Rain-out of aerosol particles in clouds

The efficiency of wet removal of gases and particles is due to the fact that the falling speed of precipitation elements greatly exceeds the dry deposition velocity of trace constituents. In discussion of removal caused by clouds and precipitation it is reasonable to differentiate processes taking place in the clouds (rain-out) and beneath the cloud base (wash-out). 

Rain-out of aerosol particles is begun at the moment of cloud formation, since at the supersaturations occurring in the atmosphere ( 1 %, which is equivalent to a relative humidity of 101 %) condensation takes place on aerosol particles, called nuclei. To understand the principle of this phenomenon (see Mason, 1957 Fletcher, 1962) let us consider a cooling air parcel. Because of the cooling the relative humidity in the parcel increases. After reaching the saturation level, cloud drops begin to form on aerosol particles in the updraft. Each particle becomes active in the 

Results of thermodynamic calculations show (e.g. Dufour and Defay, 1963 E. Meszaros, 1969) that the critical supersaturation of solution droplets formed on water soluble particles (see Subsection 4.5.1) is less than values for insoluble particles of the same size.1 Furthermore, theory indicates that larger particles arc more active in condensation processes than smaller nuclei. These theoretical considerations are in good agreement with experimental findings (e.g. Twomey 1971 and 1972) which suggests that the majority of active condensation nuclei are composed of water soluble ammonium sulfate particles with dry radii greater than 0.01 0.05 /im. 

Because the mass of ammonium sulfate and sulfuric acid particles is mainly in the size range of active condensation nuclei, it is believed that this process provides a very effective removal mechanism for the tropospheric background aerosol. However, we have to emphasize that other processes are also operating in the cloud to remove small aerosol particles, of which the most important process is the coagulation of particles and cloud drops. As we have seen (Subsection 4.1.1), thermal coagulation is particularly effective in the range of very small particles inactive in condensation. To estimate this process, let us consider a cloud in which the number concentration of drops with radius rc is Nc. If the number concentration 

1 The critical supersaturation of solution droplets may be calculated by equations [4.13] and [4.14] by using relative humidities greater than 100 %. The curves calculated in this way have a maximum at a certain supersaturation which gives the critical value (Fletcher, 1962). The critical supersaturation of insoluble particles is given by the Thompson formula, obtained by substituting x0 = 1 in equation [4.13]. 

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