In theoretical treatment, wet deposition is divided into rainout (within-cloud scavenging) and precipitation (below-cloud scavenging). In practice, however, these two processes are regarded together when developing wet deposition models.
Fine particles are incorporated into a cloud through vertical motion into its base by the lateral entrainment at its edges. A fraction of particle population contained in the rising air parcel entering the cloud at its base will contain cloud condensation nuclei (CCNs) upon which concentration of water vapor will occur. This process leads to the conversion of fine particles to cloud droplets. The process is known as nucleation scavenging.
Depletion of water vapor occurs as soon as the condensation is in place, continuing until the relative humidity reaches the saturated value of 100%. It is important then to obtain information on the distribution ofcritical supersaturation, also called supersaturation spectrum for the fine particle population in order to predict the outcome of the cloud condensation process. The number concentration of CCNs increases with supersaturation. This concentration can vary over several orders of magnitude and is dependent on proximity to sources and meteorological factors. Only a small portion of fine particles can serve as CCNs, about 20% of the total number concentrations at supersaturation of 1 % in general type of air masses as up to 35% in marine air. Thus, the term nucleation scavenging refers to the process through which some atmospheric particles, the CCNs, grow into cloud droplets in a supersaturated air parcel. The mathematical formulation of how water droplets grow in high-relative-humidity conditions and supersaturated environments (so-called Kohler equation) has been developed and is used in various deposition models.
Chemical composition of fine particles also plays a role in forming CCNs. Fine particles that consist of water-soluble, hygroscopic substances are more likely to act as CCNs. Results of various studies conclude that the primary source of CCNs in marine air is sulfate aerosol. Recent measurements indicate that sea salt CCNs can be the dominant source of activated CCNs in marine stratocumulus environment.
The capture of interstitial aerosol by cloud droplets can be an important fine-particle-removal mechanism in-cloud, in addition to nucleation scavenging.
The effect of the ice phase on in-cloud removal of fine particles by nucleation and impaction scavenging has also been studied. The transfer of particle mass from the liquid phase onto the ice phase by drop-freezing and scavenging of particle mass by riming of graupel is concluded to dominate the scavenging of particles due to the riming of snow crystals, as well as due to the impaction of particles onto ice particles. However, it should be concluded that drop nucleation dominates scavenging of particles in clouds.
Major improvement of our knowledge on the quantification of the processing of fine particles by clouds has been made during the EUROTRAC projects on Ground-Based Cloud Experiments (GCEs) and Processing of Trace Constituents in Clouds over Europe (PROCLOUD).
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