Homogeneous and Heterogeneous Ice Formation in Upper Tropospheric Conditions

卷云是一种广泛存在的大气成分，对辐射传输具有重要意义，但其形成机制尚不清楚。组成它们的冰晶是由液态水滴或水蒸气形成的固相成核而形成的。含有硫酸盐溶质的液滴均匀成核被认为是主要的过程，但是对于以这种方式形成冰的热力学条件的范围和可变性，还没有一个一致的解释。例如，在含有溶质混合物的液滴中，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，卷云形成的温度和高度可能是由溶解在水滴中的气溶胶的化学成分决定的。这个项目是一个实验室研究液滴的冻结，这些液滴由各种水溶液组成，被认为是在对流层上层发现的典型液滴。要研究的是(1)具有不同化学成分的液滴在-35至-65摄氏度的温度和70至100%的相对湿度下的均匀冻结，以及(2)含有可能存在于对流层上层的不溶性颗粒的液滴的非均匀冻结。该方法包括生成各种类型的具有良好特征的气溶胶粒子，确定它们的冰成核特性，并进行数值模拟计算，将实验室测量结果与现有的均匀和非均匀冰成核理论联系起来。这项研究将为在大尺度大气模式中预测卷云的形成提供所需的数据。

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.