Читать книгу Introduction to Nanoscience and Nanotechnology - Chris Binns - Страница 34

The presence of aerosol in the atmosphere has a significant influence on climate, its most important role being in the formation of clouds. Pure water vapor in the atmosphere is invisible but when it condenses into microscopic water droplets, suspended as an aerosol, over a region of sky a cloud is born. The process of cloud formation and how they evolve and precipitate is a complex process but an important fundamental consideration relevant to this book is that without a preexisting aerosol of particles, clouds would not form except in extremes of high supersaturation. In a purely gaseous atmosphere, even one saturated with water vapor, it is not possible for water droplets to start growing, unless there are some initial “seed” particles that water can condense onto. These seeds are referred to as cloud condensation nuclei (CCNs). The reason why pure water vapor will not form droplets is described briefly in Advanced Reading Box 2.2 but in a nutshell, although water molecules do stick together, at normal temperatures, and vapor pressures, they do not stay together long enough for a third and fourth molecule to join them and start a droplet growing. However, a water droplet above a critical size that somehow appeared would be stable and in a humid atmosphere would grow. Without CCNs there is no way to achieve the initial water droplet above the critical size. The presence of preexisting CCNs changes that and water molecules can easily condense onto them and grow to a normal cloud droplet size. These fall sufficiently slowly under gravity to be considered as suspended (see Advanced Reading Box 2.1).