-state conditions needs to occur through this
layer, the andesine surface is considered to consist of somewhat loosened and
distorted feldspar chains resulting in a porous amorphous-like structure
(Marshall, 1962; Stillings and Brantley, 1995; Zhang and Lüttge, 2007).
At pH 1, the first 5 nm of surface layer of microcline were found to be
amorphous (Lee et al., 2008). Since amorphous silica shows barely any IN
activity (see also the companion paper, Kumar et al., 2019), the loss of IN
activity of microcline in the presence of H2SO4 (Kumar et al.,
2018a) may be explained by the presence of an amorphous silica layer. Since
Thet correlates with the thickness of the leached layer reported
for the investigated feldspars, we hypothesize that amorphous surface layers
exceeding a few nanometers in depth hamper the IN activity of feldspars.
Surface charge and surface protonation
Feldspars exhibit a negative surface charge over a wide pH range with a point
of zero charge (PZC) < 2 (Karagüzel et al., 2005; Vidyadhar and
Hanumantha Rao, 2007). Bringing the feldspar surface closer to the PZC by
adding H2SO4 did not increase Thet but decreases
Fhet due to surface degradation as was shown for microcline (Kumar
et al., 2018a). This shows that surface charge is only one among several
factors influencing IN activity. We discuss the effect of surface charge on
IN activity of mineral surfaces in more detail in Sect. 4.
Surface hydroxyl groups are considered to promote IN because they can form
hydrogen bonds with water molecules and bring them in a suitable orientation
for IN. In a recent molecular simulation study, Pedevilla et al. (2017) found
that the OH density rather than a specific OH pattern is a useful descriptor
of IN ability. The relevance of surface hydroxylation is in agreement with
our observations that Na+ ions added to an aqueous feldspar
suspension decrease Thet and Fhet for all investigated
feldspars since they replace the protons (Oelkers et al., 2009) thus
decreasing the surface hydroxylation (see Fig. 2 of this paper; Kumar et al.,
2018a). Ammonium on the other hand, can form hydrogen bonds with water
molecules and therefore does not reduce the IN activity. In addition,
NH3/NH4+ is not only involved in ion exchange but also binds to the
feldspar surface (Kumar et al., 2018a). Therefore, it increases the capacity
for hydrogen bonding even more and leads to an increased IN activity. A
strong increase of proton concentration (low pH), on the other hand, hampers
or even totally impedes IN because it promotes aluminum depletion and the
formation of an amorphous silica surface layer.
Influence of sulfate
While we consider solute effects to be dominated by cations at low
concentrations, the anions seem to become more relevant at higher
concentrations. Aqueous phase complexes of dissolved aluminum with sulfate
enhance the solubility of aluminum. Surface complexes of sulfate with surface
aluminum can impede or enhance dissolution of aluminum. Bidentate surface
complexes with sulfate do not enhance surface protonation and should
stabilize aluminum at the surface, while the formation of mononuclear
complexes should facilitate surface protonation and help dissolution of
aluminum (Min et al., 2015). The effect of sulfate should therefore depend on
the bonding strength of aluminum to the feldspar surface, which depends on
the Si / Al order and Si : Al ratio. Min et al. (2015) found that the
dissolution rate of anorthite, a (Na-Ca)-feldspar with Si : Al = 1, is enhanced by the formation
of monodentate complexes between the aluminum surface sites and sulfate. This
seems to be also the case for the (Na-Ca)-feldspar andesine. On the other hand, surface complexation of
aluminum sites with sulfate seems to decrease the removal of aluminum and
block sites for IN in the case of microcline, while sanidine takes an
intermediate position.
Same as Fig. 2, but for kaolinite (5 wt %).
(a) Homogeneous ice freezing curve (dotted black line) characterized
by a constant offset Δawhom(T)=0.294. Ice
melting curve (dash-dotted black line) and the heterogeneous freezing curve
(solid black line) horizontally shifted from the ice melting curve by
Δawhet(T)=0.272 derived from the
heterogeneous freezing temperature of the suspension of the mineral in pure
water (filled black square at aw=1). Measured differences and
instrumental uncertainties in Thet and aw are smaller
than the symbol size (see Sect. 2.2). (b)Fhet
(heterogeneously frozen fraction) as a function of water activity of the
solutions. Measurement difference and uncertainty in Fhet do not
exceed ±0.1.
Kaolinite and gibbsite
Kaolinite (Al4[Si4O10](OH)8) is a layered 1 : 1 (one
tetrahedral sheet and one octahedral sheet forming stacked T–O layers)
dioctahedral phyllosilicate and one of the most abundant clay minerals found
in the Earth's crust (Murray, 1991). Each layer consists of a sheet of
SiO4 tetrahedra forming six-membered silicate rings connected via
common oxygen atoms to a sheet of AlO6 octahedra forming
four-membered aluminate rings. Weak hydrogen bonds between Al-OH and
Si–O–Si groups provide interlayer attraction preventing ions or molecules
from entering the interlayer space and constituting the non-swelling nature
of this mineral. Since ionic substitution of Si4+ with
Al3+ is absent in the regular kaolinite lattice, exchangeable ions
only occur at defects or at edges (Bibi et al., 2016). Cleavage of kaolinite
preferentially occurs along the basal plane, resulting, on one side, in a
hydroxylated Al surface with Al atoms arranged in a hexagonal pattern and on
the other side in a siloxane surface with Si–O–Si bridges forming hexagonal
rings (Bear, 1965).
Although particles of gibbsite are of rare abundance in atmospheric dusts
(Boose et al., 2016; Kaufmann et al., 2016), we investigate this mineral
because the hydroxylated Al sheets of kaolinite are structurally identical
with gibbsite. Gibbsite is an aluminum hydroxide of the general formula
Al(OH)3 that exists as different polymorphs. The basal surface of
gibbsite is comparable to the aluminol surface of kaolinite and, therefore,
often used as a model surface to elucidate the physicochemical properties of
the Al surface of kaolinite (Liu et al., 2015; Kumar et al., 2016).
