Simultaneous Measurement of Condensation and Thermal Accommodation Coefficients

当云滴通过凝结成长时，水分子从云滴周围的蒸气流到云滴表面，而凝结释放的热量则从云滴流出。经典理论把液滴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毫米。然而，新形成的云滴的尺寸通常在0.1到1mm之间，因此必须修改经典的连续介质理论，以包括气体动力学效应。该修正引入了表征热和质量传递的两个新参数，即调节系数和冷凝系数，分别用a和b表示。调节系数可以被认为是从液滴表面反弹并获得液滴温度的分子的比例。冷凝系数可以被描述为撞击液滴表面并粘在液滴上的分子的比例。一般来说，a和b不相等，必须通过实验确定，因为没有已知的方法可以从理论上推导出它们的值。虽然以前曾有人试图确定这些系数，但结果却千差万别。这有几个原因。首先，在受控条件下测量微米大小的液滴的生长速度不是一件容易的事。第二，a和b以这样一种方式联系在一起，即给定的液滴生长速率可以用a和b的许多组合来解释。第三，有证据表明，系数的值可能受到水中少量杂质的影响。这个项目的目的是利用a和b对大气压力的不同依赖来分别确定它们。在不同的温度和压力条件下，用激光照射小水滴，利用已知的米氏散射曲线峰来确定水滴的大小，观察了小水滴的生长速率。在不同化学成分的气溶胶上形成的液滴的实验将表明污染物对a和b的可能影响。已经有一些证据表明，杂质降低了这些系数的值。较低数值的作用是降低小液滴的生长速度。液滴尺寸越小，云对太阳辐射的反射率就越大。因此，污染气溶胶不仅可以增加小云滴的数量，而且还可以通过凝结延缓它们的早期生长。因此，正确处理云对太阳辐射的影响可能需要准确地了解调节系数和凝结系数。

As a cloud droplet grows by condensation, water molecules flow from the vapor surrounding the droplet to its surface and the heat released by condensation flows away from the droplet. Classical theory treats the environment of the drop 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 mm for normal sea level conditions. However, newly formed cloud droplets have sizes typically between 0.1 and 1 mm, for which 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, the accommodation coefficient and the condensation coefficient, denoted respectively by a and b. The accommodation coefficient may be thought of as the fraction of the molecules bouncing off the surface of a drop that have acquired the temperature of the drop. The condensation coefficient may be described as the fraction of the molecules hitting the surface of the drop that stick to it. In general, a and b are not equal and must be determined experimentally, because there is no known way to derive their values theoretically. Though there have been previous attempts to determine the coefficients, the results are quite variable. There are several reasons for this. First, it is no easy matter to measure the rate of growth of a micron-sized droplet in controlled conditions. Second, a and b are linked in such a way that a given droplet growth rate can be explained by many combinations of a and b. Third, there is evidence that the values of the coefficients may be influenced by small amounts of impurities in the water. This project aims to determine a and b separately by taking advantage of their different dependence on atmospheric pressure. The rates of growth of small droplets are observed under different conditions of temperature and pressure by illuminating them with a laser and employing the known peaks in the Mie scattering curve for water droplets to determine drop size. Experiments with drops formed on aerosols of different chemical composition will indicate the possible influence of contaminants on a and b. There is already some evidence that impurities lower the values of these coefficients. The effect of lower values is to reduce the growth rates of small droplets. A consequence of smaller droplet size is a greater reflectivity of the cloud for solar radiation. Therefore pollution aerosols may have the effect not only of increasing the number of small cloud droplets but also retarding their early growth by condensation. The correct treatment of the effects of clouds on solar radiation may therefore require accurate knowledge of the accommodation and condensation coefficients.