Heterogeneous Ice Nucleation in Aqueous Solutions:  the Role of Water Activity

Abstract

Heterogeneous ice nucleation experiments have been performed with four different ice nuclei (IN), namely nonadecanol, silica, silver iodide and Arizona test dust. All IN are either immersed in the droplets or located at the droplets surface. The IN were exposed to various aqueous solutions, which consist of (NH4)2SO4, H2SO4, MgCl2, NaCl, LiCl, Ca(NO3)2, K2CO3, CH3COONa, ethylene glycol, glycerol, malonic acid, PEG300 or a NaCl/malonic acid mixture. Freezing was studied using a differential scanning calorimeter and a cold finger cell. The results show that the heterogeneous ice freezing temperatures decrease with increasing solute concentration; however, the magnitude of this effect is solute dependent. In contrast, when the results are analyzed in terms of the solution water activity a very consistent behavior emerges: heterogeneous ice nucleation temperatures for all four IN converge each onto a single line, irrespective of the nature of the solute. We find that a constant offset with respect to the ice melting point curve, Deltaaw,het, can describe the observed freezing temperatures for each IN. Such a behavior is well-known for homogeneous ice nucleation from supercooled liquid droplets and has led to the development of water-activity-based ice nucleation theory. The large variety of investigated solutes together with different general types of ice nuclei studied (monolayers, ionic crystals, covalently bound network-forming compounds, and a mixture of chemically different crystallites) underlines the general applicability of water-activity-based ice nucleation theory also for heterogeneous ice nucleation in the immersion mode. Finally, the ice nucleation efficiencies of the various IN, as well as the atmospheric implication of the developed parametrization are discussed.

Ice nucleation efficiency of AgI: review and new insights

“…Nevertheless, from the immersion freezing studies compared in Fig. 2, such a relationship is not evident: freezing temperatures from experiments carried out with single crystals align well with freezing temperatures determined from emulsion experiments (Zobrist et al, 2008;Aguerd et al, 1982) conducted with irregularly shaped particles from precipitation (see Appendix A). Note that surface irregularities may be more important when the particles are exposed to air if the ice nucleation efficiency depends on the amount of water that adsorbs at surface defects or impurities (see discussion below).…”

“…The freezing experiments were carried out in a differential scanning calorimeter (DSC) immediately after preparation. In the DSC thermograms of Zobrist et al (2008) and Aguerd et al (1982), a heterogeneous and a homogeneous freezing peak appeared. The presence of the homogeneous freezing peak could be either due to the absence of crystallized AgI in some of the droplets or inactive AgI crystallites.…”

“…Their results are shown in Fig. 2 as purple triangles (Zobrist et al, 2008) and purple squares (Aguerd et al, 1982). Details of the experiments are given in Table 1.…”

“…In the studies performed by and Evans (1962, 1968) particles stick to the substrate and are completely immersed in the droplets. In the emulsion freezing studies by Zobrist et al (2008) and Aguerd et al (1982) droplets are within the oil phase so that the particles within the droplets have no access to the air-water interface.…”

Laboratory investigations of the impact of mineral dust aerosol on cold cloud formation

“…In contrast to the results of Archuleta et al (2005) who examined coated submicron mineral dust constituents, Zuberi et al (2002) who examined the freezing behavior of larger mineral dust inclusions in concentrated ammonium sulfate droplets (10-55 µm), and Zobrist et al (2008) whose studies included freezing of emulsions with distributions of included ATD, coating by SOA did not lead to a clearly detectable occurrence of heterogeneous freezing by the immersed single ATD particles as the RH w was increased. Heterogeneous freezing was recognized in the studies of Zuberri et al (2002), Archuleta et al (2005), and Zobrist et al (2008) by a clear reduction of the difference of water activity between the freezing conditions and the water activity of the solution in equilibrium with ice, compared to that required for homogeneous nucleation of the pure coating (Koop et al 2000).…”