Dependence of the heterogeneous freezing temperatures and frozen
water volume fraction on water activity
Figure 3a presents Thet, Thom and Tmelt as a
function of aw for kaolinite. The offset in aw applied to
shift the melting curve so that it passes through the freezing temperature of
the pure water case for kaolinite is Δawhet=0.272, here referred to as ThetΔaw,kaol(aw). In the presence of NH4+ solutes,
kaolinite shows an increase (≈ 2.4 K) in Thet compared
to ThetΔaw,kaol(aw) even at the lowest
investigated solute concentration (0.1 wt %; aw>0.99). The influence of NH3 on the IN activity was found to be
similar to the one of the NH4+-containing solutes. This enhancement
relative to ThetΔaw,kaol(aw) persists
over the complete concentration range probed in this study, similar to the
case of andesine. For aw<0.9, kaolinite shows an increase up to
2.9, 5.5 and 6.9 K in Thet compared to ThetΔaw,kaol(aw) in solutions of (NH4)2SO4,
NH4Cl and NH3, respectively. In contrast to the feldspars, in
Na2SO4 solutions Thet follows ThetΔaw,kaol(aw) within the error range for all investigated
concentrations.
Figure 3b shows Fhet as a function of aw for kaolinite.
Interestingly, kaolinite shows a strong enhancement in Fhet to
0.75–1.00 compared to the pure water case (Fhet≈0.52) in
the presence of NH3 and NH4+ solutes over the complete
investigated concentration range (aw=1–0.88). This enhancement
is strongest for ammonia solutions, reaching Fhet≈1
(Fhet≈1 was assumed when it was not possible to discern a
homogeneous freezing signal; see Fig. 5) at aw=0.98–0.96,
compared to Fhet≈0.5 in pure water. In contrast,
Fhet seems to be unaffected by Na2SO4 within experimental
uncertainties over the measured concentration range.
DSC thermograms of 10 wt % gibbsite suspended in ammonia
solution droplets (a) of concentrations from 0 to 1 molal
(aw=1–0.981) and (NH4)2SO4 solution
droplets (b) from 0 to 10 wt % (aw equal to 1–0.961). All curves are
normalized such that the areas under the heterogeneous and homogeneous
freezing curves sum up to the same value. Gibbsite shows no discernible
heterogeneous freezing signal in pure water and starts to show a weak
heterogeneous freezing signal when suspended in ammonia solution (dashed
black line connects the onset of heterogeneous freezing signals), while no
heterogeneous IN is observed for gibbsite suspended in (NH4)2SO4.
Figure 4 shows the DSC thermograms of the emulsion freezing experiments
carried out with 10 wt % gibbsite suspensions in pure water, in
NH3- (panel a) and in (NH4)2SO4-containing solutions
(panel b). Gibbsite, exhibiting no heterogeneous freezing signal during
emulsion freezing experiments in pure water, starts to show a weak
heterogeneous freezing signal with increasing NH3 concentration
(indicated by the dashed black line in Fig. 4a). On the other hand, no
heterogeneous freezing signal can be observed for gibbsite suspended in
(NH4)2SO4 solutions (0.05 wt %–10 wt %; aw
equal to 0.996–0.961.
Previous studies on the IN activity of kaolinite
Kaolinite has shown IN activity in various freezing modes (Hoose and
Möhler, 2012; Murray et al., 2012; Kanji et al., 2017). Many studies
investigated K-SA kaolinite from Sigma-Aldrich (Zuberi et al., 2002;
Lüönd et al., 2010; Burkert-Kohn et al., 2017), while others used the
KGa-1b kaolinite from the Clay Mineral Society with high mineralogical purity
(96 %, minor impurities of anatase, crandallite, mica, and illite)
(Murray et al., 2011; Pinti et al., 2012; Wex et al., 2014; Kaufmann et al.,
2016). Pinti et al. (2012) found with their emulsion freezing experiments
that K-SA exhibits a second freezing peak at higher temperatures with onset
at ∼248 K that could be related to impurities of feldspars (Atkinson et
al., 2013). On the other hand, KGa-1b features only one heterogeneous
freezing peak with an almost constant onset at ∼ 240 K in emulsion
freezing experiments irrespective of the kaolinite concentration in the
aqueous suspension (Pinti et al., 2012), which is an indication of the
homogeneous composition and mineralogical purity of the sample. Kaufmann et
al. (2016) evaluated the active particle fraction of kaolinite KGa-1b to be
only about 0.04 compared to values of 0.54–0.64 determined for microcline. A
lower active fraction of kaolinite compared with the feldspar microcline was
also confirmed by single particle immersion freezing experiments
(Burkert-Kohn et al., 2017).
DSC thermograms of 5 wt % kaolinite particles suspended in
ammonia solution droplets (0–4.5 molal; aw equal to 1–0.922). All
curves are normalized such that the areas under the heterogeneous and
homogeneous freezing curves sum up to the same value.
The DSC curves for kaolinite suspended in NH3 solution (Fig. 5) show
that for a certain NH3 concentration range (0.5–2 molal;
aw equal to 0.988–0.955) the heterogeneous freezing signal totally
dominates the homogeneous one, indicating that initially IN inactive
particles (contributing to the homogeneous freezing DSC signal) became IN
active after addition of ammonia. Salam et al. (2007, 2008) have previously
reported improved IN efficiency of the clay mineral montmorillonite below and
above water saturation after the particles were exposed to NH3 gas.
Which kaolinite surface is responsible for the IN activity?
As a sheet aluminosilicate, kaolinite exhibits two basal faces – the
siloxane tetrahedral and the alumina octahedral – and hydroxylated edges.
