Acid Deposition—Ch 17

What is Acid Deposition?• Acid Deposition is the falling of acids & acid forming

compounds from the atmosphere to Earth’s surface.– SO2, NOx, and particulates can react in the atmosphere to

produce acidic chemicals that can travel long distances before deposition.

• Tall smokestacks reduce local air pollution but can increase regional air pollution.

• 66% of all SO2 & 25% of all NOx comes from electric power generation that produces energy by burning fossil fuels.

• Two Forms– Dry

• acidic gases & particles

– Wet• acid rain, fog, sleet, cloud vapor & snow

– Acid deposition is also commonly referred to acid rain (which is only one type).• Natural precipitation is slightly acidic

– pH = ~5.0-5.6

• Primarily because of acid deposition, typical rain in the US is now about 10 times more acidic

– pH = Under 5.6 & often as low as ~4.3

What is Acid Deposition?

– When gas pollutants (ex. SO2 & NO2) dissolve in rain water, various acids are formed.

• CO2 + H2O H2CO3 (carbonic acid)

• SO2 + H2O H2SO3 (sulphorous acid)

• NO2 + H2O HNO2 (nitrous acid) + HNO3 (nitric acid)

What is Acid Deposition?

Acid Rain• Forms when SO2 & NOx react with H2O vapor in air• Acidity measured using pH

– Numerical measure of the concentration of H+ ions in solution

• pH less than 7 = acidic• pH greater than 7 = alkaline or basic

– Each number represents a tenfold (10x) change

• Harmful to plants/animals• Damages structures made of limestone/marble

– Remember…• Calcite reacts w/ acid

– bubbles

Acidic Precipitation

Fossil fuelsPower plants

Industrial emissionsAuto emissions

Primary PollutantsSO2

NO2

Secondary Pollutants

H2SO4 HNO2

sulfuric acid nitric acid

soilsleaching of minerals

vegetationdirect toxicity

indirect health effects

water

sedimentsleaching aluminum

acidic precipitation

What areas are affected by acid deposition?• occurs on a regional rather than global basis.

– Serious regional problems in areas downwind from coal burning power plants, smelters, factories, & lg urban areas.

• growing problem in China, parts of the former Soviet Union, India, Nigeria, Brazil, Venezuela, & Columbia.

Health & Other Effects of Acid Deposition

• Health Effects:– respiratory diseases

• bronchitis• asthma

• Other damage:– Statues & buildings

• Especially those made of limestone or marble

– metals– car finishes

• Effects on plants:– Acid rain does not usually kill trees directly.

• can damage tree foliage by making it weaker & more susceptible to being damaged even more by other stresses

– Ex. cold temperatures, diseases, insects, drought, & fungi

• The areas hardest hit by acid deposition are mountaintop forests, which tend to have thin soils without much buffering capacity.

• Nutrient leaching from soil limits the availability of nutrients to plants

• exposure to toxic substances slowly released from the soil

Environmental Effects of Acid Deposition

• Effects on animals:– Generally, the young of most species are more sensitive to

environmental conditions than adults.– Release of metals into water

• Ex. Aluminum toxicity– Al+3 released from soil builds up in water

• Contamination of fish in some lakes with highly toxic methyl mercury.– Acid shock

• pH falls below species’ range of tolerance– Lakes and streams become acidic when the water itself and its

surrounding soil cannot buffer the acid rain enough to neutralize it.• At pH 5, most fish eggs cannot hatch. • At lower pH levels, some adult fish die.• Some acid lakes have no fish.

• How seriously vegetation & aquatic life in lakes are affected depends mostly on whether its soils are acidic or basic (…their buffering capacity).

Environmental Effects of Acid Deposition

Acid Deposition

&Aquatic Systems

• Acid deposition has not been greatly reduced– New technologies

such as scrubbers have helped

• SO2 emissions lower

• NOx emissions higher

– Effects are worse than predicted

• The Clean Air Act can’t restore ecosystems

• More must be done to control acid deposit.

Solutions

Acid Deposition

Prevention Cleanup

Reduce air pollution by improving energy efficiency

Add lime to neutralize acidified lakes

Reduce coal use

Add phosphate fertilizer to neutralize acidified lakes

Increase natural gas use

Increase use of renewable energy resources

Burn low-sulfur coal

Remove SO2 particulates & NOx from smokestack gases

Remove NOx from motor vehicular exhaust

Tax emissions of SO2