Different dust particles as ice nuclei: learning from similarities and differences

For atmospheric processes, ice containing clouds are important as they cover 40% of the Earth at any time and have an important influence on both the Earth's radiation budget and the formation of precipitation. The initial step of ice formation in clouds is ice nucleation, and this process hence is of large interest. Ice nucleates either homogenously or heterogeneously, where heterogeneous freezing requires the presence of a foreign ice nucleus (IN), e.g. a mineral dust particle, which induces the freezing. Mixed phase clouds form at temperatures above those for which homogenous freezing can occur, and due to their above mentioned importance for the formation of precipitation, the heterogeneous freezing is of particular interest, and there particularly the immersion freezing (Ansmann et al., 2009).

In this paper, we will present results from measurements of heterogeneous ice nucleation of different dust particles. Immersion freezing measurements were done with LACIS (Leipzig Aerosol Cloud Interaction Simulator, Hartmann et al., 2011), examining size segregated particles consisting of either Arizona Test Dust (ATD), or Illite (NX-Illite), or one kind of a Potassium (K-) Feldspar (Microcline), or of one from two different Kaolinites (one from Fluka, one from the Clay Mineral Society (CMS, KGa-1b)). ATD and the Kaolinites were also examined with a CFDC (continuous flow diffusion chamber, Rogers et al., 2001; DeMott et al., 2010), which measured both, condensation/immersion freezing and deposition ice nucleation. Results reported here were mostly collected for particles with a mobility diameter of 300nm. Coatings with sulphuric acid, and sometimes additionally also with levoglucosan and succinic acid, were applied to the particles to examine the respective effect on the ice nucleation ability.

In general, it was found that for uncoated particles the ice nucleation ability of the majority of the dusts was very similar. The exceptions were that Microcline induced freezing at much higher temperatures than all other dusts, while the CMS Kaolinite was distinctively less ice active than the other dusts. In those cases where different sizes of particles were examined, a dependence of the freezing ability on the particle surface could be derived.

It was also found that all coatings strongly reduced deposition ice nucleation (Sullivan et al., 2010, Tobo et al., 2012), with a strong onset in the deposition ice nucleation at a relative humidity close to 95%. The effective coating thicknesses were at or above 0.5nm. The observations can be explained as follows: The coatings covered the surface of the IN, thus hindering the formation of a critical ice nucleus. At the respective deliquescence point, the coatings turned into an aqueous solution, surrounding the particle. Due to the soluble material in the solution, a freezing point depression needs to be accounted for, which, in turn, depends on the relative humidity in the vicinity of the particle (Koop and Zobrist, 2009). It could be shown that for the examined particles, deposition ice nucleation was suppressed at low relative humidity, due to the non-dissolved coatings, and immersion freezing of concentrated solutions took place at high enough relative humidity where the coatings were sufficiently dissolved.

The coatings with sulphuric acid also altered the particles ability to act as IN for immersion freezing in most cases, where usually a decline in the IN ability was observed. Here, again, an exception was observed for CMS Kaolinite particles, for which the ability to nucleate ice in the immersion freezing mode was similar for all coated and uncoated particles. Moreover, the CMS Kaolinite particles nucleated ice similar to Fluka Kaolinite particles coated with sulphuric acid. This is suggestive for the Fluka Kaolinite possessing a type of ice nucleating surface feature which is not present on the CMS Kaolinite, and which can be destroyed by reaction with sulphuric acid.

The amount of K-Feldspar contained in the different examined dusts, as given in Atkinson et al. (2013), amounts to 20%, 8% and 5% for ATD, NX-Illite and Fluka Kaolinite, respectively, while Microcline is mostly composed of it (to about 80%) and the CMS Kaolinite does not contain any detectable amount. Therefore, the amount of K-Feldspar present in the different dust samples roughly correlates with the IN activity of the dusts for immersion freezing. It was recently proposed that the ability of mineral dusts to act as IN might originate in K-Feldspar (Atkinson et al., 2013), and we will examine how much this is corroborated by our results.

Acknowledgement: This work is partly funded by the German Research Foundation DFG within the framework of the Ice Nucleation research UNiT (INUIT).

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