The overall surface charge of kaolinite in pure water is negative with a
PZC < 2. The PZC increases when kaolinite is suspended in salt solutions
(Yukselen-Aksoy and Kaya, 2011). However, this overall value determined from
bulk experiments does not need to apply to the individual faces of
kaolinite.
Recent surface force measurements have been able to probe the surface
properties of the kaolinite faces individually. These measurements evidenced
a negative charge of the siloxane surface with a PZC below pH 4 (Gupta and
Miller, 2010). Isomorphic substitution of Si(IV) for Al(III) in tetrahedral
layers is considered the main reason for the net permanent negative surface
charge, which is balanced by the adsorption of cations (e.g., Na+,
K+, Ca2+) (Cashen, 1959; Grim, 1968; Bolland et al., 1980).
The oxygen atoms on the siloxane surface are relatively weak electron donors
and incapable of forming hydrogen bonds with water molecules. Therefore, the
near-surface water molecules interact predominantly with each other so that
the surface is considered nearly hydrophobic (Jepson, 1984; Giese and van
Oss, 1993; Braggs et al., 1994).
Unlike the siloxane surface, the hydroxyl-rich surface of the Al-octahedral
layer is hydrophilic and forms hydrogen bonds with water (Schoonheydt and
Johnston, 2006; Yin et al., 2012). The alumina face was found to be
negatively charged at pH ≥ 8 and positively charged at pH ≤ 6
indicating a PZC of pH 6–8 (Gupta and Miller, 2010). The pH-dependent,
nonpermanent surface charge arises from protonation or deprotonation of surface
hydroxyl groups. While silanol groups mainly contribute to negative charge
through the formation of SiO- by deprotonation, aluminol groups
undergo both protonation at low pH and deprotonation at high pH (Abendroth,
1970; Bleam et al., 1993).
Disruption of the kaolinite sheets leads to dangling oxygen atoms at the
crystal edges that take up water molecules to transform into hydroxyl
groups. This process produces a surface rich in silanols and aluminols, which
terminate tetrahedral and octahedral layers, respectively (Brady et al.,
1996; Liu et al., 2013). The PZC of the kaolinite edge surface was found to
be below pH 4. The negative charge is considered to arise from deprotonation
of Si sites, which is only partly counteracted by positively charged Al sites
(Brady et al., 1996). The reactive hydroxyl groups, due to their charge, have
the potential to chemisorb certain other ionic species (Schoonheydt and
Johnston, 2006, 2013). Such hydroxyl groups are also present at steps and
cracks.
First principles calculations and molecular dynamics simulations have
indicated that hydrogen bonding to surface hydroxyl groups is an essential
factor for providing IN activity to mineral surfaces (Hu and Michaelides,
2007; Sosso et al., 2016a; Glatz and Sarupria, 2018). The absence of sites
for hydrogen bonding make the regular siloxane surface an unlikely candidate
for IN (Freedman, 2015). The alumina surface, on the other hand, is OH-rich
and has been probed in several modeling studies for its capability to
nucleate ice. Based on a density-functional theory study, Hu and
Michaelides (2007) concluded that the basal surfaces of kaolinite do not
support epitaxial multilayer ice growth, rather they may promote the growth
of the prism face of ice (Cox et al., 2013). Monte Carlo simulations carried
out by Croteau et al. (2008, 2009) suggested that a rigid Al surface of
kaolinite (001) is incapable of orienting water molecules into ice-like
configurations, instead trenches and surface defects were suggested to be
responsible for IN (Croteau et al., 2010a, b). Zielke et al. (2015) showed
that both the Al surface and the Si surface can nucleate ice, by
reorientation of hydroxyl groups in the former case and by ordered
arrangement of hexagonal and cubic ice layers joined at their basal planes in
the latter case. Glatz and Sarupria (2018), based on molecular dynamics
simulations on kaolinite-like surfaces, argued that lattice matching and
hydrogen bonding are necessary but not sufficient conditions for IN. Sosso et
al. (2016b) found that IN on the hydroxylated basal surface of kaolinite
proceeds exclusively via the formation of the hexagonal ice polytype but that
this process crucially depends on very small structural changes upon
kaolinite surface relaxation in the molecular dynamics simulations.
While most of these studies focused on the basal planes, experimental
evidence suggests that the edges are preferred locations for IN. Indeed, Wang
et al. (2016) found in IN experiments with a cell coupled to an environmental
scanning electron microscope that ice preferentially nucleates at the edges
of kaolinite particles when they were exposed to increasing relative
humidity. The similar response of kaolinite and feldspars to
NH3/NH4+ solutes indicates a similar chemical composition of IN
active sites on the surfaces of these two mineral types, suggesting that the
kaolinite edges, rich in both aluminol and silanol groups should be
responsible for the observed IN activity.
Similar to the hydroxylated Al layers of kaolinite, gibbsite consists of
stacked sheets of linked octahedra of aluminum hydroxide. Therefore, the
gibbsite surface is often taken as a model for the Al surface of kaolinite
(Liu et al., 2015; Kumar et al., 2016). The PZC of gibbsite falls in the pH
range from 7.5 to 11.3 (Kosmulski, 2009; Liu et al., 2015), thus in a
similar, yet somewhat higher, range than the one of the Al surface of
kaolinite (pH 6–8). DSC thermograms of the emulsion freezing experiments
with gibbsite (Fig. 4) show no heterogeneous freezing signal in pure water
and (NH4)2SO4 solutions up to 10 wt %. Only when suspended in
NH3 solutions (0.05–1 molal, aw equal to 0.996–0.981), a weak
heterogeneous freezing signal developed. This indicates that NH3
hydrogen bonded to the hydroxylated Al surface provide IN activity to
gibbsite, while the positively charged NH4+ does not adsorb on the
positively charged Al surface. Given the inability of gibbsite to nucleate
ice in water and (NH4)2SO4, we conclude that the IN activity of
kaolinite rather stems from the edges as suggested by Wang et al. (2016) and
Croteau et al. (2010a). This conclusion is further supported by the finding
that kaolinite indeed adsorbs NH3 by forming hydrogen bonds with the
hydroxyl groups at the edges (James and Harward, 1962).
