Aerosols are solid or liquid microscopic particles that exist within the atmosphere and are composed of a number of different natural or man-made substances. They are formed through the dispersal of material at the Earth’s surface or the reaction of gases in the atmosphere. The three main types of aerosol are volcanic, desert dust, and anthropogenic (human-made). Scientists believe that aerosols do impact the climate; however, due to the variability of location and short lifespan of aerosols it is difficult to measure concentrations and clearly assess the full effect on our climate.
Aerosols interact with the climate in two ways, directly and indirectly. Directly, aerosols scatter sunlight back into space, thus cooling the planet. For example, it has been estimated that over the eastern United States pollution in this form has reduced the annual crop growing season by one week. Indirectly, aerosols can affect the climate by changing the properties of clouds. Aerosols are necessary for cloud formation, acting as ?seeds? to start the formation of cloud droplets. Increases in aerosol concentrations cause water in a cloud to spread over many more particles, each of which is correspondingly smaller. Smaller particles fall more slowly in the atmosphere and decrease the amount of overall rainfall. Therefore, the changing of aerosols in the atmosphere can alter both the frequency of cloud occurrence as well as the thickness and rainfall amounts.
Most aerosols form a thin haze in the lower atmosphere, where they are typically washed out of the air by rain within a week; however, volcanic aerosols are quite different. After a particularly large volcanic explosion, droplets of sulfuric acid form in the stratosphere and winds spread the aerosols across the globe. Because it does not rain in the stratosphere, these aerosols can remain for up to two years. Volcanic aerosols reflect sunlight, thereby cooling the Earth’s surface. The relative coolness experienced in 1993 is thought to have been caused by the stratospheric aerosol layer produced by the Mt. Pinatubo eruption in 1991. Four years after the eruption, remnants still remained in the atmosphere.
Unlike volcanic aerosols, desert dust aerosols both absorb and reflect sunlight. Once in the atmosphere, minerals within the dust absorb sunlight, warming the surrounding air. This warm air prevents storm clouds from forming, deterring rainfall and furthering desertification. Sunlight reflected by the dust aerosols is prevented from reaching the Earth’s surface, creating a cooling effect similar to that of volcanic aerosols. Large grains of airborne sand fall to the ground while smaller particles can remain suspended in the air for a week or more. These smaller particles can be swept downwind over thousands of miles; desert dust from North Africa often travels widely over the Atlantic Ocean.
Approximately 10% of the total amount of aerosols in our atmosphere is anthropogenic. Much of this stems from sulfate aerosols created through the burning of fossil fuels, such as coal and oil, although smoke from burning forests can also be a factor. Major contributors of sulfate aerosols into the atmosphere include Europe, eastern North America, and eastern Asia. At current production levels, human-made sulfate aerosols are thought to outweigh those naturally produced. These aerosols also reflect sunlight, surviving in the atmosphere for 3 to 5 days. They can also enter clouds where they increase the number of cloud droplets but decrease the size of the droplets, causing the clouds to reflect more sunlight than they normally would. Scientists also believe that these added aerosols help polluted clouds to last longer and reflect more sunlight than non-polluted clouds.
Aerosols are thought to have the opposite effect of their climate changing counterpart, greenhouse gases. While greenhouse gases are expected to cause global warming, aerosols tend to produce regional cooling, particularly in areas near or downwind of industrial areas. Various computer models suggest that the cooling effect of man-made aerosols could potentially offset the warming effect by increased greenhouse gas concentrations by 30%. However, there is not enough research at this time to truly indicate what the long term outcome of these effects will be.
As part of a larger NOAA effort to monitor and study global aerosol concentrations, this page was created to educate the public on this unique atmospheric substance. It provides detailed information on aerosols, links to references, and pages that discuss aerosol ?hot spots.? Images of aerosol concentrations throughout the globe are also included.
Atmospheric Aerosols: What Are They, and Why Are They So Important?
This NASA fact page describes the three main types of aerosols. Other issues, such as how best to remove aerosols from the atmosphere and how to use aerosols as ?atmospheric tracers,? are also discussed.
Aerosols and Climate Change
This NASA Earth Observatory web page provides background on aerosols and briefly describes some of the connections between aerosols, cloud formation, and global warming. Various scientific studies that have contributed to the understanding of aerosols are also discussed.
All the World’s a Stage?For Dust
Animation of dust storms throughout the world is provided on this NASA website, along with helpful links and detailed information about how dust storms can contribute to the problem of aerosol pollution.
Data & Maps
Aerosols Group (AERO)
Since the mid 1970’s, the Geophysical Monitoring for Climate Change (GMCC) program—an initiative of the National Oceanic and Atmospheric Administration—has been measuring aerosols in the atmosphere from various stations around the globe. The program’s website lists much of the information gathered during its study, describes the instruments and measurements used, and provides other detailed scientific research.
Aerosols Increase Clouds, Brighten Atlantic Skies
Two maps from NASA’s Earth Observatory shows the interaction of clouds and aerosols over the Atlantic Ocean between June and August of 2004.
For the Classroom
Consumer Aerosols Products Council (CAPCO): Aerosol Facts
CAPCO provides free kits to assist educators in teaching their students about aerosols. The website also provides several fact sheets about aerosols, along with an entertaining and educational video.
University of Washington, Department of Atmospheric Sciences K-12 Pages: Danger in the Air!
Designed to collect and look at particulate matter that is in polluted air, this experiment helps students understand many of the concepts behind aerosols, including what they are, how they form, and what impact they may have on climate.
Buchdahl, Joe. What are Aerosols? Encyclopedia of the Atmospheric Environment, 2000.
Hardin, Mary and Ralph Kahn. Aerosols and Climate Change. NASA Earth Observatory.
NASA. Atmospheric Aerosols: What Are They, and Why Are They So Important? NASA Facts Online, August 1996.