Studies of Aerosol Scavenging by Ice Particles in Clouds

本研究的目的是确定在降雪中沉积的气溶胶和气体中，有多少部分是由于通过轮状机制（无论是通过雪晶轮状机制还是通过高山表面轮状机制）去除的液态云水造成的。本研究将重点研究大气气溶胶的各种微量成分是如何分布在云中三种水物质成分之间的：冰、液态水和水蒸气。研究结果将利用在研究地点获得的云的微物理特性知识进行解释。将根据观测到的气象条件和使用一个简单的冰晶轨迹和生长模型来估计冰粒生长的微物理条件。对所发生的大气条件的了解以及对云滴、冰粒和气溶胶颗粒的仔细现场表征，应能使我们更好地了解是什么清除和生长过程导致了最终观测到的冰晶化学成分。大气化学的关键问题涉及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。在冰冻条件下，这些被清除的物质中有很大一部分沉积在冰霜中，冰霜是在预先形成的雪晶或地球表面物体上的过冷雾或云滴冻结产生的冰。据信，在降雪条件下，气溶胶和气体的清除主要是通过液滴的收集和随后在雾凇中的沉积，而不是在雪晶中进行的。关于相互竞争的沉积机制的相对重要性的数据将有助于了解自然界中化学元素的全球循环。特别是，这些知识应该有助于了解在高海拔山区酸性物质沉积的条件。对高海拔地区森林破坏的认识提出了一个问题，即是什么机制导致这些影响在面积范围上如此严格地划定。对大气沉积过程的详细了解将有助于厘清酸、氧化剂或其他空气传播成分的作用。

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.