Development of Thet(a) and
Fhet(b) for 5 wt % kaolinite suspended in water,
10 wt % (NH4)2SO4, 0.1 wt % (NH4)2SO4,
0.005 molal ammonia solutions over a period of 5 days. Data points depict
mean (with error bars representing min to max) for Thet and
Fhet measured for two aging experiments performed with two
independently aged suspensions.
Aging effect
We carried out immersion freezing experiments with kaolinite (5 wt %)
suspended in pure water, NH3 solution (0.005 molal, aw=0.999), and (NH4)2SO4 solutions (0.1 wt % and 10 wt %;
aw=0.996, aw=0.961, respectively) over a period of
5 days to assess the effect of aging on the IN efficiency of kaolinite.
Figure 6 shows Thet (panel a) and Fhet (panel b) as a
function of time. Thet remains stable over the measured time period
within the experimental measurement uncertainties. Similarly, Fhet
remains constant except in the pure water case where it shows a slight
increase.
The dissolution rate of kaolinite depends on pH with the lowest values of
10-14–10-13 moles (of Si) m-2 s-1 realized at near-neutral conditions (Carroll and Walther, 1990; Huertas et al., 1999). At
acidic conditions the dissolution increases because of H+ attack at
the Si–O–Al linkages of the edges leading to the liberation of aluminum
ions into the solution (Xiao and Lasaga, 1994; Fitzgerald et al., 1997). At
alkaline conditions, dissolution is considered to occur dominantly at the
basal octahedral face via deprotonation of aluminum sites (Huertas et al.,
1999; Naderi Khorshidi et al., 2018). At near-neutral conditions, both
mechanisms are inefficient leading to a minimum in the dissolution rate.
Between pH 5 and 10, kaolinite dissolution is accompanied by the
precipitation of an aluminum hydroxide phase (Huertas et al., 1999).
The pH of the solutions used for the aging experiments with kaolinite are in
the slow dissolution near-neutral regime. Since there is no decrease in IN
efficiency during the 5 days of aging in the aqueous solutions of ammonia and
ammonium sulfate, surface alteration during this time period seems to be
minor. Indeed, with a dissolution rate of
10-14–10-13 moles m-2 s-1, the Si dissolved during
the 5 days of the experiments should be well below a monolayer and should not
lead to a significant alteration of the surface composition. Nevertheless,
when dissolution occurs preferentially on chemically more reactive sites that
are likewise IN active, destruction of these sites might lead to a decrease
in IN activity (Kumar et al., 2019). However, the increase of IN activity in
pure water rather suggests the generation of new IN active sites due to
surface changes in the presence of water.
Micas
Muscovite (KAl2(AlSi3O10)(OH)2) and biotite
(K(Mg,Fe)3AlSi3O10(OH)2), both quite soft (on Mohs scale 2–3),
are 2 : 1 layer phyllosilicates (one octahedral sheet sandwiched between
two tetrahedral sheets, forming stacked T–O–T layers) belonging to the mica
group with nearly perfect basal cleavage (Bower et al., 2016; Christenson and
Thomson, 2016). The tetrahedral layer consists of Si and Al at a ratio of
3 : 1. The Al is randomly ordered avoiding Al–O–Al arrangements. The
presence of Al in the tetrahedral layer introduces a negative charge that is
neutralized by potassium ions acting as the bridge between the T-O-T
layers. Muscovite and biotite differ in their octahedral layers, which in the
case of muscovite is dioctahedral and occupied by Al3+. Biotite is
part of a solid solution series within the mica group with trioctahedral
layers occupied by Fe2+ and Mg2+. The end-members of the
series are pure iron biotite called annite and pure magnesium biotite called
phlogopite (Bray et al., 2015), while biotites take intermediate positions in
terms of Fe : Mg ratios (Fleet et al., 2003).
DSC thermograms of 5 wt % (a, b) and
10 wt % (c) muscovite particles suspended in ammonia
(0–5.5 molal; aw equal to 1–0.912) and
ammonium sulfate (0–1 wt %; aw equal to
1–0.988) droplets of different solution
concentrations (given as inserts). All curves are normalized such that the
areas under the heterogeneous and homogeneous freezing curves sum up to the
same value. The dashed black line connects the heterogeneous freezing onset
temperatures (Thet) of the emulsions. With increasing ammonia
concentration heterogeneous IN efficiency starts to develop, as can be
noticed from the appearance of Thet, which was absent in pure
water. The intensity of the heterogeneous freezing signal, hence
Fhet, becomes more prominent in 10 wt % suspensions (see
Table 1).
DSC thermograms of 5 wt % (a) and
10 wt % (b) biotite particles suspended in ammonia solution
droplets (0–5.5 molal; aw equal to
1–0.912). All curves are normalized such that the
areas under the heterogeneous and homogeneous freezing curves sum up to the
same value. In contrast to muscovite, biotite becomes IN active only at very
high concentrations of ammonia. In addition, the intensity of the
heterogeneous freezing signal (hence Fhet) is low, but becomes more
prominent in 10 wt % suspensions (see Table 2).
Heterogeneous freezing of aqueous solution droplets containing
micas
Figure 7 shows DSC thermograms of muscovite particles suspended in
(NH4)2SO4 and NH3 solution droplets of increasing
concentration, and Fig. 8 the same for biotite in NH3 solution
droplets. Thet and Fhet from these measurements for
muscovite and biotite are summarized in Tables 1 and 2, respectively.
