Acid Deposition

• Lake Barkevatn in Norway used to have healthy stocks of trout and perch.

• As a result of acid rain, the trout stock died out in the mid-1970s and the perch stock at the beginning of the 1980s.

• Unpolluted rain is itself slightly acidic with a pH level of about 5.6.

• Results from the dissolution of CO2 (an acidic oxide) in water droplets

CO2 (g) + H2O(l) H2CO3 (aq)

H2CO3 (aq) H+ (aq) + HCO3-(aq)

CO2 (g) + H2O(l) H+ (aq) + HCO3- (aq)

Acid Deposition

• The deposition of acids from the atmosphere in precipitation that falls as rain, sleet, snow, and particulates.

• Fog & dew can also be acidic.

• Mainly results from nitrogen oxides and sulfur oxides.

• These graphs compare nitrate concentrations & pH levels in the Eastern US with less populated areas that have fewer coal-burning power plants.

• Coal contains sulfur in the form of the mineral pyrite FeS2

4 FeS2 + 11 O2 2 Fe2O3 + 8 SO2

• A 1000 megawatt coal-fired electricity generating plant burns ~700 tons of coal per hr

• @4% S• 56 tons SO2 generated per hour

• 490,560 tons SO2 per year

Other Sources of Sulfur

• Volcanoes

• Microbes & plants

• Smelters

Smelters

• Nonferrous smelting facilities are large point sources (individual sites emitting large amounts of a pollutant) of SO2

• Many useful metals such as copper & nickel occur in nature as sulfide ores.

• In these smelters, ores are reduced to free metals.

• First step in the process is to remove the sulfur from the sulfide (roasting).

The Smelting Process

Acid Rain Is Not a New Phenomenon

• First recognized by Robert Angus Smith in the mid-1800s in Great Britain

• In the 1950s Swedish scientists began to observe pH values of precipitation lower than 5.2.

• In the 1960s low pH values were discovered in the north Eastern U.S.

Isopleths of annual rainfall pH 1955-56 & 1965-66

Isopleths of annual rainfall pH 1972-73 & 1975-76

Note the increased concentration from west to east.

Inputs of NOx to Norway 1985-93

Inputs of SOx to Norway 1985-93

Effects of Acid Rain

Effects on Aquatic Life

• In late 1960s– fish populations were found to be declining in

lakes in the Adirondack Mountains.– Fish populations were declining in lakes in

Sweden & Norway– Salamander populations were declining rapidly

in the northeastern U.S.

Precipitation pH Data for Dec. 23, 1997-Jan 20, 1998

New York State

Effects on Aquatic Life

• By 1970s in Adirondack Mountains– more than 50% of lakes >600 meters had pH<5– 90% of lakes contained no fish

• By 1990s in Sweden– 14,000 lakes unable to support lake organisms

sensitive to acidic conditions– 2,200 lakes nearly lifeless

Neutralization & Acid-sensitivity

• Different rivers, lakes & soils are not equally affected by or sensitive to acid deposition.

• Natural water bodies may contain substances that buffer the pH.

• The buffering capacity is mainly determined by the amount of bicarbonate (HCO3

-) and carbonate (CO32-) in the water.

Natural Buffers

• If soils and the underlying rock are rich in calcium carbonate (limestone), then the deposited acids are partially neutralized.CaCO3 + H2SO4 Ca2+ +SO4

2- + HCO3- + H+

• This leads to a delay in the acidification of these aquatic systems.

Natural Buffers

• Natural waters underlain by rocks containing little CaCO3 are more susceptible to acidification.– Adirondacks– Fennoscandinavian region

• Rocks rich in silicate minerals are abundant in these areas.

Human Intervention

• Sweden & Norway are carrying out a program to neutralize the acid in lakes by adding lime to the lakes.

• Stimulates the natural process involving the reaction of acid with carbonate.

• An expensive temporary cure.

• Analogous to taking antacid tablets to neutralize stomach acid!

Effects on Forest Ecosystems

• Forest damage has been extensive in Europe.

• In Germany, the 75% of the fir trees were found to be affected in 1983.

Declining Forests

• The crown density of the spruce at right is normal.

• The crown density of tree at left is lower

Declining Forests

• Leaves and needles can be damaged directly

• Trees damaged indirectly by loss of nutrients from the soil via leaching.– In very acidic conditions,

Al becomes soluble & is released from soil

– at high concentration it is toxic & damages roots

Destruction of Our History

• Both SO2 and acid rain react with buildings, statues & monuments made of limestone & marble.

• Calcium carbonate is the chemical composition of these materials.

Reactions of CaCO3

• Calcium carbonate dissolves in acid rain

CaCO3(s) + H+(aq) Ca2+ (aq) + HCO3- (aq)

• Calcium carbonate reacts with SO2 to form gypsum (CaSO4.2H2O) which is also water soluble.

German sandstone statue from 1702 photographed in 1908 (left) & in 1969 (right).

Gargoyles