A more complete definition of aerosols.

   Aerosol :

• particles may be solid or liquid;
• range in size from 0.01 microns to several tens of microns.
• originate from natural or anthropogenic (human-made) sources.

 

 

The composition of atmospheric particulate matter varies widely and may consist of various chemical species such as sulfate, mineral dust, organics, soot, etc. Key examples of important tropospheric aerosols are smoke from large-scale fires (biomass combustion), dust plumes from semi-arid regions, and sulfates from industrial polluted regions. Under normal circumstances, the majority of aerosols form a thin haze in the lower atmosphere (troposphere), where they are washed out of the air by rain within about a week. Aerosols are also found in a part of the atmosphere just above the troposphere (called the "stratosphere"). A severe volcanic eruption, such as Mount Pinatubo 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 remain there for many months, producing beautiful sunsets around the globe, and possibly causing summer temperatures to be cooler than normal. Scientists estimate that Mount Pinatubo injected  about 20 million tons of sulfur dioxide into the atmosphere, cooling average global temperatures over the following year by about half a degree. It is now widely accepted that tropospheric aerosols cause the largest uncertainty in predicting the global change (IPCC, 1995).

	Bush Fires in Southern Mozambique. Easterly winds from the Mozambique Channel blow the smoke from many large fires in the southern part of this island. Neighboring views show the smoke moving over and around the Chimanimani Mountains, into Zimbabwe on the African plateau (NASA photograph).	Large dust storm in China developed in April 16. Its interaction with a meteorological system carried the dust far out into the Pacific Ocean. On April 25, dust from this event reached the west coast of North America (SeaWiFs photograph).
	Space Shuttle photograph of fires in the east coast of Mozambique in southern Africa. The source of the burning in Africa is the savanna grass (NASA photograph).	Biomass burning in Central Saudi Arabia. The biomass burning is thought to be a seasonal occurrence, possibly having to do with agricultural infestation (NASA photograph).
	Saharan Air Layer in Canary Islands. Dust is originated in dry regions of Africa and covers much of the tropical ocean between Africa and the Caribbean during summer months (SeaWiFs photograph).	Space Shuttle photograph of burning in the tropical rainforest ofBrazil. Estimates suggest that on the average, about 30 million acres of tropical rainforest in Brazil burn each year (NASA Photograph).