E2 acid deposition
• State what is meant by the term acid
deposition and outline its origins.
• Discuss the environmental effects of acid
deposition and possible methods to
counteract them.
acid deposition
• Acid deposition refers to the process by which
acidic particles, gases and precipitation leave the
atmosphere as they are brought back down to
earth e.g. on the ground, on trees, buildings,
inside plants or animals (e.g. your lungs).
• There are two types of acid deposition:
 wet deposition: acid rain (pH less than 5.6), fog and snow
 dry deposition: acidic gases such as SO2 and salts
(when acidic gas reacts with an alkali)
• All rain water is naturally acidic owing to the
presence of dissolved carbon dioxide, which
reacts with water to form carbonic acid:
• CO2(g)+H2O(l)⇄H2CO3
• Sulfur dioxide dissolves in water to form
sulfurous acid H2SO3
• H2O(l)+SO2 (g)→H2SO3
• Sulfuric acid H2SO4 (aq) is formed when the
sulfur dioxide is oxidized to sulfur trioxide
which then dissolves in water.
acid deposition
NOx: formed as a result of high temperatures in internal combustion
engines, i.e. cars and jet engines.
1. production of nitrogen oxides: N2 (g) + O2 (g)  2NO (g)
2NO (g) + O2 (g)  2NO2 (g)
2. the following equations show two ways in which nitric acid is
formed in the atmosphere
2 NO2 (g) + H2O (l)  HNO3 (aq) + HNO2 (aq) (=nitrous acid)
or
4 NO2 (g) + 2 H2O (l) + O2 (g) 4 HNO3 (aq)
acid deposition
SOx: from burning of coal which contains sulphur/smelting
plants
1. production of oxides: S (g) + O2 (g)  SO2 (g)
2SO2 (g) + O2 (g)  2SO3 (g)
2. formation of atmospheric sulphuric acid and sulfurous acid
SO3 (g) + H2O (l) 
SO2 (g) + H2O (l) 
H2SO4 (aq)
H2SO3(aq)
acid deposition
humans
buildings
 irritation of the mucus membranes and lung tissue when
breathing in fine droplets of acid rain;
 increase in risk of respiratory illnesses like asthma and
bronchitis;
 acidic water also dissolves and leaches poisonous ions like
Al3+ (linked with Alzheimer disease) and Pb2+.
 corrosion of materials such as marble and dolomite
(CaCO3.MgCO3): equation:
CaCO3 (s) + H2SO4 (aq)  CaSO4(s) + H2O(l) + CO2 (g)
 faster corrosion of iron and steel structures in buildings or
bridges.
acid deposition
aquatic life
vegetation
increased levels of aluminum ions dissolved from the soil
kills fish as it effects the function of the gills; a lot of fish,
algae, insect larvae even plankton cannot survive in
water below a certain pH.
 increased soil acidity leaches important nutrients (e.g.
Ca2+/K+/ Mg2+) out of the top soil
 Mg2+is necessary to make chlorophyll so removal of
this ion results in lowering rate of photosynthesis and
reducing growth of plants and crop yields.
 Increased concentration of Al3+ which damages roots
 stunted growth,
 thinning of tree tops,
 yellowing and loss of leaves.
Effects on water
• The nitric acid present in acid rain poses a
particular problem as the nitrates present can
lead to eutrophication. The nitrate ions
promote excessive plant growth causing
plants to need more oxygen to be available in
the water supply, which can in turn lead to
other plant deaths.
Control strategies
• Acid deposition can be controlled by reducing
the level of emission of nitrogen and sulfur
oxide. The possible method is to switch to
alternative energy sources, such as wind, solar
or tidal energy, or to reduce the demand for
fossil fuels by using more public transport or
more efficient energy transfer systems.
The role of ammonia in acid deposition
• Ammonia is present in the atmosphere from
both natural and synthetic sources. It is
produced naturally by animal livestock and by
the action of certain bacteria and also from
artificial fertilizers.
• NH3+HNO3→NH4NO3
• 2NH3+H2SO4→(NH4)2SO4
acid deposition
• cutting down emissions of nitrogen oxides by using catalytic
converters; thermal exhaust systems;
• use of low sulphur fuels; use of these fuels still release
sulphur dioxide
• removal of sulphur oxides from exhaust fumes (Flue Gas
Desulphurization);
• use of alternative sources of energy which do not involve
fossil fuels and the production of very high temperatures.
• dealing with acid rain itself: neutralization of lakes using
calcium carbonate or calcium oxide,
e.g. CaO +
H2SO4 (aq)  CaSO4 (s) + H2O (l)