Experimental and Theoretical Studies of Cloud-Particle Growth Kinetics

当云滴或冰晶通过凝结生长时，水分子从颗粒周围的水蒸气流向其表面，凝结释放的热量从颗粒流出。经典理论将颗粒所处的环境视为一个连续介质，从而用扩散方程来描述传热传质过程。对于比分子平均自由程大得多的水滴来说，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微米到1微米之间。为此，必须对经典的连续介质理论进行修改，以包括气体动力学效应。该修正引入了两个新的参数来表征热量和质量的传递，称为冷凝系数和调节系数。凝结系数可以用分子撞击附着在其上的颗粒表面的比例来描述。调节系数可以被认为是从颗粒表面反弹回来的分子中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来确定，因为没有已知的方法可以从理论上得出它们的值。之前对它们进行评估的尝试给出的结果差异很大。本项目探索了一种新的方法，在控制温度、压力和湿度的条件下，用电动悬浮将颗粒悬浮起来，用激光脉冲对其进行加热，并从温度变化和蒸发质量损失中推导出调节和凝结系数。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。使用掺入这些材料的液滴进行的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，以评估动力学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，准确计算云对太阳辐射的影响可能需要了解动力学系数。

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.