A suspension of particles, liquid or solid, in a gas. The term implies a degree of permanence in the suspension, which puts a rough upper limit on particle size at a few tens of micrometers at most (1 micrometer = 0.00004 in). Thus in proper use the term connotes the ensemble of the particles and the suspending gas.

The atmospheric aerosol has two major components, generally referred to as coarse and fine particles, with different sources and different composition. Coarse particles result from mechanical processes, such as grinding. The smaller particles are ground, the more surface they have per unit of mass. Creating new surface requires energy, so the smallest average size that can be created by such processes is limited by the available energy. It is rare for such mechanically generated particles to be less than 1 μm (0.00004 in.) in diameter. Fine particles, on the other hand, are formed by condensation...