Studies of Aerosol Scavenging by Ice Particles in Clouds

这项研究的目的是确定降雪中沉积的气溶胶和气体中有多大比例是通过结圈机制去除液态云水所致，无论是通过雪晶结圈还是通过高山表面结圈。这项研究将侧重于大气气溶胶的各种微量成分如何以及在哪里分布在云中的三种水物质成分之间：冰、液态水和水蒸气。结果将使用在研究地点获得的云的微物理性质的知识来解释。对冰粒生长的微物理条件的估计将根据观测到的气象条件和使用简单的冰晶轨迹和生长模型进行。对发生的大气条件的了解和对云滴、冰粒和气溶胶粒子的仔细现场描述应有助于更好地了解最终观测到的冰晶的化学成分是什么清除和生长过程。大气化学的关键问题与空气中的气溶胶粒子和气体被云和雨清除并沉积在地球表面的机制有关。在冰冻条件下，这些被清除的物质中有很大一部分被沉积在雾化冰中，雾化冰是由于过冷的雾或云滴在预制的雪晶或地球表面的物体上冻结而产生的冰。人们认为，在降雪条件下，气溶胶和气体的清除主要是通过收集液滴和随后在雾层中沉积来进行的，而不是通过雪晶进行。关于相互竞争的沉积机制的相对重要性的数据将有助于理解自然界中化学元素的全球循环。特别是，这种知识应该有助于理解在高山海拔地区酸性物质沉积的条件。对最高海拔地区森林破坏的认识引发了问题，即是什么机制导致这些影响在面积上被如此明显地划定。对大气沉积过程的详细了解应该有助于理清酸、氧化剂或其他空气成分的作用。

The objective of this research is to determine what fraction of the aerosols and gases deposited in snowfall is due to liquid cloud water removal by the riming mechanism, whether by snow crystal riming or by high mountain-surface riming. This study will focus on how and where the various trace constituents of the atmospheric aerosol are distributed among the three water substance components within the cloud: ice, liquid water and water vapor. The results will be interpreted using acquired knowledge of the microphysical properties of the cloud at the study site. Estimates of the microphysical conditions within which the ice particles grew will be made from observed meteorological conditions and the use of a simple ice crystal trajectory and growth model. Knowledge of the atmospheric conditions which occur and careful on-site characterization of the cloud droplets, ice particles and aerosol particles should permit a better understanding of what scavenging and growth processes lead to the final observed ice crystal chemical composition. Key questions of atmospheric chemistry pertain to the mechanisms by which airborne aerosol particles and gases are scavenged by clouds and rain, and deposited at the earth's surface. Under freezing conditions a significant fraction of these scavenged materials is deposited in rime ice, which is the ice that results from freezing of supercooled fog or cloud droplets either on preformed snow crystals or on objects at the earth's surface. The scavenging of aerosols and gases under snowing conditions is believed to occur predominantly by collection in liquid droplets and subsequent deposition in the rime, rather than in the snow crystals. Data on the relative importance of competing deposition mechanisms will contribute to the understanding of the global cycling of the chemical elements in nature. In particular, this knowledge should help to understand the conditions governing the deposition of acidic materials at high mountain elevations. The recognition of forest damage at the highest elevations has raised questions about what mechanisms cause these effects to be so sharply delimited in areal extent. A detailed understanding of atmospheric deposition processes should help to sort out the roles of acids, oxidants, or other airborne constituents.