The role of aerosols in Earth’s radiation budget is complex. Depending on the type of aerosol, they can either reflect or absorb incoming solar radiation. When they reflect incoming radiation that energy is no longer available for use in the Earth–atmosphere system, and can result in an overall cooling effect. Certain types of aerosols, however, called dark aerosols, tend to absorb incoming solar radiation (An example of a dark aerosol would be soot, or the incomplete burning of hydrocarbons.) This radiation is then radiated in all directions warming the atmosphere in the vicinity of the soot, but cooling Earth’s surface.

Of all the solar radiation that reaches the top of Earth’s atmosphere, 30% is reflected back into space. This 30% is called Earth’s albedo. Of the 6% that is reflected by Earth’s atmosphere, some is reflected by aerosols. Of the 16% of incoming solar radiation that is absorbed by the atmosphere, some is absorbed by aerosols while the remainder is absorbed by various gaseous molecules in the atmosphere.

As we have seen, tropospheric aerosols including those produced by humans as well as natural aerosols like sand have roles in Earth’s radiation budget. However, they remain in the atmosphere for only a few days or weeks. But when certain aerosols reach the stratosphere, they can persist for years.

The Role of Volcanoes

The aerosols emitted by strong volcanic eruptions tend to persist in the atmosphere for years, changing Earth’s radiation budget for that period. Unlike human produced aerosols, or other natural aerosols that settle out of the atmosphere in a matter of days, volcanic eruptions can spew aerosols high into the stratosphere, where the winds can spread their effects worldwide. The stable air in the stratosphere that lacks strong vertical motions helps prevent some types of these aerosols from settling quickly. Some examples of volcanic eruptions that have apparently had an effect on Earth’s radiation budget include Tambora, Krakatoa, and most recently Pinatubo. After Tambora erupted in 1815, much of the world experienced the “year without a summer.” In 1816, crops failed throughout much of North America and Europe. Snowfalls were reported in New England in the middle of summer. The cold was attributed to the effects of Tambora. (See http://www.islandnet.com/~see/weather/history/1816.htm for more on “The year without a summer.”) In 1883, Krakatoa, another Indonesian volcano erupted. Only Tambora is known to have been a bigger eruption in modern times. Again, cooling, though not as pronounced as after Tambora, was reported in many locations for the following years. More recentlty, Mount Pinatubo, in the Philippines erupted in 1991. The following two years saw a 1¼ centigrade drop in average temperatures worldwide. The drop in temperatures may have been even more pronounced were it not for the 1991-1992 and 1993 El Niño events. (See http://www.cotf.edu/ete/modules/elnino/elnino.html). Although volcanoes emit a lot of solid particulate aerosols into the atmosphere, these tend to settle out in a relatively short period of time. Major cooling is thought to be largely due to large amounts of a liquid aerosol, sulfuric acid (H2SO4) that reflects incoming solar radiation back to space. Some volcanoes emit great amounts of sulfuric acid.

URLs: Earth's Radiation Budget

http://www.cotf.edu/ete/modules/ozone/ozatmo.html

http://www.ouh.nl/open/dja/klimaat/system/crucial_role_of_aerosols_a.htm

http://denali.frontier.iarc.uaf.edu:8080/~cecile/Professional/CloudMicrostructure/CloudMicrostructureImpact.html

Cloud Radiation Processes
http://climate.gsfc.nasa.gov/research/clouds.php

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