The impact of (bio-)organic substances on the ice nucleation activity of the K-feldspar microcline in aqueous solutions
Authors: Klumpp Kristian , Marcolli Claudia , Peter Thomas 
Published: March 2022

Abstract. Potassium feldspars (K-feldspars), such as microcline, are considered key
dust minerals inciting ice nucleation in mixed-phase clouds. Besides the
high ice nucleation activity of microcline, recent studies also revealed a
high sensitivity of microcline to interaction with solutes on its
surface. Here, we investigate the effect of organic and bio-organic
substances on the ice nucleation activity of microcline, with the aim to
better understand the underlying surface interactions. We performed
immersion freezing experiments with microcline in solutions of three
carboxylic acids, five amino acids, and two polyols to represent these
compound classes. By means of a differential scanning calorimeter we
investigated the freezing of emulsified droplets of microcline suspended in
various solutions. Depending on the type of solute, different effects were
observed. In the case of carboxylic acids (acetic, oxalic, and citric acid),
the measured heterogeneous onset temperatures, Thet, showed no
significant deviation from the behavior predicted by the water activity
criterion, Thet(aw)=Tmelt(aw+Δaw), which
relates Thet with the melting point temperature Tmelt via a constant
water activity offset Δaw. While this behavior could be
interpreted as a lack of interaction of the solute molecules with the
surface, the carboxylic acids caused the fraction of heterogeneously frozen
water, Fhet(aw), to decrease by up to 40 % with increasing solute
concentrations. In combination, unaltered Thet(aw) and reduced
Fhet(aw) suggest that active sites were largely deactivated by the
acid molecules, but amongst those remaining active are also the best sites
with the highest Thet. A deviation from this behavior is citric acid,
which showed not only a decrease in Fhet, but also a decrease in
Thet of up to 4 K for water activities below 0.99, pointing to a
depletion of the best active sites by interactions with the citrate ions.
When neutralized solutions of the acids were used instead, the decrease in
Fhet became even more pronounced. The slope of Thet(aw) was
different for each of the neutralized acid solutions. In the case of amino
acid solutions, we found a decrease in Thet (up to 10 K), significantly
below the Δaw criterion, as well as a reduction in Fhet (up
to 60 %). Finally, in the case of the investigated polyols, no significant
deviation of Thet from the Δaw criterion was observed, and
no significant deviation of Fhet in comparison to a pure water
suspension was found. Furthermore, we measured the effects of aging on the
ice nucleation activity in experiments with microcline suspended in
solutions for up to 7 d, and tested the reversibility of the
interaction with the solutes after aging for 10 d. For citric acid, an
ongoing irreversible degradation of the ice nucleation activity was
observed, whereas the amino acids showed completely reversible effects. In
summary, our experiments demonstrate a remarkable sensitivity of microcline
ice nucleation activity to surface interactions with various solutes,
underscoring the importance of the history of such particles from the source to
frozen cloud droplets in the atmosphere.