Minuscule solid particles (sing. nucleus) suspended in the atmosphere. Three types of atmospheric nuclei are distinguished: Aitken nuclei, of radii less than 0.1 μm‎ (Heggs et al. (1991) J. Geophys. Res. 96, 18), large nuclei, radii 0.2–1 μm‎, and giant nuclei, with radii greater than 1 μm‎. See Clarke (1992) J. Atmos. Chem. 14.

These nuclei may be scraps of dust, from volcanic eruptions or dust storms, or salt crystals, or given off when bubbles burst at the surface of the sea. Atmospheric nuclei can scatter sunlight enough to lower temperatures, if enough are present, and play an important role in cloud formation; see Kulmala et al. (2001) Tellus B 53, 4.