Experimental and Theoretical Studies of Cloud-Particle Growth Kinetics

当云滴或冰晶通过凝结生长时，水分子从微粒周围的蒸气流到其表面，而凝结释放的热量则从微粒流出。经典理论把粒子的环境看作一个连续体，因此传热和传质可以用扩散方程来描述。对于比分子平均自由程大得多的水滴，这是一个有效的近似，在正常海平面条件下，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到1微米之间。因此，经典的连续介质理论必须加以修正，以包括气体动力学效应。修正引入了表征传热和质量传递的两个新参数，称为冷凝系数和调节系数。冷凝系数可以被描述为附着在粒子表面的分子的比例。调节系数可以被认为是在获得了该粒子的温度的情况下，从粒子表面反弹的分子的比例。一般来说，这些所谓的动力学系数必须通过实验来确定，因为没有已知的方法可以从理论上推导出它们的值。以前对它们进行评估的尝试给出了差异很大的结果。本课题采用了一种新的解决方法，在温度、压力和湿度可控的条件下，通过电动悬浮悬浮粒子，用激光能量脉冲加热粒子，通过温度变化和蒸发质量损失推导出调节系数和冷凝系数。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。用掺杂这些材料的液滴进行实验将确定它们对动力学系数的影响。将使用数值云模式进行敏感性试验，以评估动力系数及其变率的重要性。这项工作是了解大气中云的形成、发展和消散的基础。此外，云对太阳辐射的影响的准确计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.