Atmospheric deposition

Many kinds of particulates and gases are deposited from the atmosphere to the surfaces of terrestrial and aquatic ecosystems. Wet deposition refers to deposition occurring while it is raining or snowing, whereas dry deposition occurs in the time intervals between precipitation events.

Relatively large particles suspended in the atmosphere, such as dust entrained by strong winds blowing over fields or emitted from industrial smokestacks, may settle gravitationally to nearby surfaces at ground level. Particulates smaller than about 0.5 microns in diameter, however, do not settle in this manner because they behave aerodynamically like gases. Nevertheless, they may be impaction-filtered from the atmosphere when an air mass passes through a physically complex structure. For example, the large mass of foliage of a mature conifer forest provides an extremely dense and complex surface. As such, a conifer canopy is relatively effective at removing particulates of all sizes from the atmosphere, including those smaller than 0.5 microns. Forest canopies dominated by hardwood (or angiosperm) trees are also effective at doing this, but somewhat less-so than conifers.

Dry deposition also includes the removal of certain gases from the atmosphere. For example, carbon dioxide gas is absorbed by plants and fixed by photosynthesis into simple sugars. Plants are also rather effective at absorbing certain gaseous pollutants such as sulfur dioxide , nitric oxide, nitrogen dioxide, and ozone . Most of this gaseous uptake occurs by absorption through the numerous tiny pores in leaves known as stomata. These same gases are also dry-deposited by absorption by moist soil , rocks, and water surfaces.

Wet deposition involves substances that are dissolved in rainwater and snow. In general, the most abundant substances dissolved in precipitation water are sulfate, nitrate, calcium, magnesium, and ammonium. At places close to the ocean, sodium and chloride derived from sea-salt aerosols are also abundant in precipitation water. Acidic precipitation occurs whenever the concentrations of the anions (negatively charged ions) sulfate and nitrate occur in much larger concentrations than the cations (positively charged ions) calcium, magnesium, and ammonium. In such cases, the cation "deficit" is made up by hydrogen ions going into solution, creating solutions that may have an acidity less than pH 4. (Note that the ion concentrations in these cases are expressed in units of equivalents, which are molar concentrations multiplied by the number of charges on the ion.)

The deposition of acidified snow or rain can result in the acidification of vulnerable surface waters. This is particularly the case of streams, rivers, ponds, and lakes that have low concentrations of alkalinity and consequently little capacity for neutralizing inputs of acidity. Acidification may also be caused by the dry deposition of certain gases, especially sulfur dioxide and oxides of nitrogen. After they are dry-deposited, these gases become oxidized in soil or water into the ions sulfate and nitrate, respectively, a process accompanied by the production of equivalent amounts of hydrogen ions. In some environments the gaseous concentrations of sulfur dioxide and oxides of nitrogen are relatively high, particularly in areas polluted by gaseous emissions from large numbers of automobiles, power plants , or other industries. In those circumstances the dry deposition of acidifying substances will be a much more important cause of environmental acidification than the wet deposition of acidic precipitation.

[>Bill Freedman Ph.D. ]

RESOURCES

BOOKS

Freedman, B. Environmental Ecology. San Diego, CA: Academic Press, 1995.

Joskow, P.L., A.D. Ellerman, R. Schmalensee, M. Juan-Pablo, and E.M. Bailey, editors. Markets for Clean Air: The U.S. Acid Rain Program. Cambridge, UK: Cambridge University Press, 2000.

ORGANIZATIONS

United States Environmental Protection Agency, Clean Air Markets Division, 1200 Pennsylvania Avenue, NW, Washington, D.C. USA 20460 (202) 564-9150, Email: , http://www.epa.gov/airmarkets/acidrain/

Worldwatch Institute, 1776 Massachusetts Ave., N.W., Washington, D.C. USA 20036-1904 (202) 452-1999, Fax: (202) 296-7365, Email: [email protected], http://www.worldwatch.org/