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.

中文翻译：

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高岭土矿物高岭石和埃洛石的冰核特性比较

摘要。大气中尘埃颗粒上的非均质冰核是云中冰形成的关键机制。然而，对于有效冰核化的粒子表面条件知之甚少。在这项研究中，我们展示了使用差示扫描量热法对乳化矿物粉尘悬浮液进行浸没冷冻实验的结果，其中涉及两种化学性质相同但形态不同的高岭土矿物高岭石和埃洛石。高岭石以板状形态出现，而埃洛石主要形成管状结构。我们研究了六种不同的埃洛石和两种不同的高岭石样品。我们的结果表明，平均而言，埃洛石样品表现出比高岭石样品更高的冰核 (IN) 活性，而且在冻结起始温度和非均质冻结部分方面也具有更高的多样性。在短暂研磨样品后重复冷冻实验导致冷冻起始温度和埃洛石样品的不均匀冷冻部分降低，使其IN活性更接近高岭石的活性。为了解释这些发现，冷冻实验得到了动态蒸汽吸附 (DVS) 测量、浆料孔隙冰融化实验以及研磨前后的透射电子显微镜 (TEM) 的补充。这些测量结果证明了铣削对管的破坏，并为埃洛石的管状结构对其 IN 活性的影响提供了证据。我们将 OH-Al-O-Si-OH 功能化边缘确定为最有可能形成冰核的位置，因为边缘的高几何多样性最好地解释了埃洛石 IN 活性的高多样性。我们假设埃洛石管和高岭石薄片中的层堆叠和堆叠数量会影响冷冻温度，较厚的堆叠有可能在较高温度下冻结水。通过将高岭土矿物与蒙脱石和长石进行比较，进一步支持了边缘构成高岭土矿物的 IN 活性部分的观点，所有这些在氨和含铵溶液的存在下都表现出增强的 IN 活性。由于 OH-Al-O-Si-OH 功能化边缘表面是高岭土颗粒与蒙脱石和长石唯一的共同表面类型，