Atmospheric ice nuclei in the Arctic

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The Arctic climate is changing twice as fast as the global average and these dramatic changes are evident in the decreases in sea ice extent over the last few decades. The lowest sea ice cover to date was recorded in 2007 and recent data suggests sea ice cover this year may be even lower. Clouds play a major role in the Arctic climate and therefore influence the extent of sea ice, but our understanding of these clouds is very poor. Low level, visually thick, clouds in much of the world tend to have a cooling effect, because they reflect sunlight back into space that would otherwise be absorbed at the surface. However, in the Arctic this albedo effect is not as important because the surface, often being covered in snow and ice, is already highly reflective and Arctic clouds therefore tend to warm instead of cooling. Warming in the Arctic can, in turn, lead to sea ice break-up which exposes dark underlying sea water. The sea water absorbs more of the sun's energy, thus amplifying the original warming. Hence, small changes in cloud properties or coverage can lead to dramatic changes in the Arctic climate; this is where the proposed research project comes in. A large portion of clouds, including those found in the Arctic region, are categorized as mixed phase clouds. This means they contain both supercooled water droplets and ice crystals (for a demonstration of supercooled water see: http://www.youtube.com/watch?v=0JtBZGXd5zo). Liquid cloud droplets can exist in a supercooled state well below zero degrees centigrade without freezing. Freezing will, however, be observed if the droplets contain a particle known as an ice nucleus that can catalyze ice formation and growth. Ice formation dramatically alters a cloud's properties and therefore its influence on climate. At lower latitudes, ice nuclei are typically made up of desert dusts, soot or even bacteria. But the composition and source of ice nuclei in the Arctic environment remains a mystery. A likely source of ice nuclei in the Arctic is the ocean. Particles emitted at the sea surface, through the action of waves breaking and bubble bursting, may serve as ice nuclei when they are lofted into the atmosphere and are incorporated in cloud droplets. This source of ice nuclei has not yet been quantified. We will be the first to make measurements of ice nuclei in the central Arctic region. We will make measurements of ice nuclei in the surface layers of the sea from a research ship as well as measuring airborne ice nuclei from the BAe-146 research aircraft.The sea's surface contains a wide range of bacteria, viruses, plankton and other materials which are ejected into the atmosphere and may cause ice to form. We will use state-of-the-art equipment developed at Leeds to measure how well sea-derived particles and particles sampled in the atmosphere nucleate ice. We will piggy back on a NERC funded project called ACACCIA, which not only represents excellent value for money (since the ship and aircraft are already paid for under ACCACIA), but is a unique opportunity to access this remote region. Results from the proposed study will build upon previous work performed in the Murray laboratory and generate quantitative results that can be directly used to improve computer-based cloud, aerosol and climate models. Our results will further our understanding of these mysterious and important mixed phase clouds and, in turn, the global climate.