Abstract
Two types of particles exist in the atmosphere, primary and secondary particles. While primary particles such as soot, mineral dust, sea salt particles or pollen are introduced directly as particles into the atmosphere, secondary particles are formed in the atmosphere by condensation of gases. The formation of such new aerosol particles takes place frequently and at a broad variety of atmospheric conditions and geographic locations. A considerable fraction of the atmospheric particles is formed by such nucleation processes. The newly formed particles may grow by condensation to sizes where they are large enough to act as cloud condensation nuclei and therefore may affect cloud properties. The fundamental processes of aerosol nucleation are described and typical atmospheric observations are discussed. Two recent studies are introduced that potentially change our current understanding of atmospheric nucleation substantially.
Highlights
When a cloud forms in the atmosphere, water molecules condense
A large fraction of all the particles that float in the atmosphere can act as cloud condensation nuclei already at very small supersaturations of around 100.05% to 100.5% relative humidity
In the troposphere there are always enough particles around that can act as cloud condensation nuclei so that clouds never form in the atmosphere via the formation of new, pure water droplets
Summary
When a cloud forms in the atmosphere, water molecules condense. In cloud formation processes, typically an air parcel is ascending and temperature drops and relative humidity rises due to the expansion of the air parcel. This is, because it is energetically more favourable for the water molecules to condense on pre-existing particle surfaces, rather than forming new particles on their own due to the involved surface energy These particles on which the water condenses are called the cloud condensation nuclei (CCN). In the troposphere there are always enough particles around that can act as cloud condensation nuclei so that clouds never form in the atmosphere via the formation of new, pure water droplets. Supersaturation alone is not enough, it must be highly supersaturated such that on the one hand the nucleation barrier (see below) can be overcome and on the other hand the nucleating molecules have to cluster faster than being lost to the pre-existing aerosol particles that are always abundant in the atmosphere This condition is only reached occasionally and only for a few substances. Because the aerosol influences climate directly [20] and indirectly via the cloud formation processes [21], as well as because of adverse health effects of ultrafine particles [22,23], it is very important to gain a deeper understanding of nucleation processes and to include these processes in global climate and chemistry models
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