Glossary

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

A

acid deposition

The process by which acidic particles, gases, and precipitation leave the atmosphere. More commonly referred to as acid rain, acid deposition has two components: wet and dry deposition.

acid rain

The result of sulfur dioxide (SO2) and nitrogen oxides (NOx) reacting in the atmosphere with water and returning to earth as rain, fog, or snow. Broadly used to include both wet and dry deposition. The acid rain page provides a great deal of information about this issue.

Al

Aluminum; a metal that is toxic to trees and fish

allowance

A tradeable permit to emit a specific amount of a pollutant. For example, under the Acid Rain Program, one allowance permits the emissions of one ton of sulfur dioxide (SO2).

anions

Negatively charged molecule such as sulfate (SO4(2-)) and nitrate (NO3-). In combination with hydrogen (H+), these molecules act as strong acids.

acid neutralizing capacity (ANC)

A measure of the ability for water or soil to neutralize added acids. This is done by the reaction of hydrogen ions with inorganic or organic bases such as bicarbonate (HCO3-) or organic ions.

acidification

Refers to reducing something's pH, making it more acidic; also means the loss of ANC.

adsorb

To take up and hold (a gas, liquid, or dissolved substance) in a thin layer of molecules on the surface of a solid substance.

B

buffering capacity

The resistance of water or soil to changes in pH.

base cations

Positively charged ions such as magnesium, sodium, potassium, and calcium that increase pH of water (make it less acidic) when released to solution through mineral weathering and exchange reactions.

C

Ca(2+)

Calcium; a base cation that helps to reduce acidification

chronic acidification

Generally refers to surface waters that remain acidified (ANC<0) regardless of variations in hydrologic conditions (precipitation, stream flow, etc.).

D

deposition

The processes by which chemical constituents move from the atmosphere to the earth's surface. These processes include precipitation (wet deposition, such as rain or cloud fog), as well as particle and gas deposition (dry deposition).

dose response functions

The relationship between the effects (response) on an organism or system and the amount (dose) of some material to which the organism/system is exposed.

dry deposition

The settling of gases and particles out of the atmosphere. Dry deposition is a component of acid deposition, more commonly referred to as acid rain.

E

eutrophication

A reduction in the amount of oxygen dissolved in water. The symptoms of eutrophication include blooms of algae (both toxic and non-toxic), declines in the health of fish and shellfish, loss of seagrass beds and coral reefs, and ecological changes in food webs.

F

G

H

I

J

K

L

leaching

Process by which water removes chemicals from soil through chemical reactions and the downward movement of water.

M

Mg(2+)

Magnesium; a base cation that helps to reduce acidification

mineral weathering

The physical and chemical breakdown of rocks that releases ions such as calcium and aluminum.

MW

Megawatt; a unit for describing how much electricity a power plant can generate. The Acid Rain Program includes virtually all units in the US that can generate over 25 MW.

N

nitrogen fixation

The process in which bacteria convert biologically unusable nitrogen gas (N2) into biologically usable ammonia (NH3) and nitrates (NO3-).

nitrogen oxides (NOx)

A group of gases that cause acid rain and other environmental problems, such as smog and eutrophication of coastal waters. Burning fossil fuels, such as coal and gasoline, releases NOx into the atmosphere. Various programs are reducing NOx emissions, including the Acid Rain Program and NOx cap and trade programs.

O

P

pH

A scale that denotes how acidic or basic a substance is. Pure water has a pH of 7.0 and is neither acidic nor basic. For more information, see the pH page.

precipitation

Water in the form of rain, sleet, or snow (wet deposition).

Q

R

S

sulfur dioxide (SO2)

A gas that causes acid rain. Burning fossil fuels, such as coal, releases SO2 into the atmosphere. Various EPA programs are reducing SO2 emissions, including the Acid Rain Program.

T

U

V

W

wet deposition

The process by which chemicals are removed from the atmosphere and deposited on the Earth's surface via rain, sleet, snow, cloudwater, and fog.

X

Y

Z

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