Rainwater is naturally slightly acidic due to carbon dioxide in the air. However, when sulfur and nitrogen oxides from fossil fuel combustion dissolve in rainwater, they form strong acids like sulfuric and nitric acid, making the rainwater much more acidic. This phenomenon is called acid deposition. It can occur through wet deposition as acid rain or snow, or dry deposition of acidic particles. Primary pollutants like sulfur dioxide and nitrogen oxides directly cause acid deposition, which can damage forests and aquatic ecosystems by inhibiting growth, killing microbes and fish, and leaching nutrients from soils. International agreements have aimed to reduce acid deposition through domestic policies like installing pollution control devices on power plants and vehicles.

1. 6.4 Acid Deposition (Rain)

2. The pH Scale…What is acidic?

3. Acidity • Rain is naturally slightly
acidic pH 5.6
• Caused by carbon
dioxide in the air
dissolving into the water
forming carbonic acid

4. https://www.youtube.com/watch?v
=Nf8cuvl62Vc • However, when sulfur
and nitrogen oxides
dissolve in the rain they
form sulfuric and nitric
acid and can have a
very low pH (1-3)
• pH scale is logarithmic,
pH1 is x10 more acidic
that pH 2, and x100
stronger than pH 3

5. Acid Deposition (Process)

6. Chemistry of acidified precipitations
 Acid deposition can be either wet or dry:
 Wet deposition - acidic rain, snow, or other
precipitation
 Dry deposition - acidic gas or dry particles,
not mixed with water
 Pollutants can be classified as either primary or
secondary:
 Primary pollutants - those directly emitted by
a factory or automobile
 SO2 - sulfur dioxide
 NO and NO2, usually identified as NOx

7. Primary pollutants by source

8. Chemistry Cont.
 Secondary pollutants - primary pollutants react
with other substances in the atmosphere and
create different pollutants
 H2SO3 - sulfurous acid
 H2SO4 - sulfuric acid
 HNO3 - nitric acid

9. Effects of acid deposition on
environment
 Direct effects (Know 1):
 Inhibits embryonic development of fish
 Chlorophyll loss & yellowing of tree leaves and
buds → diminished growth
 Thinning of cuticle (the waxy coating on
needles)
 Symbiotic root microbes killed (i.e. Rhizobium
spp. and other beneficial fungi)

10. Effects of acid deposition on
environment
 Toxic effects (Know 1):
 Aluminum (Al) leaches out of soil into streams
 Al disrupts salt, water, and oxygen regulating
mechanisms in fish
 Al can also adhere to fish gills, causing
suffocation
 Lichens sensitive to SO2 gases and used as
indicator species

11. Effects of acid deposition on
environment
 Nutrient effects (Know 1):
 Soil particles can’t retain Ca, Mg, K, and
other nutrients in acidic environment, so
those nutrients are leached out of soil and
not available to trees
 Dissolved Al ions damage root hairs (the
smallest roots, which are the most
effective at absorbing nutrients), so the
trees are unable to absorb as many
nutrients
 N-fixing bacteria don’t function as well, so
less N is added to soil matrix

12. Effects of acid deposition on
environment
 Damage to human infrastructure:
 acid rain dissolves limestone structures,
especially buildings and statues

13. Acid deposition is regional
 Acid precipitation falls
back to Earth rather than
entering stratospheric jet
stream
 Most areas are
downwind of pollution
sources
 Canadian forests
damaged by coal-fired
power plants in USA
 Scandinavian and
German forests

14. Potential problem areas
because of sensitive soils
Potential problem areas because
of air pollution: emissions leading
to acid deposition
Current problem areas
(including lakes and rivers)

15. Reducing the effects of acid
deposition
 find out about the following techniques:
 Liming lakes to neutralise acidity.
 Precombustion (before burning) techniques.
 End of pipe measures (after burning).
 Of course we could always reduce our use of
fossil fuels to help solve this problem.
 Nuclear power? Electric cars?

16. Pollution management strategies
for acid deposition
 See Table 15.7 on p.298 of the IB ESS 2010 Course
Companion
 Replace
 Switch to renewable energy sources (reduce fossil fuel
use)
 Increase energy efficiency (better light bulbs and
appliances)
 More public transportation (fewer automobiles on the
road)
 Use low-sulfur fuels
 Regulate
 Install ‘scrubbers’ on smokestacks of coal-fired power
plants to remove SO2
 Catalytic converters installed on automobiles (required

17. Management cont.
 Restore
 Add limestone to acidified lakes and streams
 Using limestone or calcium carbonate (CaCO3) can
neutralize (buffer) the impact of acids.
 Freshwater ecosystems much more vulnerable
 Expensive and hard to determine how much to add

19.  Remember, this is not a Global problem but a
regional one most serious in
 Mountain top forests
 Areas with high auto traffic
 Areas downwind from industrial centers

20. CASE STUDY
Research the effect of acid deposition and
intergovernmental agreements or legislation and their
effectiveness
 In groups you are going to create a poster to
show the impact and pollution management
strategies in one of the cases below.
 Canada affected by acid deposition from the
USA.
 Sweden and Norway affected by acid deposition
from Poland, Germany and UK.
 China