Homogeneous and Heterogeneous Ice Formation in Upper Tropospheric Conditions

卷云是一种分布广泛的大气成分，对辐射传输具有重要意义，但其形成机制尚不清楚。 冰晶是由液态水滴或水蒸气的固相成核而形成的。 含有硫酸盐溶质的液滴的均质成核被认为是主要过程，但是对于冰以这种方式形成的热力学条件的范围和变化性还没有一致的解释。 例如，含有溶质混合物的液滴中的冰的成核显然发生在比冻结单一溶质液滴所需的温度更高的温度下。 因此，卷云形成时的温度和海拔高度，可以由溶解在液滴中的气溶胶的化学成分决定。 该项目是对由各种水溶液组成的液滴的冻结进行的实验室研究，这些水溶液被认为是对流层上部发现的典型水溶液。 要研究的是（1）在-35 ℃和-65 ℃之间的温度和70%和100%之间的相对湿度下具有不同化学成分的溶液液滴的均匀冷冻，和（2）含有可能存在于对流层上部的那种不溶性颗粒的溶液液滴的非均匀冷冻。 该方法涉及生成各种类型的气溶胶颗粒，确定其冰成核特性，并进行数值模拟计算，将实验室测量与现有的均质和非均质冰成核理论联系起来。 这项研究将提供在大尺度大气模型中预测卷云形成所需的数据。

Cirrus clouds are a widespread atmospheric constituent with major importance for radiative transfer, yet the mechanism of their formation is not well understood. The ice crystals of which they are composed form by nucleation of the solid phase from liquid water droplets or water vapor. Homogeneous nucleation of droplets containing sulfate solutes is thought to be the dominant process, but there is not yet a consistent explanation for the range and variability of thermodynamic conditions at which ice forms in this way. For example, nucleation of ice in droplets containing a mixture of solutes evidently occurs at warmer temperatures than required to freeze single-solute droplets. Therefore the temperature, and hence the altitude, at which cirrus clouds form may be determined by the chemical composition of aerosols dissolved within the droplets. This project is a laboratory study of the freezing of droplets composed of various aqueous solutions thought to be typical of those found in the upper troposphere. To be investigated are (1) the homogeneous freezing of solution droplets having different chemical compositions at temperatures between -35 and -65 degrees C and relative humidities between 70 and 100%, and (2) the heterogeneous freezing of solution droplets containing insoluble particles of the kind that might be present in the upper troposphere. The approach involves generation of well-characterized aerosol particles of various types, determining their ice-nucleating properties, and performing numerical modeling calculations to relate the laboratory measurements to existing theories of homogeneous and heterogeneous ice nucleation. The study will provide the data needed for predicting cirrus cloud formation in large-scale atmospheric models.