Ice Formation Processes in Upper Tropospheric Cloud Conditions

DeMott/摘要本研究的目的是确定大气气溶胶的化学成分如何影响卷云的形成。需要检验的假设是，在低于-35摄氏度的温度下，溶液液滴存在于100%相对湿度以下时，通过均匀的冻结成核形成冰的理论是一致的，并且部分可溶大气气溶胶的不可溶组分在不那么严格的条件下很容易成为非均相冻结核。将使用连续流动扩散室对温度低至-60摄氏度的温度进行系统的实验室测量，以确定与大气相关的硫酸盐气溶胶形成的溶液液滴中均匀冻结成核的起始溶质浓度条件。将对气雾剂进行重复测量，包括特征明确的不溶性气雾剂成分。可溶和部分可溶气溶胶分布的验证测量将在模拟绝热气团上升的云室中进行。实验室数据将与现有的对流层上层气溶胶和冰核组成数据一起用于数值研究卷云中的冰形成。预计这一结果将与气溶胶通过控制云的微物理成分影响对流层上层云的辐射特性的作用有关。

Demott/Abstract The objective of this research are to determine how the chemical composition of atmospheric aerosols influences the formation of cirrus clouds. The Hypotheses to be tested are that ice formation by homogeneous freezing nucleation occurs in agreement with theory for solution droplets existing below 100% relative humidity at temperatures below -35 degrees Celsius, and that the insoluble components of partially- soluble atmospheric aerosols readily serve as heterogeneous freezing nuclei under less stringent conditions. Systematic laboratory measurements to temperatures as cold as -60 degrees Celsius will be made using a continuous flow diffusion chamber to determine the onset solute concentration conditions for homogeneous freezing nucleation in solution droplets formed on sulfate aerosols of atmospheric relevance. Measurements will be repeated for aerosols including well-characterized insoluble aerosol components. Validation measurements for distributions of soluble and partially-soluble aerosols will be made in a cloud chamber which simulates adiabatic air parcel ascent. The laboratory data will be used together with existing data on upper tropospheric aerosol and ice nuclei composition to numerically investigate ice formation in cirrus clouds. The results are expected to be relevant to the role of aerosols in influencing the radiative properties of upper tropospheric clouds through controls on cloud microphysical composition.