Many people, when they hear the word “aerosol,” think of a chemical aerosol spray from a can of air freshener or hair spray. The word actually applies to any solid or liquid particles suspended in air. Because they are so small, aerosols tend to float (e.g., clouds), though like everything else they are subjected to gravity, falling at a rate of about four inches (10 centimeters) every 24 hours, unless washed away more quickly by rain.