Wash-out of trace constituents below the cloud base

M3 Wash-out of trace constituents below the cloud base 

Once a cloud is formed there are two possibilities concerning its future fate. One possibility is that the cloud partially or totally evaporates. In this case absorbed trace constituents again become airborne. However, a new aerosol spectrum is produced in this way compared to the size distribution before cloud formation, since 

2 It is to be noted that the laboratory experiments of Berry (see Beiike and Gravenhorst, 1978) do not clearly support this conclusion. 

The other obvious possibility is that materials absorbed are carried by precipitation to the surface of the Earth, that is they are definitively removed from the air. There is no intention here to discuss the formation of precipitation. We only mention that it is believed (Fletcher, 1962) that, in winter layer clouds with small liquid water content, ice crystals play an important role in precipitation formation, while in summer convective clouds the coalescence of large drops with smaller ones is the dominant process. At the same time we have to emphasize that the wet removal of trace constituents is continued by falling precipitation elements (snow crystals, raindrops) below the cloud base. This removal mechanism is called washout. 

Aerosol particles below the cloud base are captured by precipitation elements due to gravitational coagulation. This type of coagulation is caused by the difference between falling speeds of the aerosol particles and the raindrops or snow crystals. In other words, this means that precipitation elements overtake the particles. The air molecules go around the falling drops (or crystals) while large particles are impacted against the drops due to their inertia. For this reason precipitation elements are considered to be small impactors (see Subsection 4.1.2). 

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