Quantifying the efficiency with which solid mineral particles nucleate ice when immersed in supercooled water droplets

由冰粒和过冷液滴组成的云被称为混合相云，存在于-37摄氏度以上的温度，覆盖了地球的大部分地区。这些云层既通过捕获发出的红外辐射来使地球变暖，又通过将从太阳传入的可见光反射回太空来冷却地球，从而影响气候。越来越明显的是，混合相云对大气中存在的粒子(称为气溶胶)的数量和类型非常敏感。过去已经做了大量工作，以了解气溶胶对完全由液滴组成的云的作用，政府间气候变化专门委员会(气专委)试图量化这种影响，尽管存在很大的不确定性。然而，气溶胶在冰层形成中扮演的角色仍然非常不确定，尽管有证据表明影响很大，但气溶胶在冰层形成中所起的作用仍然非常不确定，政府间气候变化专门委员会无法评估这种强迫。能够催化冰粒形成的气溶胶被称为冰核；然而，它们的特性、浓度、全球分布以及它们成核冰的效率目前都没有得到很好的量化。一类重要的冰核是矿物尘埃，它从撒哈拉沙漠等干旱地区输送到全球各地(主要提案中的卫星图片戏剧性地说明了这一点)，并已知会在行星范围内影响云的形成。不幸的是，尽管我们知道冰的形成对云很重要，但我们的知识太贫乏，无法在气候模型中准确地模拟云的形成，这限制了它们的准确性，这对政策制定者来说是一个明显的问题。此外，自20世纪60年代以来，北大西洋地区矿物粉尘的数量增加了2-4倍，这可能与人类活动对云中结冰的影响未知有关。然而，在模拟云中冰的形成及其对气候的影响方面正在取得进展。在第一个全球模型敏感性研究中，Lohmann和Diehl(J.ATM.科学文献p968,2006)研究了两种粘土矿物(大气尘埃的常见成分)对层状混合相云的影响，发现辐射强迫在1.0到2.1Wm-2之间。因此，矿物尘埃通过冰核在一种云类型中的辐射强迫可与1750年以来通过人类活动排放的二氧化碳(1.7W·m-2)的强迫相当。此外，它们还证明了气候对沙尘类型的敏感性。虽然这些全球模型研究表明，尘埃形成的冰很重要，但作者承认，他们的模型研究建立在不充分的实验室数据基础上。现有数据仅涵盖已知存在于大气中的一小部分矿物粉尘，根据液滴中存在的粉尘数量很难对存在的矿物粉尘进行量化。这种情况需要通过专门的实验室实验来改善。在这项提议中，概述了一系列实验，在这些实验中，将利用默里实验室开发的现有设备来确定各种矿物粉尘在浸入水滴时成核冰的速度。这些数据将为理解任何天然矿物粉尘的成冰特性提供基础，因为天然粉尘是矿物的混合物。这一假设随后将在一系列天然尘埃中得到验证。在这项提案中，为一位著名的研究员(莎拉·布罗德利)寻求12个月的博士后职位。

Clouds composed of both ice particles and supercooled liquid droplets, known as mixed phase clouds, exist at temperatures above ~-37oC and cover a large portion of the planet. These clouds impact on climate by both simultaneously warming the planet by trapping outgoing infrared radiation and cooling the planet by reflecting incoming visible light from the sun back to space. It is becoming increasingly apparent that mixed phase clouds are very sensitive to the number and type of particles, known as aerosols, present in the atmosphere. A lot of work has been done in the past to understand the role of aerosols on clouds that are entirely composed of liquid droplets and the Intergovernmental Panel on Climate Change (IPCC) attempted to quantify this impact, albeit with large uncertainties. However, the role that aerosols play in ice formation, which dramatically alters the properties of a cloud, remains very uncertain and the IPCC were not in a position to assess this forcing despite evidence that the impact is large. Aerosols that can catalyse ice particle formation are known as ice nuclei; however, their identity, concentration, global distribution and the efficiency with which they nucleate ice are all poorly quantified at present. An important class of ice nuclei is mineral dust which is transported over globally from arid regions such as the Sahara (dramatically illustrated by the satellite picture in the main proposal) and is known to impact cloud formation on a planetary scale. Unfortunately, while we know that ice formation is important for clouds, our knowledge is too poor to accurately model cloud formation in climate models and this limits their accuracy, which is an obvious problem for policy makers. Furthermore, there has been a two-four fold increase in mineral dust in the North Atlantic region since the 1960's and this is possibly linked to human activities with an unknown impact on ice formation in clouds. However, progress in modelling ice formation and its impact on climate in clouds is being made. In the first global modelling sensitivity study of its kind, Lohmann and Diehl (J. Atm. Sci., p968, 2006) studied the impact of two clay minerals (common components of atmospheric dust) on stratiform mixed phase clouds and found the radiative forcing to be between 1.0 and 2.1 W m-2. Hence radiative forcing by mineral dust through ice nucleation in just one cloud type is comparable to the forcing by CO2 (1.7 W m-2) emitted through human activity since 1750. In addition they also demonstrated that climate is sensitive to the dust type. While these global model studies show that ice formation by dust is important the authors acknowledge that their modelling studies are based inadequate laboratory data. The existing data only covers a narrow range of mineral dusts known to be in the atmosphere and that which exists is poorly quantified in terms of amounts of dust present in droplets. This situation needs to be improved through dedicated laboratory experiments. In this proposal a set of experiments are outlined in which existing equipment, developed in Murray's laboratory, will be made use of to determine the rate at which various mineral dusts nucleate ice when immersed within aqueous droplets. This data will provide the basis for understanding the ice nucleating properties of any natural mineral dust since natural dusts are a mixture of minerals. This hypothesis will then be tested with a range of natural dusts. In this proposal funds are sought for a 12 month post-doctoral position for a named researcher (Sarah Broadley).