Interestingly, when suspended in pure water, neither muscovite nor biotite
exhibit a discernible heterogeneous freezing signal in the DSC thermograms.
Considering the particle size distributions of the two minerals, which peaks
at about 335 nm in the case of the muscovite sample (Kaufmann et al., 2016) and
is bimodal with maxima at 241 nm and 1.7 µm for biotite (see the
Supplement), the lack of a heterogeneous freezing signal cannot be ascribed
to the predominance of empty emulsion droplets. Indeed, both start to develop
IN activity when immersed in ammonia or ammonium solutions (indicated by the
dashed black lines in Figs. 7 and 8). The frozen fraction, Fhet, is
a strong function of NH3 or NH4+ concentration. For
muscovite (5 wt %), Fhet increases from 0.138 in
0.005 molal NH3 (aw=0.997) to 0.387 in
4.5 molal NH3 (aw=0.922) solutions. In the case of
biotite (5 wt %), suspensions do not reveal a heterogeneous freezing
signal in pure water or in dilute ammonia solutions. A well discernable
heterogeneous freezing signal appears only in concentrated ammonia
(4.5 molal, aw=0.922, Fig. 8a). For 10 wt % biotite
suspensions, the heterogeneous freezing signal becomes visible already in
2 molal ammonia (aw=0.968; Table 2), yet the signals are
clearly weaker than those from muscovite suspensions at similar solute
concentrations.
Summary of the freezing experiments with emulsified aqueous solution
droplets containing muscovite (5 wt % and 10 wt %). Note that the
absolute uncertainty in Fhet may be up to ±0.1. Only
Fhet values above this threshold are clear evidence of
heterogeneous IN.
a molality; b 10 wt % muscovite
suspended in 5.5 molal NH3 solution; n/a: no Thet and
Fhet can be reported due to absence of a discernible heterogeneous
freezing signal in the emulsion freezing experiments.
Summary of the freezing experiments with emulsified aqueous solution
droplets containing biotite (5 wt % and 10 wt %). Note that the
absolute uncertainty in Fhet may be up to ±0.1. Only
Fhet above this threshold are clear evidence of heterogeneous IN.
* 10 wt % biotite suspended in 5.5 molal NH3
solution; n/a: no Thet and Fhet can be reported due to
absence of a discernible heterogeneous freezing signal in the emulsion
freezing experiments
Previous studies on IN activities of micas
Micas have been tested for IN activity in numerous studies, but with
very diverse outcomes. Older studies have reported dentritic ice growth on the
basal planes of freshly cleaved micas (muscovite and fluorophlogopite:
KMg3AlSi3O10F2) when they were exposed to water
saturation at cold temperatures (Bryant et al., 1959; Hallett, 1961;
Layton and Harris, 1963). Shen et al. (1977) showed that fluorophlogopite
and muscovite particles (44–74 µm) induce IN in bulk freezing
experiments with onset temperatures of 272 and 268 K, respectively.
Steinke (2013) investigated the freezing of water droplets on a
muscovite surface in a cold stage and found IN activity around 250 K but
with a much lower active site density compared with clay minerals.
Atkinson et al. (2013), on the other hand, observed hardly any IN
activity in immersion mode for a non-specified mica. Campbell et al. (2015)
found freezing close to the homogeneous freezing temperature for droplets on
muscovite. Surface imperfections on the basal plane of muscovite were found
to promote IN on muscovite exposed to water vapor below the threshold
temperature for homogeneous IN but had no effect when the surface was
immersed in water above this threshold temperature. Overall, the IN activity
of micas seems to be much lower than that of clay minerals such as kaolinite
despite their similar structure.
IN activity in relation to the surface properties of micas
The basal surfaces of micas are dominated by Si–O–Si and Al–O–Si bridges
exhibiting little hydroxylation. Due to the isomorphic substitution of Si(IV)
for Al(III) in the tetrahedral layer, the basal surfaces are hydrophilic and
carry a permanent negative charge that is neutralized by K+ ions
(Zhao et al., 2008; Yan et al., 2013). When suspended in electrolyte
solutions, K+ exposed to the surface undergo ion exchange within
seconds (Cho and Komarneni, 2009; de Poel et al., 2017; Lee et al., 2017).
In pure water, surface K+, and to a lesser degree also interlayer
potassium ions, are lost to the suspension and replaced by protons introducing
hydroxylation to the basal surface and widening the muscovite lamellae
(Banfield and Eggleton, 1990). At the edges of the plate-like particles,
the broken bonds of the disrupted sheets are saturated by –OH, resulting in
a hydrophilic, hydroxylated surface with a surface charge that depends on pH
(Zhao et al., 2008).
Micas slowly dissolve when suspended in water accompanied by precipitation of
metal (hydr)oxides and kaolinite, which may form nanometer coatings on the
mica surface blocking active sites (Pachana et al., 2012). The
dissolution occurs with similar pH dependence as observed for feldspars with
the lowest rates at near-neutral conditions (Oelkers et al., 2008; Bray et
al., 2015; Lammers et al., 2017). Dissolution may occur via etch pits on the
basal surface, but the dominating process seems to be corrosion of the edge
surfaces (Oelkers et al., 2008; Pachana et al., 2012).
The Si dissolution rate of muscovite is about 10-13 to
10-12 moles m-2 s-1 at neutral conditions (Brady and Walther, 1989; Lammers
et al., 2017), which is sufficient to lead to surface alterations within the
timescale of our experiments. The initial dissolution of Al is higher than
the one of Si (Pachana et al., 2012), confirming that edges
dissolve more readily than the basal faces. The basal plane of muscovite
carries negative charge independent of pH while the edges carry a negative
charge at high pH and a positive charge at low pH with a PZC at pH 7–8. The
dissolution rate of biotite is 10-12 to 10-11 moles m-2 s-1 at neutral conditions. The initial dissolution at pH 6 was observed
to be congruent with respect to Si and Al, but Mg and Fe positioned in the
octahedral layer dissolved at a higher rate, resulting in a metal-depleted
disordered octahedral layer (Pachana et al., 2012; Bray et al., 2015).
