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

Abstract

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 NH4+ 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 aw equal to 0.88–1.0. Using differential scanning calorimetry (DSC) on emulsified micron-sized droplets, we derived onset temperatures of heterogeneous (Thet) and homogeneous (Thom) freezing as well as heterogeneously frozen water volume fractions (Fhet). Suspensions in pure water of andesine, sanidine and kaolinite yield Thet 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., Thet≈Thom≈237.0 K). The presence of NH3 and/or NH4+ 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 NH4+ salts, Thet 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 Na2SO4 cause the Thet of feldspars to clearly fall below the water-activity-based immersion freezing description (Δaw= const.) even in very dilute Na2SO4 solutions, while Thet of kaolinite follows the Δaw= 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 (K+, Na+, Ca2+) 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 (Δaw= const.). We hypothesize that adsorption of NH3 and NH4+ on the feldspar surface rather than ion exchange is the main reason for the anomalous increased Thet in dilute solutions of NH3 or NH4+ salts. This is supported by the response of kaolinite to NH3 or NH4+, 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.

Ice nucleation activity of silicates and aluminosilicates in pure water and aqueous solutions – Part 2: Quartz and amorphous silica

“…Arid and semiarid regions are the main sources of mineral dust (e.g., Saharan and Gobi deserts) (Laurent et al, 2006(Laurent et al, , 2008, which can have atmospheric lifetimes of several days (Huneeus et al, 2011). The dust, while being transported, can interact with a variety of trace gases (Usher et al, 2003;Kolb et al, 2010), which can lead to changes in the surface physicochemical properties.…”

Ejection of Dust From the Ocean as a Potential Source of Marine Ice Nucleating Particles

“…Our measurements were collected within a few hours of dust addition, but longer ocean residence times could potentially alter the structure and/or composition of the dust through dissolution and/or disrupting active sites, though Perkins et al (2020) found that incubation in sea salt had little effect upon the IN‐activity of ATD once colligative effects had been considered. However, feldspar is a major contributor to mineral dust INPs, and their ice nucleation activity can be reduced by the presence of aqueous salt (Kumar et al, 2019; Whale et al, 2018), and thus, more investigation is needed to understand the effects of suspension in seawater on the IN‐activity of mineral dust. Seawater also contains small (<0.2 μm) organic ice nucleating entities (INE) from diatom or bacterial exudates that freeze at temperatures ≥ −8°C (McCluskey, Hill, Humphries, et al, 2018; Wilson et al, 2015).…”

The effect of (NH&lt;sub&gt;4&lt;/sub&gt;)&lt;sub&gt;2&lt;/sub&gt;SO&lt;sub&gt;4&lt;/sub&gt; on the freezing properties of non-mineral dust ice nucleating substances of atmospheric relevance

“…Lithium iodide was found to increase the freezing temperature of kaolinite particles in one study (Reischel and Vali, 1975), but not in a more recent study (Ren et al, 2020). Other inorganic salts including, NaOH and NaCl, decrease the freezing temperatures of some types mineral dusts (Kumar et al, 2019a(Kumar et al, , 2018(Kumar et al, , 2019bReischel and Vali, 1975;Whale et al, 2015). Inorganic acids either decrease the ice nucleation ability of mineral dust particles or have little effect, depending on the type of acid, exposure time, concentration of the acid, and type of mineral dust (Kumar et al, 2018;Burkert-Kohn et al, 2017;Sullivan et al, 2010b;Tobo et al, 2012;Augustin-Bauditz et al, 2014;Wex et al, 2014;Sullivan et al, 2010a;Link et al, 2020).…”

“…For comparison purposes, we also investigated the effect of (NH4)2SO4 on the ice nucleating ability of four types of mineral dust (Arizona Test Dust, K-rich feldspar, montmorillonite, and kaolinite). The effect of (NH4)2SO4 on the ice nucleating ability of Arizona Test Dust, K-rich feldspar, and kaolinite, has been studied before (Kumar et al, 2018;Whale et al, 2018;Kumar et al, 2019b), but the effect on montmorillonite has not.…”