Investigating the discrepancy between wet-suspension- and dry-dispersion-derived ice nucleation efficiency of mineral particles

Abstract

Abstract. Cloud chamber investigations into ice nucleation by mineral particles were compared with results from cold-stage droplet freezing experiments. Kaolinite, NX-illite, and K-feldspar were examined, and K-feldspar was revealed to be the most ice-active mineral particle sample, in agreement with recent cold-stage studies. The ice nucleation efficiencies, as quantified using the ice-active surface site density method, were found to be in agreement with previous studies for the lower temperatures; however, at higher temperatures the efficiency was between a factor of 10 and 1000 higher than those inferred from cold-stage experiments. Numerical process modelling of cloud formation during the experiments, using the cold-stage-derived parameterisations to initiate the ice phase, revealed the cold-stage-derived parameterisations to consistently underpredict the number of ice crystals relative to that observed. We suggest the reason for the underestimation of ice in the model is that the slope of the cold-stage-derived ice-active surface site density vs. temperature curves are too steep, which results in an underestimation of the number of ice crystals at higher temperatures during the expansion. These ice crystals suppress further freezing due to the Bergeron–Findeison process. A coagulation model was used to investigate the idea that the mineral particles coagulate in suspension. This model suggests that coagulation during the experiments may be sufficient to significantly remove the particles for the suspension by sedimentation or reduce the total particle surface area available for ice nucleation to take place. Aggregation was confirmed to take place in mineral suspensions using dynamic light-scattering measurements. However, it is not proven that aggregation of the mineral particles is able to reduce the surface area available for ice nucleation. The implication is that the mineral particles may be more important at nucleating ice at high temperatures than previously thought.

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

“…This may be a decrease in freezing temperatures by almost 3 K over about eight freezing runs (H10) or an abrupt jump to lower freezing temperature by almost 2 K from one freezing run to the next (H11), or a slow increase in freezing temperature by 4.5 K over 35 freezing cycles (H12). Such features are clearly nonstochastic and must have been due to modifications (deteriorations or improvements) of the site, which might be due to coagulation, settling, or breakup of aggregates (Emersic et al, 2015).…”

“…However, it is not clear whether these active sites originate from a specific mineral component or even biogenic components in the dust sample (Conen et al, 2011;Tobo et al, 2014;O'Sullivan et al, 2014). Moreover, the activity of sites could be influenced by coagulation or the breakup of aggregates (Emersic et al, 2015).…”

Ice nucleation efficiency of natural dust samples in the immersion mode

“…The first group consists of the most frequently found minerals in the natural dust samples, namely quartz, muscovite, and the carbonates calcite, dolomite/dolostone, and ankerite. The second group are Kfeldspars (adularia, microcline, orthoclase, and sanidine), which proved to have a high ice nucleation efficiency (Atkinson et al, 2013;Emersic et al, 2015;Zolles et al, 2015;Harrison et al, 2016;Peckhaus et al, 2016). The third group consists of (Na, Ca)-feldspars (anorthite, labradorite, albite (pericline), and plagioclase, not further specified), which have been reported to be quite efficient as INPs (Atkinson et al, 2013;Augustin-Bauditz et al, 2014;Zolles et al, 2015;Peckhaus et al, 2016), though less efficient than Kfeldspars.…”

Enhanced ice nucleation efficiency of microcline immersed in dilute NH<sub>3</sub> and NH<sub>4</sub><sup>+</sup>-containing solutions

“…Aggregation and coagulation may reduce the surface area available for ice nucleation and decrease the heterogeneously frozen fraction (Emersic et al, 2015). The dissociation of silanol groups is the primary factor governing the surface charge of feldspar resulting in a negative Atmos.…”