From AMS Glossary
Any one of the many microscopic particles in the atmosphere that serve as condensation nuclei for droplet growth in the large supersaturations (greater than a few hundred percent over water) produced during the rapid, near adiabatic expansion produced in an Aitken dust-counter.
These nuclei, often numbering many tens of thousands per cubic centimeter in city air, are both solid and liquid particles with diameters on the order of tenths of microns or smaller. Because of the excessive supersaturations that accompany expansions of the air sample in an Aitken dust- counter, the nucleus spectrum observed with this instrument does not correspond to that observed in natural cloud condensation processes, where supersaturations larger than one per cent over water are probably rare. On the other hand, Aitken nuclei play an important role in determining the local electrical conductivity of the air, because they capture small ions, becoming large ions with much lower mobility in the earth's fair-weather electric field. In air containing large numbers of Aitken nuclei, the small ion population is small, the large ion population is large, and the air conductivity is low. Either nucleus may also be a protoparticle for larger particles such as cloud condensation nuclei, the subset of Aitken nuclei responsible for the formation of cloud droplets.