The absence of a heterogeneous freezing signal in emulsion freezing
experiments suggests a low density of IN active sites. Thus, the IN activity
seems to stem from very special features that are rare, while the regular
mica surfaces are inactive despite the hydrophilicity of the basal surfaces
and the dense hydroxylation of the edges, both characteristics of IN active
surfaces. Indeed, sum-frequency-generation (SFG) spectra showed that water
adsorbed at full monolayer coverage (90 % RH) forms an ice-like film on
the basal muscovite surface (Miranda et al., 1998). Yet, this film does not
seem to grow readily into bulk ice. The surface electric field of the
negatively charged muscovite surface orders water molecules with the protons
pointing towards the surface. SFG spectra show that this ordering decreases
when the water freezes and increases again when the ice melts, suggesting
that the ordered water layer adsorbed on the basal mica surface is unable to
match ice (Anim-Danso et al., 2016). This finding is in line
with Abdelmonem et al. (2017) who found that IN was not promoted by the
presence of an ordered water layer on the hydroxylated sapphire
(α-Al2O3) surface. On the contrary, IN was observed to occur
at slightly warmer temperature close to the PZC when the water molecules were
disordered. While at low and high pH, the aligning of the water molecules on
the positively or negatively charged surface, respectively, was detrimental
to IN.
While the absence of IN activity at the edges of biotite may be ascribed to
the fast disintegration of this surface in the presence of water, it is not
clear whether surface alteration at the edges of muscovite are sufficient to
explain their lack of IN activity in pure water. In the presence of
NH3 and (NH4)2SO4 solutions, both investigated micas show
IN activity in the emulsion freezing experiments. This might be due to
adsorption of NH3/NH4+ on the basal planes increasing the
number of sites available for hydrogen bonding or due to adsorption at the
edges that may influence the dissolution rate.
IN activities in terms of Thet and Fhet together
with surface properties of minerals investigated in this study at
near-neutral conditions (representative for pure water), microcline from
Kumar et al. (2018a) and quartz from Kumar et al. (2019).
Mineral Surface functional groups at neutral conditionspH of PZCDissolution rate (moles Si m-2 s-1)Suspension concentration (wt %)Thet (K) pure waterFhet pure waterFeldspars Si-O-Si, Si-O-Al, Si-OH-Al, Si-OH, Si-O-, Al-OH, Al-OH2+aMicrocline (K-feldspar)<2h4×10-14–8×10-14 atpH ∼ 6p2252.10.74Sanidine (K-feldspar)∼2×10-13 atpH ∼ 6p2241.20.42Andesine (Na-Ca-feldspar)<2i10-12–10-11 atpH ∼ 8q2242.80.65Kaolinite (clay mineral) 10-14–10-13 atpH ∼ 7r,s5240.30.52Basal Si faceSi-O-Sib< 4k,lBasel Al faceAl-OH1/2-, Al-OH21/2+, Al2-OHc6–8k,lEdgesSi-O-, Si-OH, Al-OH, Al-O-, Al-OH2+, Al-(OH)2c,d<4k,mMuscovite (mica) 10-13–10-12 atpH ∼ 6t,u10n/an/aBasal faceSi-O-Si, Al-O-Sie< 4l,nEdgesSi-OH, SiAl-O1/2-, Al2-OH, Al-OH1/2-, Al-OH21/2+e7–8l,nGibbsite Al-OH1/2-, Al-OH21/2+, Al2-OHf7.5–11.3o10-14–10-13 atpH ∼ 7v10n/an/aQuartz Si-O-, Si-OH, Si-(OH)2,2h10-13–10-12 atpH ∼ 7w,x1–9milled: 247–2510.7–0.92Si-(OH)O-g5long-term aged: ∼ 239 K∼ 0.1
a: Teng et al., (2001); b: Schoonheydt and Johnston, (2006);
c: Brady et al. (1996); d: Liu et al. (2013); e: Yan et al. (2011);
f: Hiemstra et al. (1999); g: Liu et al. (2014); h: Vidyadhar and Hanumantha
Rao (2007); i: Karagüzel et al. (2005); k: Gupta and Miller (2010);
l: Kumar et al. (2017); m: Liu et al. (2014); n: Zhao et al. (2008);
o: Kosmulski (2009); p: Crundwell (2015); q: Gudbrandsson et al. (2014);
r: Carroll and Walther (1990); s: Huertas et al. (1999); t: Brady and Walther
(1989); u: Lammers et al. (2017); v: Dietzel and Böhme (2005); w: Brady
and Walther (1990); x: Berger et al. (1994); n/a: no Thet and
Fhet can be reported due to the absence of a discernible
heterogeneous freezing signal in the emulsion freezing experiments
Summary of IN activity of mineral surfacesInfluence of surface properties
Table 3 summarizes the IN activity of the minerals investigated in this study
together with results of microcline from Part 1 (Kumar et al., 2018a) and
quartz results from Part 2 (Kumar et al., 2019) and relates them to selected
surface properties of the investigated minerals, namely the prevalent surface
groups, the dissolution rates at near-neutral conditions and the PZC.
