One of the components of an atmospheric air parcel, comprised of minute solids particles part of which is almost certainly water.
[The Character and Origins of Smog Aerosols; v9 - Advances in Environmental Science and Technology Series; Hidey, ed.; page 234; 1980; Wiley; New York.] [Environmental Particles; v1 - Environmental Analytical and Physical Chemistry Series; pages 7, 30, 33;1992.]
Source: Atmospheric Chemistry Glossary