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

Abstract

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 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 NH3, (NH4)2SO4, NH4Cl and Na2SO4, with solute concentrations corresponding to water activities a w equal to 0.88-1.0. Using differential scanning calorimetry (DSC) on emulsified micron-sized droplets, we derived onset temperatures of heterogeneous ( T het) and homogeneous ( T hom) freezing as well as heterogeneously frozen water volume fractions ( F het). Suspensions in pure water of andesine, sanidine and kaolinite yield T het 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 NH3 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 NH3 or NH 4 + salts,