Experimental and Theoretical Studies of Cloud-Particle Growth Kinetics

当云滴或冰晶通过冷凝生长时，水分子从围绕粒子的蒸汽流向其表面，冷凝释放的热量从粒子中流出。 经典理论将粒子所处的环境视为连续介质，因此热质传递由扩散方程描述。 对于比分子平均自由程大得多的液滴，这是一个有效的近似值，在正常海平面条件下，分子平均自由程约为0.06微米。 然而，新形成的液滴的尺寸通常在0.1微米和1微米之间。 对于这些，经典的，连续理论必须修改，包括气体动力学效应。 该修正引入了两个新的表征传热传质的参数，称为凝结系数和调节系数。 凝结系数可以被描述为击中颗粒表面并粘附在其上的分子的分数。适应系数可以被认为是从颗粒表面反弹并获得颗粒温度的分子的分数。 一般来说，这些所谓的动力学系数必须通过实验确定，因为没有已知的方法从理论上推导出它们的值。 以前试图评估它们的结果差异很大。 该项目采用一种新的方法来解决这个问题，通过在温度，压力和湿度的控制条件下通过电动悬浮来悬浮颗粒，用激光能量脉冲加热它们，并从温度变化和蒸发引起的质量损失中推导出调节和冷凝系数。 最近的注意力集中在微量挥发性溶质对云滴生长和蒸发的可能影响上。 使用掺杂有这些材料的液滴的实验将确定它们对动力学系数的影响。 将使用数值云模型进行敏感性试验，以评价动力系数的重要性及其变异性。 这项工作是理解大气中云的形成、发展和消散的基础。 此外，云对太阳辐射的影响的准确核算可能需要动力学系数的知识。

As a cloud droplet or ice crystal grows by condensation, water molecules flow from the vapor surrounding the particle to its surface and the heat released by condensation flows away from the particle. Classical theory treats the environment of the particle as a continuum, so that the heat and mass transfer are described by the diffusion equation. This is a valid approximation for drops that are much larger than the molecular mean free path, which is about 0.06 microns for normal sea level conditions. However, newly formed droplets have sizes typically between 0.1 and 1 microns. For these, the classical, continuum theory must be modified to include gas kinetic effects. The modification introduces two new parameters that characterize the transfer of heat and mass, called the condensation coefficient and the accommodation coefficient. The condensation coefficient may be described as the fraction of the molecules hitting the surface of the particle that stick to it. The accommodation coefficient may be thought of as the fraction of the molecules bouncing off the surface of a particle that have acquired the temperature of the particle. In general, these so-called kinetic coefficients must be determined experimentally, because there is no known way to derive their values theoretically. Previous attempts to evaluate them have given widely variable results. This project approaches the problem a new way, by suspending the particles by electrodynamic levitation under controlled conditions of temperature, pressure, and humidity, heating them with pulses of laser energy, and deducing the accommodation and condensation coefficients from the temperature change and the loss of mass by evaporation. Recent attention has focused on the possible influence of trace amounts of volatile solutes on cloud droplet growth and evaporation. Experiments using droplets doped with these materials will determine their effect on the kinetic coefficients. Sensitivity tests using a numerical cloud model will be carried out to evaluate the significance of the kinetic coefficients and their variability. This work is fundamental for understanding the formation, development, and dissipation of clouds in the atmosphere. Furthermore, an accurate accounting of the effects of clouds on solar radiation may require knowledge of the kinetic coefficients.