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
What is Acid Deposition?
– 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)
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
Primary Pollutants
SO2
NO2
Secondary Pollutants
H2SO4
HNO2
sulfuric acid nitric acid
acidic precipitation
Fossil fuels
Power plants
Industrial emissions
Auto emissions
vegetation
direct toxicity
indirect health effects
soils
leaching of minerals
water
sediments
leaching aluminum
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
Environmental Effects of Acid Deposition
• 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).
Acid
Deposition
&
Aquatic
Systems
Solutions
Acid Deposition
Prevention
Reduce air pollution
by improving
energy efficiency
Cleanup
• Acid deposition has
not been greatly
reduced
Add lime to
neutralize
acidified lakes
Reduce coal use
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
Add phosphate
fertilizer to
neutralize
acidified lakes
– 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.