Although all investigated minerals possess hydroxylated surfaces, which are
generally thought to promote IN (Hu and Michaelides, 2007; Sosso et al.,
2016a; Glatz and Sarupria, 2018) (see Table 3), not all of them proved to be
IN active in our emulsion freezing experiments. Namely, the gibbsite and mica
(muscovite and biotite) samples did not show any heterogeneous freezing
signal in pure water. Zeta potential measurements of these minerals show that
the hydroxylated edges of muscovite and the surface of gibbsite carry a
positive charge at neutral conditions due to protonation of aluminol groups
(see Table 3). This is in contrast to the surfaces of quartz, feldspars and
the hydroxylated edges of kaolinite, which all carry negative charge at
neutral conditions due to partial deprotonation of the silanol groups. Thus,
positively charged (due to protonation) hydroxylated surfaces seem to have a
tendency to be IN inactive, while negatively charged surfaces tend to be IN
active. However, long-term aged quartz surfaces (7 months; see Kumar et al.,
2019) show hardly any IN activity while milled quartz surfaces are highly IN
active, although the quartz surface is in both cases negatively charged and
highly hydroxylated (Turci et al., 2016). On the other hand, AgI (Edwards and
Evans, 1962; Marcolli et al., 2016) and sapphire (Abdelmonem et al., 2017)
show the highest IN activity close to the point of zero charge. Conversely,
pyroelectric LiTaO3 and SrTiO3 promote IN when their surfaces
are positively charged (Ehre et al., 2010). This shows that the OH surface
density together with the surface charge are not sufficient to predict IN
activity.
Dissolution and growth in pure water
The surfaces of the investigated feldspars exhibit similar chemical
compositions although with different Si : Al ratios and Si / Al
ordering, which both influence the aluminum depletion and the dissolution
rate. The initial dissolution of most feldspars is incongruent with
preferential dissolution of aluminum, resulting in a Si-rich amorphous layer.
We hypothesize that the surface remains well-ordered over a longer period of
time for microcline due to its lowest dissolution rate (compared to the other
investigated feldspars). Indeed, our emulsion freezing experiments on a
microcline suspension (2 wt %) in pure water, which was aged for
6 months, showed no significant change in IN efficiency compared with a
freshly prepared sample (aged sample: Thet=251.4 K,
Fhet=0.71, see Fig. S15 in the Supplement; fresh sample:
Thet=252.1 K, Fhet=0.74; Kumar et al., 2018a).
Peckhaus et al. (2016) observed a decrease in IN activity by 2 K for a
K-feldspar over a time span of 5 months. This implies that the high initial
IN activity of microcline degrades only slowly within days or months (Kumar
et al., 2018a). Hydrogen bonding to the feldspar surface is a prerequisite to
arrange water molecules in a suitable configuration for IN, but at the same
time it is the first step to the disintegration of the feldspar surface.
Feldspar dissolution occurs via protonation of Al–O–Si bridges followed by
removal of Al3+ and protonation of the dangling bonds resulting in
a silanol-rich surface. As the dissolution proceeds, the Al-depleted Si layer
becomes thicker and more amorphous-like.
When the surface of milled quartz grows or dissolves, active sites are lost
and the IN activity decreases (Kumar et al., 2019). Under growth conditions,
i.e., when the dissolved Si concentration exceeds the saturation
concentration with respect to quartz, a siliceous layer forms on the quartz
surface within about a day, which hampers the IN activity of quartz. When
this layer is washed away with pure water after several days of aging, the IN
activity is restored. On timescales of months, quartz slowly grows within
this layer resulting in an intact, grown quartz surface that is barely IN
active. This suggests that the IN activity of silica mainly stems from
activation through grinding and is absent in regular, grown quartz surfaces.
We ascribe the absence of IN on the highly hydroxylated grown quartz surface
to networks of interconnected hydrogen bonds that are too strong to be
disrupted by water molecules. This implies that for a hydroxylated surface to
be IN active the hydrogen bonds must be available for bonding to water
molecules in order to direct them into an ice-like pattern.
Annotated summary: heterogeneous IN onset temperatures,
Thet, of various mineral dusts (2 wt %–10 wt %) as a
function of water activity in NH3, (NH4)2SO4, NH4Cl
and Na2SO4 solutions (color coded) for aw equal to
1.0–0.92. (a) Results for microcline from Kumar et al. (2018a).
(b–g) Feldspars, kaolinite, micas and gibbsite investigated in the
present study. (h) Results for quartz from Kumar et al. (2019).
Dash-dotted black lines: ice melting point curves. Dotted black lines:
homogeneous ice freezing curves for supercooled aqueous solutions. Solid
black lines: expected heterogeneous freezing curves if the presence of
solutes did not change the mineral surface properties. Lines result by
horizontally shifting from the ice melting curve by a constant Δawhet emanating from the heterogeneous freezing
temperatures of the suspensions of the minerals in pure water (at aw=1). Specifically, Δawmicrocline=0.296,
Δawsanidine=0.264, Δawandesine=0.254, Δawkaolinite=0.272 and Δawquartz=0.221.
Influence of solutes
Figure 9 reviews the influence of solutes on the heterogeneous IN onset
temperatures of the investigated aluminosilicates and quartz. The presence of
NH3/NH4+ in the suspensions increases Thet of all
investigated feldspars and kaolinite (Fig. 9, point 1). Moreover, it provides
IN activity to the investigated micas and gibbsite, which showed no IN
activity in our emulsion freezing experiments performed in pure water
(Fig. 9, point 4). The increase in Thet of kaolinite with no
exchangeable cations confirms the findings from Kumar et al. (2018a) that the
enhanced IN activity in the presence of NH3/NH4+ is not due to ion
exchange of NH4+ with the native cations but stems from the
adsorption of NH3/NH4+ on the mineral surface. Water molecules can
form hydrogen bonds with NH groups of adsorbed NH3/NH4+ that may
promote their arrangement into an ice-like pattern. Moreover, this adsorption
can decrease the feldspar dissolution by stabilizing the surface. While
NH3/NH4+ also interact with silica surfaces, these interactions do
not lead to an increase in Thet. Rather, a decrease is observed in
the presence of NH3 due to the enhanced dissolution of quartz under
alkaline conditions (Fig. 9, point 5).
