Heterogeneous freezing of water droplets containing kaolinite particles
Heterogeneous freezing of water droplets containing kaolinite particles
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DOI：
10.5194/acp-11-4191-2011
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发表时间：
2011-01-01
影响因子：
6.3
通讯作者：
Wills, R. H.
中科院分区：
文献类型：
--
作者：
Murray, B. J.;Broadley, S. L.;Wills, R. H.
由冰粒和过冷液态水滴组成的云存在于类似于236 K的温度以上。这些混合相云强烈影响气候，对可以催化冻结的固体颗粒的存在非常敏感。在本文中，我们描述的实验，以确定在粘土矿物高岭石成核冰的条件下，当浸入水滴。这是第一次浸没模式实验，其中高岭石的冰成核能力已被确定为粘土表面积，冷却速率的函数，也在恒温下。含有已知量粘土矿物的水滴负载在疏水性表面上，并以0.8至10 K min（-1）的速率冷却或保持在恒定的零度以下温度。通过光学显微镜测定单个10-50 μ m直径液滴冻结的时间和温度。对于10 K min（-1）的冷却速率，随着液滴中高岭石的浓度从0.005wt%增加到1wt%，10-40 μ m直径液滴的中值成核温度从接近均匀成核极限（236 K）增加到240.8 +/-0.6 K。这些数据表明，冻结的可能性与高岭石包裹体的表面积成比例。我们还表明，在恒定的温度下，随着时间的推移，液滴的数量呈指数下降，因为它们冻结。恒定的冷却速率的实验是一致的随机，奇异和修改奇异的描述的异质成核，然而，在冷却过程中，在恒定的温度下的冻结可以调和最好的随机方法。我们报告温度依赖的成核速率系数（每单位面积每单位时间的成核事件）的高岭石，并提出了一个一般的参数化浸泡成核，这可能是适合云建模一旦成核由其他重要的冰成核物种是量化的未来。
Clouds composed of both ice particles and supercooled liquid water droplets exist at temperatures above similar to 236 K. These mixed phase clouds, which strongly impact climate, are very sensitive to the presence of solid particles that can catalyse freezing. In this paper we describe experiments to determine the conditions at which the clay mineral kaolinite nucleates ice when immersed within water droplets. These are the first immersion mode experiments in which the ice nucleating ability of kaolinite has been determined as a function of clay surface area, cooling rate and also at constant temperatures. Water droplets containing a known amount of clay mineral were supported on a hydrophobic surface and cooled at rates of between 0.8 and 10 K min(-1) or held at constant sub-zero temperatures. The time and temperature at which individual 10-50 mu m diameter droplets froze were determined by optical microscopy. For a cooling rate of 10 K min(-1), the median nucleation temperature of 10-40 mu m diameter droplets increased from close to the homogeneous nucleation limit (236 K) to 240.8 +/- 0.6 K as the concentration of kaolinite in the droplets was increased from 0.005 wt% to 1 wt%. This data shows that the probability of freezing scales with surface area of the kaolinite inclusions. We also show that at a constant temperature the number of liquid droplets decreases exponentially as they freeze over time. The constant cooling rate experiments are consistent with the stochastic, singular and modified singular descriptions of heterogeneous nucleation; however, freezing during cooling and at constant temperature can be reconciled best with the stochastic approach. We report temperature dependent nucleation rate coefficients (nucleation events per unit time per unit area) for kaolinite and present a general parameterisation for immersion nucleation which may be suitable for cloud modelling once nucleation by other important ice nucleating species is quantified in the future.