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Acid DepositionAcid rain, snow, fog, and dry

deposition

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Acid Deposition

• Is the process by which acidic particles leave

the atmosphere

• Can be

– Wet deposition in the form of rain, sleet, snow,

fog, hail

– Dry deposition, includes acidic gases and particles

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pH scale

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• Measure of acidity/alkalinity of solution

• 7 = neutral

• Less than 7 = acidic

• More than 7 = alkaline

• Logarithmic scale– Change from 7 to 6 means

10X more acidic

pH scale

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Rain is naturally acidic

• pH of about 5.6

• CO2 + H2O � H2CO3

– Naturally occurring or anthropogenic CO2

combines with water to form carbonic acid

• Any precipitation with a pH of less than 5.6 is considered acidic.

Formation of Acids

• Primary pollutants (SO2, NOx, particulates) as discussed earlier

• Primary pollutants converted to secondary pollutants

• Secondary pollutants may deposit directly (dry deposition) or react with water in atmosphere to form acid (wet deposition)

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Conversion of SO2 to acid

Primary: S + O2 � SO2

Secondary: SO2 + O2 � SO3

SO2 + H2O � H2SO3

2H2SO3 + O2 � 2H2SO4

SO3 + H2O � 2H2SO3

23 April 2009 Acid-Rain.ppt 6

Conversion of NO to acid

Primary: N2 + O2 � 2NO

Secondary:2NO + O2 � 2NO2

3NO2 + H2O � 2HNO3 + NO

4NO + O2 + H2O � 4HNO2

23 April 2009 Acid-Rain.ppt 7 8

Areas prone to acid deposition

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• In the US: Northeastern States & Ohio Valley

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Areas prone to acid deposition• Outside the US: Eastern Europe, Russia, China

Acid-Rain.ppt 10

Some statistics

• Most acidic precipitation recorded in

Wheeling, WV, with a pH of 1.5

• Los Angeles often has fogs with a pH of 2.2-4.0

• Average pH of rainwater in this part of Florida

is around 4.8

• 20-60% of deposition can be dry, depending

on location

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Effects of Acid Deposition

Aquatic environments (lakes, rivers, streams, etc.)

– Aquatic species may die, reducing diversity and abundance

of aquatic ecosystem

– Which can be detrimental to other animals who rely on

aquatic species.

– HNO3 can lead to eutrophication

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Effects of Acid Deposition

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Plant Life• Deposition directly damages leaves and needles• SO2 blocks stomata on leaves, preventing photosynthesis• Reduces ability to withstand cold, reproduce

Effects of Acid DepositionSoil & Plants

• Acid deposition leaches nutrients from soil

– Ca2+, Mg2+ and K+ are replaced by H+, and so are no longer

available to plants

– Mg2+ required for photosynthesis

• Reacts with rock to release Al3+ into soil

– Damages plant roots

– Reduces ability to uptake water and nutrients

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Effects of Acid Deposition

Effects on materials

• Reaction between acids and materials like

marble and limestone (CaCO3), concrete or

iron

2CaCO3 + SO2 + O2 � CaSO4 + 2 CO2

CaCO3 + H2SO4 � CaSO4 + H2O + CO2

Fe + H2SO4 � FeSO4 + H2

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Effects of Acid Deposition

1910-age about 400 yrs 1984-age about 484 yrs

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Effects of Acid Deposition

Acid-Rain.ppt 19

Effects of Acid Deposition

Humans

• Breathing of acidic pollutants can harm

humans, as discussed earlier

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Control Strategies

• Control of emissions, as

discussed earlier

• Reduce demand for fossil

fuels by

– Increasing use of

alternative energy

sources

– Increasing public transit

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Control Strategies

• Lakes in limestone rich environments have

natural defense

• During “liming,” limestone is added to lakes or

streams without this defense to counteract

acidification

• Only provides short term control

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Is it getting better?

• Clean Air Acts 1970 & 1990

• “Concentrations of major pollutants decreased 32%,” between 1970-1997.• Costs of compliance = $436 billion

• Health benefits = $2.7 – 14.6 trillion

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The End

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