Formation of cloud droplets Heterogeneous nucleation

As seen from the Kohler equation (4.223), the condensation of water vapor into preexisting droplets is only possible when the saturation ratio is larger than one. However, the mass transfer of water molecules is possible for 6 1 when the solid (aqueous particles are not existing) particle surface provides a large enough affinity to H2O. This property is called hygroscopicity. Each crystalline water-soluble surface dissolves or deliquesces at a certain RH (Table 4.15). 

When pHjO P o, water condenses onto the activated CCN and the particle volume roughly increases by at least two orders of magnitude to form mist, fog and/or cloud droplets. This process is called heterogeneous nucleation. The supersaturation is always a result of the adiabatic cooling of the air parcel. Because of the release of the condensation heat and the entrainment of dryer air from surrounding droplet formations (or cloud extensions) is limited. 

From a mass transfer point of view, the heterogeneous nucleation is also a nucleation scavenging of PM, the first process of in-cloud scavenging, following by a gas uptake. Junge (1963) described it by the simple equation  

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