Laboratory investigations of ice formation in the Earth's atmosphere

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.