Technically, an aerosol is a suspension of fine solid particles or liquid droplets in a gas. Examples are clouds, and air pollution such as smog and smoke. In general conversation, aerosol usually refers to an aerosol spray can or the output of such a can. The word aerosol derives from the fact that matter "floating" in air is a suspension (a mixture in which solid or liquid or combined solid–liquid particles are suspended in a fluid). To differentiate suspensions from true solutions, the term sol evolved—originally meant to cover dispersions of tiny (sub-microscopic) particles in a liquid. With studies of dispersions in air, the term aerosol evolved and now embraces both liquid droplets, solid particles, and combinations of these. The Earth atmosphere contains aerosols of various types and concentrations, including quantities of:
By far the most common aerosols in the atmosphere are clouds, which normally consist of suspensions of water droplets or ice particles of greater or lesser density.
Aerosols can be found in urban Ecosystems in various forms, for example: