A comprehensive laboratory study on the immersion freezing behavior of illite NX particles: a comparison of 17 ice nucleation measurement techniques
A comprehensive laboratory study on the immersion freezing behavior of illite NX particles: a comparison of 17 ice nucleation measurement techniques
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DOI：
10.5194/acp-15-2489-2015
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发表时间：
2015-01-01
影响因子：
6.3
通讯作者：
Yamashita, K.
中科院分区：
文献类型：
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作者：
Hiranuma, N.;Augustin-Bauditz, S.;Yamashita, K.
沉浸冻结是混合相云中冰晶形成的最相关的异质冰核形成机制。近年来，越来越多的利用各种仪器的实验室实验研究了与大气相关的冰核粒子的沉浸冻结活性。然而，对这些实验室结果进行相互比较是一项艰巨的任务，因为研究人员使用了不同的冰核（IN）测量方法来得出这些结果。一个仍然存在的挑战是探索这些技术的敏感性和准确性，并了解与这些技术相关的实验参数如何潜在地影响或使冰核结果产生偏差。
在冰核研究单元（INUIT）的框架内，我们将一种富含伊利石的样品（伊利石NX）作为大气矿物粉尘粒子的代表性替代物分发给研究人员，以便他们使用不同的冰核测量方法进行沉浸冻结实验，并获得作为粒子浓度、温度（T）、冷却速率和成核时间函数的冰核数据。共有17种测量方法参与了数据的相互比较。使用7种仪器进行的实验是在冷却前将测试样品预先悬浮在水中，而另外10种仪器则是先让水蒸气凝结在干分散的粒子上，然后进行沉浸冻结。使用冰核活性表面位点密度n(s)对所得的综合沉浸冻结数据集进行评估，以建立一个具有代表性的n(s)(T)谱，该谱涵盖了较宽的温度范围（-37°C < T < -11°C），并且n(s)涵盖了9个数量级。
一般来说，17种沉浸冻结测量技术在温度方面大约8°C的范围内，相对于n(s)偏差3个数量级。此外，我们有证据表明，伊利石NX粒子以n(s)表示的沉浸冻结效率相对独立于液滴大小、悬浮粒子质量、粒子大小和冻结过程中的冷却速率。富含伊利石的黏土矿物粒子的沉浸冻结效率对温度有很强的依赖性，对时间和大小的依赖性较弱，这使得n(s)参数化仅作为温度的函数。我们还对n(s)(T)谱进行了表征，并确定了在-20°C到 -27°C之间斜率较陡的一段，在该段中我们测试的粉尘的大部分活性位点可能引发沉浸冻结。在 -27°C以下是一个斜率较缓的区域。虽然在 -27°C以下不同仪器之间的一致性是合理的，但对于这种矿物粉尘，悬浮和干分散粒子测量的温度相关的冰核活性似乎有不同的趋势，特别是在较高温度下。例如，在 -27°C到 -18°C之间，以n(s)表示的冰核活性对于湿悬浮样品的平均值较小，而对于干分散气溶胶样品的平均值较大。只有使用湿悬浮样品进行测量的仪器能够测量 -18°C以上的冰核活性。对 -27°C到 -18°C之间偏差的一种可能解释进行了讨论。提供了基于比表面积的n(s)(T)和基于几何表面积的n(s)(T)在线性和对数空间的多重指数分布拟合。这些通过使用相同参考样品进行约束的新拟合将有助于比较本研究未包括的冰核测量方法以及未来冰核仪器的冰核数据。
Immersion freezing is the most relevant heterogeneous ice nucleation mechanism through which ice crystals are formed in mixed-phase clouds. In recent years, an increasing number of laboratory experiments utilizing a variety of instruments have examined immersion freezing activity of atmospherically relevant ice-nucleating particles. However, an intercomparison of these laboratory results is a difficult task because investigators have used different ice nucleation (IN) measurement methods to produce these results. A remaining challenge is to explore the sensitivity and accuracy of these techniques and to understand how the IN results are potentially influenced or biased by experimental parameters associated with these techniques.Within the framework of INUIT (Ice Nuclei Research Unit), we distributed an illite-rich sample (illite NX) as a representative surrogate for atmospheric mineral dust particles to investigators to perform immersion freezing experiments using different IN measurement methods and to obtain IN data as a function of particle concentration, temperature (T), cooling rate and nucleation time. A total of 17 measurement methods were involved in the data intercomparison. Experiments with seven instruments started with the test sample pre-suspended in water before cooling, while 10 other instruments employed water vapor condensation onto dry-dispersed particles followed by immersion freezing. The resulting comprehensive immersion freezing data set was evaluated using the ice nucleation active surface-site density, n(s), to develop a representative n(s)(T) spectrum that spans a wide temperature range (-37 degrees C < T < -11 degrees C) and covers 9 orders of magnitude in n(s).In general, the 17 immersion freezing measurement techniques deviate, within a range of about 8 degrees C in terms of temperature, by 3 orders of magnitude with respect to n(s). In addition, we show evidence that the immersion freezing efficiency expressed in n(s) of illite NX particles is relatively independent of droplet size, particle mass in suspension, particle size and cooling rate during freezing. A strong temperature dependence and weak time and size dependence of the immersion freezing efficiency of illite-rich clay mineral particles enabled the n(s) parameterization solely as a function of temperature. We also characterized the n(s)(T) spectra and identified a section with a steep slope between -20 and -27 degrees C, where a large fraction of active sites of our test dust may trigger immersion freezing. This slope was followed by a region with a gentler slope at temperatures below -27 degrees C. While the agreement between different instruments was reasonable below similar to -27 degrees C, there seemed to be a different trend in the temperature-dependent ice nucleation activity from the suspension and dry-dispersed particle measurements for this mineral dust, in particular at higher temperatures. For instance, the ice nucleation activity expressed in n(s) was smaller for the average of the wet suspended samples and higher for the average of the dry-dispersed aerosol samples between about -27 and -18 degrees C. Only instruments making measurements with wet suspended samples were able to measure ice nucleation above -18 degrees C. A possible explanation for the deviation between -27 and -18 degrees C is discussed. Multiple exponential distribution fits in both linear and log space for both specific surface area-based n(s)(T) and geometric surface area-based n(s)(T) are provided. These new fits, constrained by using identical reference samples, will help to compare IN measurement methods that are not included in the present study and IN data from future IN instruments.