Ice nucleation activity of silicates and aluminosilicates in pure water and aqueous solutions – Part 3: Aluminosilicates

摘要

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, NHsub3/sub or NH 4 + 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 NHsub3/sub , (NHsub4/sub)sub2/subSOsub4/sub , NHsub4/subCl and Nasub2/subSOsub4/sub , with solute concentrations corresponding to water activities asubw/sub equal to 0.88–1.0. Using differential scanning calorimetry (DSC) on emulsified micron-sized droplets, we derived onset temperatures of heterogeneous ( Tsubhet/sub ) and homogeneous ( Tsubhom/sub ) freezing as well as heterogeneously frozen water volume fractions ( Fsubhet/sub ). Suspensions in pure water of andesine, sanidine and kaolinite yield Tsubhet/sub 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., T het ≈ T hom ≈ 237.0 K). The presence of NHsub3/sub and/or NH 4 + 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 NHsub3/sub or NH 4 + salts, Tsubhet/sub 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 Nasub2/subSOsub4/sub cause the Tsubhet/sub of feldspars to clearly fall below the water-activity-based immersion freezing description ( Δasubw/sub= const.) even in very dilute Nasub2/subSOsub4/sub solutions, while Tsubhet/sub of kaolinite follows the Δasubw/sub= 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 ( Ksup+/sup , Nasup+/sup , Casup2+/sup ) 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 ( Δasubw/sub= const.). We hypothesize that adsorption of NHsub3/sub and NH 4 + on the feldspar surface rather than ion exchange is the main reason for the anomalous increased Tsubhet/sub in dilute solutions of NHsub3/sub or NH 4 + salts. This is supported by the response of kaolinite to NHsub3/sub or NH 4 + , 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.