Preprints
https://doi.org/10.5194/acp-2022-489
https://doi.org/10.5194/acp-2022-489
 
01 Aug 2022
01 Aug 2022
Status: this preprint is currently under review for the journal ACP.

Comparing the ice nucleation properties of the kaolin minerals kaolinite and halloysite

Kristian Klumpp, Claudia Marcolli, Ana Alonso-Hellweg, and Thomas Peter Kristian Klumpp et al.
  • Institute for Atmospheric and Climate Sciences, ETH Zurich, Zurich, 8092, Switzerland

Abstract. Heterogeneous ice nucleation on dust particles in the atmosphere is a key mechanism for ice formation in clouds. However, the conditions of a particle surface for efficient ice nucleation are poorly understood. In this study we present results of immersion freezing experiments using differential scanning calorimetry on emulsified mineral dust suspensions, involving the two chemically identical, but morphologically different kaolin minerals kaolinite and halloysite. Kaolinite occurs in a platy morphology, while halloysites form predominantly tubular structures. We investigated six different halloysite and two different kaolinite samples. Our results show that, on average, the halloysite samples exhibit a higher ice nucleation (IN) activity, than the kaolinite samples, but also a higher diversity in terms of freezing onset temperatures and heterogeneously frozen fraction. Repeating the freezing experiments after shortly milling the samples led to a decrease in freezing onset temperatures and in the heterogeneously frozen fraction of the halloysite samples, bringing their IN activity closer to that of the kaolinites. To interpret these findings, the freezing experiments were complemented by dynamic vapour sorption (DVS) measurements, pore ice melting experiments with slurries, and transmission electron microscopy (TEM) before and after milling. These measurements demonstrate the destruction of tubes by milling and provide evidence for the influence of the tubular structure of the halloysites on their IN activity. We identify the OH–Al–O–Si–OH functionalized edges as the most likely site for ice nucleation, as the high geometric diversity of the edges best accounts for the high diversity in IN activity of halloysites. We hypothesize that the stacking of layers and the number of stacks in halloysite tubes and kaolinite platelets affect the freezing temperature, with thicker stacks having the potential to freeze water at higher temperatures. The notion that the edges constitute the IN-active part of kaolin minerals is further supported by comparing kaolin minerals with montmorillonites and feldspars, all of which exhibit enhanced IN activity in the presence of ammonia and ammonium-containing solutions. As OH–Al–O–Si–OH functionalized edge surfaces are the only surface type kaolin particles have in common with montmorillonites and feldspars, the common feature of IN activity enhancement in ammoniated solutions can only be explained by ice nucleation occurring at the edges of kaolin minerals.

Kristian Klumpp et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2022-489', Anonymous Referee #1, 03 Oct 2022
  • RC2: 'Comment on acp-2022-489', Anonymous Referee #2, 24 Oct 2022

Kristian Klumpp et al.

Kristian Klumpp et al.

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Short summary
The prerequisites of a particle surface for efficient ice nucleation are still poorly understood. This study compares the ice nucleation activity of two chemically identical but morphologically different minerals kaolinite and halloysite. We observe, on average higher ice nucleation activities for halloysite than kaolinite, but also higher diversity between individual samples. We identify the particle edges as the most likely site for ice nucleation.
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