Enhanced Ice Nucleation Efficiency of Mineral Dust Particles in Ammonia/Ammonium Solution Droplets

Abstract

Clouds play an important role in the Earth’s radiative budget and hence climate. However, large uncertainties still exist in our understanding of the formation of cold, high-altitude cirrus and warmer, lower-altitude mixed-phase clouds. Ice formation in the atmosphere happens via both homogeneous ice nucleation (IN) occurring in an aqueous liquid droplet and heterogeneous IN induced by a foreign particle termed ice nucleating particle (INP) at warmer temperatures than homogeneous IN. Natural mineral dust (typically a complex mixture of several minerals) is a well-established class of INPs and is known to play a key role in atmospheric IN, although, we still lack a comprehensive knowledge of which properties make a mineral dust particle an efficient INP. Given that both sub- and supermicron-sized dust particles are efficiently transported over long distances, they can undergo physical and chemical changes due to e.g. photochemical aging, chemical reactions with trace gases and inorganic species. Such interactions can cause a complete change in the IN efficiency of potential INPs. It is difficult to resolve the interactions of individual solutes with mineral surfaces in a mixture of several minerals. Therefore, this PhD thesis focuses on understanding fundamental effects of solutes on the IN efficiency of single minerals (viz. aluminosilicates and silicas) and elucidating the interactions of mineral surfaces with pure water and inorganic solutions on a microphysical scale. For this purpose, a differential scanning calorimeter was used to detect immersion freezing (INP submerged in aqueous droplets) of emulsified aqueous droplets containing suspended particles of single minerals, namely K-feldspars (microcline and sanidine), (Na, Ca)-feldspar (andesine), kaolinite, quartz, micas (muscovite and biotite), gibbsite and amorphous silica. The suspensions were either prepared with pure water or aqueous solutions of sulfuric acid, ammonia and several inorganic salts, namely - ammonium sulfate, ammonium chloride, ammonium nitrate, ammonium bisulfate, sodium sulfate, potassium chloride, potassium sulfate and sodium hydroxide. Results show that heterogeneous IN onset temperatures for various minerals deviate from the previously established water-activity-based immersion freezing description because of the interactions between the INP surface and solutes. Interestingly, a general enhancement in IN efficiency (in terms of both, freezing onset temperatures (Thet) and heterogeneously frozen water volume fraction (Fhet)) of aluminosilicates is observed in the presence of dilute ammonia and ammonium salts. In addition, aqueous suspensions of micas and gibbsite, which did not show a heterogeneous freezing signal in emulsion freezing experiments in pure water, developed IN activity in the presence of ammonia. In contrast, Thet of quartz (a crystalline form of silica) follows the water-activity-based immersion freezing description, yet its Fhet is slightly enhanced in ammonium sulfate, ammonium bisulfate and sodium sulfate but decreased in ammonia and sodium hydroxide. Hence, the silicas respond to externally added solutes quite differently compared to the aluminosilicates. In addition, IN efficiency is drastically hampered under severe pH conditions (e.g. feldspars under acidic and alkaline conditions; quartz under alkaline conditions). The IN activity of quartz particles disappeared almost completely after aging for 7 months in pure water in a glass vial. During this time quartz slowly grew by incorporating silicic acid leached from the glass vial. Conversely, the synthesized amorphous silica samples showed no discernable heterogeneous freezing signal unless they were milled. We therefore suggest that milling, hence creation of defects, rather than crystallinity is the prime reason for the IN activity of silica surfaces. In summary, these investigations show that a high surface density of OH and NH groups providing sites for hydrogen bonding, surface charge influencing the orientation of water molecules, surface degradation due to the slow dissolution of the minerals and surface defects introduced by milling are all relevant factors for the IN activity of aluminosilicate and silica surfaces. Implications for IN on airborne dust consisting of aluminosilicates and silicas, especially when they undergo atmospheric aging, might be multifold. Therefore, the chemical exposure history of the particles is a relevant determinant of the IN efficiency of airborne dust.