Chemical and
Physical Transformations of Aluminosilicate Clay Minerals Due to Acid
Treatment and Consequences for Heterogeneous Ice Nucleation
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Abstract
Mineral dust aerosol is one of the
largest contributors to global
ice nuclei, but physical and chemical processing of dust during atmospheric
transport can alter its ice nucleation activity. In particular, several
recent studies have noted that sulfuric and nitric acids inhibit heterogeneous
ice nucleation in the regime below liquid water saturation in aluminosilicate
clay minerals. We have exposed kaolinite, KGa-1b and KGa-2, and montmorillonite,
STx-1b and SWy-2, to aqueous sulfuric and nitric acid to determine
the physical and chemical changes that are responsible for the observed
deactivation. To characterize the changes to the samples upon acid
treatment, we use X-ray diffraction, transmission electron microscopy,
and inductively coupled plasma–atomic emission spectroscopy.
We find that the reaction of kaolinite and montmorillonite with aqueous
sulfuric acid results in the formation of hydrated aluminum sulfate.
In addition, sulfuric and nitric acids induce large structural changes
in montmorillonite. We additionally report the supersaturation with
respect to ice required for the onset of ice nucleation for these
acid-treated species. On the basis of lattice spacing arguments, we
explain how the chemical and physical changes observed upon acid treatment
could lead to the observed reduction in ice nucleation activity