Laboratory investigations of ice formation in the Earth's atmosphere

地球大气层中形成的云层在地球气候中扮演着重要角色。它们都可以反射来自太阳的入射光线，从而使地球降温，并通过捕获热量来隔绝行星表面。云还提供了一种可以发生化学反应的介质，从而改变了大气的化学成分。除其他因素外，云层影响大气和气候的方式取决于单个云滴和冰晶的物理性质。然而，这项提议的中心话题，如云中冰的形成等过程，人们对此知之甚少。事实上，我们对云中冰形成的了解程度非常低，以至于政府间气候变化专门委员会(IPCC)在其最新的气候变化评估中没有将冰云包括在内，尽管冰核无疑对气候有重大影响。这项提案中描述的工作将通过一系列实验室实验开始解决基本科学知识的匮乏问题。这里提出的实验室实验主要分为两大类。在第一组中，建议研究在大气条件下形成的冰的晶体结构(冰中水分子的排列)。在最近的一项重大发现中，人们发现(Murray等人，自然，v434，p202,2005)，液态水可以冻结成冰，其晶体结构以前预计不会在地球较低的大气(海拔50公里)形成。六角冰是大气中常见的一种冰，它的晶体结构形成了雪花的六角形。这项提议涉及的一种不寻常的冰类型被称为立方冰，它与六角形冰具有一些不同的物理性质，因此，立方冰可能会强烈影响云的形成方式。本文建议研究与大气有关的成分的溶液液滴冻结时冰的结晶结构。用来做这件事的方法通常不适用于大气科学问题。如果这一提议成功，BJM将为英国大气科学界带来这一新颖而重要的方法论。在第二组实验中，建议研究固体不可溶颗粒对大气中冰云形成的影响。众所周知，如果大气中的纯净水滴被冷却，它将保持液态，直到达到约-38摄氏度。然而，水的结冰温度通常比零下38摄氏度高得多，因为结冰通常是由固体或颗粒引起的。只有在没有固体表面的情况下，液滴才能在非常低的温度下保持液体。固体颗粒对冰云形成的影响很难量化，部分原因是人们对常见大气颗粒的冰启动特性没有很好的了解。显然，如果我们想要更好地理解冰云及其对气候的影响，就需要对这些粒子的冰启动特性有一个详细的基础知识。这里建议开发一种方法，能够量化烟尘、矿物粉尘和陨石颗粒的替代物在浸泡在与大气有关的溶液液滴中时的结冰特性。这将用光学显微镜来测量带有固体包裹体的液滴中的冰形成。这些冰形成研究的结果将被用来在数值模式中限制冰的形成，以便评估特定粒子类型对云形成的影响。

Clouds that form in the Earth's atmosphere play an important role in the planet's climate. They can both reflect incoming light from the sun, thus cooling the planet, and insulate the surface of the planet by trapping heat. Clouds also alter the chemistry of the atmosphere by providing a medium in which, or on which, reactions can take place. The way in which clouds influence the atmosphere and climate depend, amongst other factors, on the physical properties of individual cloud droplets and ice crystals. However, processes such as ice formation in clouds, the central topic of this proposal, are very poorly understood. In fact our level of understanding of ice formation in clouds is so low that the Intergovernmental Panel on Climate Change (IPCC) does not include ice clouds in their most recent climate change assessment, even though ice nucleation undoubtedly has a significant impact on climate. The work described in this proposal will begin to address this paucity of basic scientific knowledge through a series of laboratory experiments. The laboratory experiments that are proposed here fall into two main categories. In the first set it is proposed to investigate the crystalline structure of ice (the arrangement of water molecules in ice) that forms under atmospheric conditions. In a recent major discovery it was found (Murray et al., Nature, v434, p202, 2005) that liquid water can freeze to ice with a crystal structure that was previously not expected to form in the Earth's lower atmosphere (altitude <50 km). Hexagonal ice is the 'normal' type of ice encountered in the atmosphere and its crystal structure gives rise to the hexagonal shape of snow flakes. The unusual type of ice that this proposal is concerned with is known as cubic ice and has some different physical properties to those of hexagonal ice, hence, cubic ice may strongly influence the way in which clouds form. It is proposed here to investigate the crystalline structure of ice when solution droplets of atmospherically relevant compositions freeze. The methodologies employed to do this are not typically applied to atmospheric science problems. If this proposal is successful, BJM will bring this novel and important methodology to the UK atmospheric science community. In the second set of experiments it is proposed to investigate the impact solid insoluble particles have on the formation of ice clouds in the atmosphere. It is well established that if a pure water droplet in the atmosphere is cooled, it will remain liquid until it reaches about -38oC. However, water often freezes at much higher temperatures than -38oC, because freezing is often induced by a solid object or particle. Only in the absence of solid surfaces can droplets stay liquid to very low temperature. The impact of solid particles on ice cloud formation is very poorly quantified, in part, because the ice initiating properties of common atmospheric particles are not well understood. Clearly, if we are to improve our understanding of ice clouds and their impact on climate, a detailed fundamental knowledge of the ice initiating properties of these particles is required. It is proposed here to develop a methodology capable of quantifying the ice forming properties of soot, mineral dust and proxies of meteoric particles when immersed in solution droplets of atmospheric relevance. This will be done with an optical microscope to measure ice formation in droplets with solid inclusions. The results from these ice initiation studies will be used to constrain ice formation in a numerical model in order to asses the impact of a particular particle type on the formation of clouds.