Acid rain is caused by NOx and SOx emissions from the combustion of fossil fuels which react with water vapor and oxygen in the air to form acids. These acids can fall to earth long distances from the source as wet or dry deposition, damaging lakes and forests. Lakes experience decreased biodiversity and fish mortality while forests show needle loss, inhibited growth, and increased soil acidity. Management strategies include reducing emissions through cleaner fuels and regulations, adding neutralizing agents to smokestacks, and remediating ecosystems. However, regulations are costly to implement and maintain, and international agreements are difficult to enforce.

1. Topic 6
Atmospheric Systems and Societies
6.4 Acid Deposition
http://www.youtube.com/watch?v=v09KnqiYi-c

2. Acid Rain
• NOx and SOx may either fall to the ground as
particles, aerosols or gases (dry deposition) or
else forms acid rain (wet deposition)
• The longer they remain suspended in the air,
the more likely they will be oxidised to HNO3
and H2SO4
• Wet deposition may occur thousands of
kilometres from the pollution source
(especially now that chimney stacks may be
hundreds of metres tall)

3. Effects of Acid Rain
• Lakes
– Reduced biodiversity
– Acidity may cause death of fish fry
– Decreased turbidity
– Growth of white moss on lake beds
– Increased concentrations of dissolved metals
(including Cd2+, Cu2+, Al3+, Zn2+, Pb2+) – this
makes them bioavailable to water plants and
also allows them to enter drinking water
• Forests
– Damage to lipids and membranes leading to
plant death
– SOx interferes with photosynthesis especially in
coniferous plants
– Acidity causes conifers to shed needles too
rapidly
– Low soil pH causes damage to root hairs and
and inhibits water and nutrient uptake

4. Distribution of Acid Deposition
• It is a global rather than localised problem, since
deposition may occur thousands of kilometres
from source
• There are natural sources of acidity, including
plants which acidify soils (heather and bog
moss), volcanic eruptions and the dissolution of
carbon dioxide in precipitation (carbonic acid)
• There also exist natural sources of buffering,
including chalk and limestone deposits
• Conversely, granite deposits are naturally acidic
and have low buffering capacity
• The areas most affected by anthropogenic acid
deposition are Sweden, Norway, Denmark,
Scotland, Germany, The Netherlands, Austria
and The East Coast of the U.S.
• This appears to be largely due to prevailing
winds and levels of rainfall
• They are also areas rich in lakes and rivers with
generally thin soils with low buffering capacity

5. Management Strategies
• Altering the human activity
– Burn less fossil fuels
– Reduce numbers of private vehicles
– Increased use of public transport and bikes
– Encourage carpooling
– Encourage pedestrianisation
– Switch to low emission fuels (or electric cars?)
• Evaluation
– It is hypocritical for MEDCs to deny sources of energy to
developing economies
– We live in a fossil fuel dependent economy
– Fossil fuel producers are strong lobbyists

6. Management Strategies
• Regulating pollution at the source
– Remove sulphur before combustion or SOx after
combustion (scrubbing)
– Burn coal with a neutralising agent (such as limestone)
– Use catalytic converters to change NOx back to N2
• Evaluation
– Expensive technologies
– Costs of these technologies are passed to the consumer
– Catalysers are very effective if used correctly
– Technologies are costly to maintain

7. Management Strategies
• Cleanup and regulation of damaged ecosystems
– Use chemicals to buffer soil or water and allow
decomposition of plants to offset the acidification process
– Allow recolonisation of damaged areas
– International agreements are necessary since the polluter
and the victim of the pollution may be different countries
(e.g. The Gothenburg Protocol, 1999)
• Evaluation
– It is always better to treat the cause rather than symptoms
– It acidification continues then remediation must continue
indefinitely
– International agreements are difficult to reach and police

8. Questions
1. Describe how acid rain is caused
2. Describe the effects of acid deposition on
living things
3. Describe and evaluate strategies to control
acid rain