Ice nucleation efficiency of clay minerals in the immersion mode

Abstract

Abstract. Emulsion and bulk freezing experiments were performed to investigate immersion ice nucleation on clay minerals in pure water, using various kaolinites, montmorillonites, illites as well as natural dust from the Hoggar Mountains in the Saharan region. Differential scanning calorimeter measurements were performed on three different kaolinites (KGa-1b, KGa-2 and K-SA), two illites (Illite NX and Illite SE) and four natural and acid-treated montmorillonites (SWy-2, STx-1b, KSF and K-10). The emulsion experiments provide information on the average freezing behaviour characterized by the average nucleation sites. These experiments revealed one to sometimes two distinct heterogeneous freezing peaks, which suggest the presence of a low number of qualitatively distinct average nucleation site classes. We refer to the peak at the lowest temperature as "standard peak" and to the one occurring in only some clay mineral types at higher temperatures as "special peak". Conversely, freezing in bulk samples is not initiated by the average nucleation sites, but by a very low number of "best sites". The kaolinites and montmorillonites showed quite narrow standard peaks with onset temperatures 238 K < T std on < 242 K and best sites with averaged median freezing temperature T best med = 257 K, but only some featuring a special peak (i.e. KSF, K-10, K-SA and SWy-2) with freezing onsets in the range 240-248 K. The illites showed broad standard peaks with freezing onsets at 244 K < T std on < 246 K and best sites with averaged median freezing temperature T best med = 262 K. The large difference between freezing temperatures of standard and best sites shows that characterizing ice nucleation efficiencies of dust particles on the basis of freezing onset temperatures from bulk experiments, as has been done in some atmospheric studies, is not appropriate. Our investigations demonstrate that immersion freezing temperatures of clay minerals strongly depend on the amount of clay mineral present per droplet and on the exact type (location of collection and pre-treatment) of the clay mineral. We suggest that apparently contradictory results obtained by different groups with different setups are indeed in good agreement when only clay minerals of the same type and amount per droplet are compared. The natural sample from the Hoggar Mountains, a region whose dusts have been shown to be composed mainly of illite, showed very similar freezing characteristics (standard and best) to the illites. Relating the concentration of best IN to the dust concentration in the atmosphere suggested that the best IN in the Hoggar sample would be common enough downwind of their source region to account for ambient IN number densities in the temperature range of 250-260 K at least during dust events.

Refreeze experiments with water droplets containing different types of ice nuclei interpreted by classical nucleation theory

“…Then, 80 vol % of this mixture and 20 vol % of aqueous suspension were vigorously stirred to obtain an emulsion as described by Pinti et al (2012). This suspension was subjected to repeated freezing cycles.…”

“…In the conventional manner, a lower limit for j het was obtained by assuming that the whole sample surface is active at nucleating ice. For Hoggar Mountain dust and ATD, the available surface area per sample was calculated based on the mass present in the sample and the BET (Brunauer-Emmett-Teller) surface area, namely 46.3 m 2 g −1 for Hoggar Mountain dust (Pinti et al, 2012) and 85 m 2 g −1 for the ATD sample (Bedjanian et al, 2013).…”

“…The best nucleation sites probed in the refreeze experiments with bulk samples are active from 260 to 268 K, i.e., at distinctly higher temperatures than the average sites probed in the emulsion experiments, which nucleate ice below 252 K. In contrast to the bulk measurements, no memory effect was observed for ATD emulsions. Hoggar Mountain dust is a mixture of various minerals which nucleate ice at quite different temperatures (Pinti et al, 2012;Kaufmann et al, 2016), giving rise to the broad freezing signal starting below 257 K with the freezing of single large emulsion droplets as shown in panel (a). Again, there is no overlap in freezing temperatures between emulsion measurements and the refreeze experiments performed with large single droplets which froze from 258 to 265 K. With an onset of 255 K, the heterogeneous freezing peak of the emulsion made from the birch pollen washing water exhibits a clear overlap with the freezing temperatures observed for bulk measurements, which indicates that the ice nucleation active macromolecules present in the birch pollen washing water contain nucleation sites of quite uniform quality.…”

“…Hoggar Mountain dust collected from the Sahara (Pinti et al, 2012) was chosen to represent natural mineral dusts. It is a mixture of minerals originating from a source region of dust aerosols with high shares of clay minerals.…”

“…Zobrist et al (2007) performed refreeze experiments with water droplets coated by a nonadecanol monolayer, which arranges itself in a 2-D crystalline lattice on the water surface. The structural match of this 2-D crystal with the ice lattice has been considered as a key reason for the good ice nucleation ability of long-chain alcohol monolayers (Popovitz-Biro et al, 1994;Majewski et al, 1995;Knopf and Forrester, 2011). The refreeze experiments by Zobrist et al (2007) are reevaluated here assuming that instead of the whole monolayer only an active site in it is responsible for ice nucleation.…”