Aerosols are tiny particles suspended in the air.
They occur naturally, originating from volcanoes, dust storms, forest and grassland fires, emissions from living vegetation, and sea spray. Human activities, such as the burning of fossil fuels and the alteration of natural surface cover, also generate aerosols. A severe volcanic event, such as the Pinatubo eruption in the Philippines in 1991, can put large amounts of aerosol into the stratosphere. Since it does not rain in the stratosphere, these aerosols can last for many months, producing beautiful sunsets around the globe, and possibly causing summer temperatures to be cooler than normal. Aerosols tend to cause cooling of the surface below them, since most aerosols are bright particles that reflect sunlight back to space, reducing the amount of solar radiation that can be absorbed at the surface.