Ice nucleation activity of silicates and aluminosilicates in pure water and aqueous solutions – Part 1: The K-feldspar microcline
Authors: Kumar Anand , Marcolli Claudia , Luo Beiping , Peter Thomas 
Published: May 2018

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