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1/30/2012
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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
23 April 2009 Acid-Rain.ppt 18
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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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