The alkali salt Na2SO4 strongly decreases the IN activity of
feldspars (Fig. 9, point 2) but does not influence that of kaolinite (Fig. 9,
point 3). This indicates that alkali ions influence the IN activity of
feldspars through ion exchange, which does not take place in kaolinite.
Dissolved Na+ may take the positions of the protons on the surface
and thus impede IN. This shows that the replacement of the charge balancing
cations with protons taking place in pure water is crucial for the IN
activity of feldspars.
Influence of milling
Milling increases the IN activity of quartz (Fig. 9, point 6), while the
growth of an intact quartz layer on top of the milled quartz surface strongly
hampers it (Fig. 9, point 7). Milling leads to an increase in surface
irregularities and defects, which may lead to changes in the abundance and
distribution of surface functional groups. These changes, in general, enhance
IN efficiency but the enhancement depends on the type of mineral surface.
Zolles et al. (2015) showed that additional milling of K-feldspar leads to
only a slight increase in its high IN efficiency. Kaufmann et al. (2016)
performed emulsion freezing experiments with a dust sample that was
ground-collected in Antarctica (48 % feldspars, 24 % quartz) and
observed an increase of the already high IN activity of the sieved sample
when it was additionally milled. We ascribe the high IN activity of the
sieved sample to the presence of the feldspars, whereas an additional
increase after milling is most likely due to activation of the quartz
component. Together with the finding that K-feldspars aged in pure water over
months (see Sect. 4.2) kept their IN activity, we conclude that mechanical
activation, e.g., due to milling, is not a prerequisite for the IN activity
of feldspars. This is in contrast to our findings for quartz surfaces
presented here (Fig. 9) and in Kumar et al. (2019). Boose et al. (2016) found
a slightly higher active site density of the milled fraction of their Atacama
dust sample (65 % feldspars, 10 % quartz) compared with the sieved
fraction (51 % feldspars, 16 % quartz). On the other hand, they
observed a decreased IN activity of their Israel dust sample after milling
(80 % calcite, 6 % quartz) compared with the sieved fraction
(67 % calcite, 7 % quartz). A natural quartz sample from Zolles et
al. (2015) showed a very strong enhancement in IN efficiency due to milling.
Moreover, milled samples from Boose et al. (2016) collected in Australia and
Morocco, which primarily consisted of quartz, showed a very high IN activity,
corroborating our results on quartz and amorphous silica in the companion
paper (Kumar et al., 2019). Therefore, it seems likely that milling is a
prerequisite for the IN activity of quartz. A natural process, closest to
milling, would be dry erosion, yet it is unclear whether the forces exerted
by fracturing during erosion are sufficient to generate similar defects as
milling.
Conclusions and atmospheric implications
Immersion freezing experiments with aluminosilicates, namely feldspars,
kaolinite, micas and gibbsite presented in this study and in Kumar et
al. (2019) have shown that modifications of the mineral surfaces in the
presence of water and solutes influence their IN activity.
The interaction of water with mineral surfaces is complex and manifold. It
depends not only on the type of mineral but also on the exposure time of the
surfaces to water and solutes. Factors influencing the IN ability of
aluminosilicate surfaces are adsorption and ion exchange with solute
molecules influencing the density of OH and NH groups that provide sites
for hydrogen bonding with water molecules, permanent and pH-dependent
surface charge influencing the orientation of water molecules and surface
changes generated by the slow dissolution of the minerals. Therefore, for an
improved understanding of IN, the specific surface properties of each
mineral and the processes modifying the mineral surfaces in water and
aqueous solutions need to be investigated by experimental surface
techniques. In addition, molecular dynamics simulations of IN may contribute
to clarifying these processes, but only if interactions of the mineral
surface with water, e.g., surface ion exchange, protonation or deprotonation of
the surface and slow dissolution, are taken into account.
Aluminosilicates constitute the majority of airborne mineral dust. Dust
particles can be exposed to reactive gases like NH3 or SO2
during long-range transport, resulting in a coating and surface modification
(Sullivan et al., 2007; Kolb et al., 2010; Fitzgerald et al., 2015). The
modifications introduced by chemical coatings depend on the particle
mineralogy, transport pathway and exposure duration (Matsuki et al., 2005;
Sullivan et al., 2007; Rodríguez et al., 2011). Our results suggest that
the IN activity of dust aerosols not only depend on their composition but
also on their chemical exposure history. In Part 1 (Kumar et al., 2018a) we
discussed in detail the fate of microcline in atmospheric solution droplets
(especially (NH4)2SO4 and H2SO4) following various
atmospheric air parcel trajectories with increasing moisture. This discussion
is applicable to the aluminosilicates shown in this study and can hence be
extended. A coating of aqueous (NH4)2SO4 solution may enhance the
IN efficiency in condensation mode of aluminosilicates in general, but only
when sulfuric acid and ammonia are deposited concomitantly. Aluminosilicates,
especially feldspars, are sensitive to highly acidic conditions due to
enhanced dissolution. This might hamper their relevance in the case of
long-range transport when they are exposed to acidic air masses (e.g.,
acquiring sulfuric acid coatings prior to neutralization by ammonia).
Moreover, in Part 2 (Kumar et al., 2019) we have shown that the relevance of
quartz particles as atmospheric INPs is uncertain. To assess the IN activity
of naturally eroded quartz, IN experiments need to be carried out with
quartz-rich natural dust samples that are just sieved and not milled.
Data availability
The data for freshly prepared sanidine, andesine and
kaolinite suspensions in water or aqueous solutions (Figs. 2–3) and aging
tests on kaolinite (Fig. 6) presented in this publication are available at
the following repository: 10.3929/ethz-b-000260067 (Kumar et al.,
2018b).
The supplement related to this article is available online at: https://doi.org/10.5194/acp-19-6059-2019-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.
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; Peter Brack for providing various minerals. 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.
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