6.0 Acid Deposition

6.1 Introduction

     Acid deposition is a
general name for a number of phenomena, namely acid rain, acid fog and acid
mist. It occurs during the combustion of fossil fuels and other
industrial processes undergo complex chemical reactions and emitted gases in
the atmosphere. These gases it then emitted to the Earth as wet deposition or
dry deposition.  Acid deposition can
cause harmful effect on environment.

     For wet deposition, it occurs when the
acidic chemicals in the air is blew by wing and transport to the areas where
the weather is wet. Then, these acidic chemicals will fall to the ground in the
form of rain, sleet, fog, snow or mist which is the type of wet deposition.
This will cause the acid to be remove from atmosphere and deposit on the
Earth’s surface. These acids are toxic and when they flow through the ground,
most of the aquatic life, animals and plants will be affected.

     Besides that, for dry deposition, it
occurs when the acidic chemicals in the air is blew by wing and transport to
the areas where the weather is dry. Almost
50% of the acidic chemicals in the atmosphere fall back through dry deposition.
 Theses acidic chemicals slip into dust or smoke and fall to the ground as dry
particles. They will stick to the buildings, cars, houses and trees.

     Acid deposition is usually caused by
the accumulation of acids or acidic compounds on the Earth’s surface. The main chemical precursors
are sulphur dioxide (SO2)
and nitrogen oxides (NOx). When these two compounds react with water, oxygen,
carbon dioxide, and sunlight in the atmosphere to form sulphuric acids (H2SO4) and nitric acids (HNO3) which is the primary agents of acid deposition.

     More recently, acid precipitation or acid
rain that is more acidic than normal has been linked to air pollution. Acid
rain damage historic artworks, statues, and classic architecture and erodes
names, dates and marking on cemetery gravestones. It also impairs the
production of fish.


6.2 Causes

   Emission of chemical gases such as sulphur
dioxide (SO2) and nitrogen oxides (NOX) into the atmosphere
are major contributor to acid deposition. This mainly cause by man-made
sources. SO2 and NOX gases are emitted through human
activities. These chemical gases can be released from factories, particularly
in hear and power generation facilities, large industrial boilers, action of
bacteria on fertilisers or vehicle exhaust.

     There gases are reactive and they will react with
the water, oxygen or other chemicals substances in the atmosphere to form more acidic
pollutants. This reaction can cause the formation of several acidic compounds
like nitric acid, sulphuric acid and ammonium nitrate. Acid deposition in this area should be
very high.

     On the other hand, a small amount of chemical gases
is come from natural sources. For example, they can come from volcanic gases which
contain sulphur dioxide. Most of these gases are come from the use of combustion
of fossil fuels to generate electricity. Also, natural source like lightning strikes
can produces oxides of nitrogen that will react with the water to form acid rain.


     SO2 and NOX can
be blew by wind and transport over long distances to other areas. This result in
the spreading of acid deposition and cause negative impact on environment, buildings
and living things.






of hydrocarbon fuels produce sulphur dioxide (SO2).

+ O2 ? SO2

+ H2O (rain water) + O2 (Oxygen in air) ? H2SO4
(Acid Deposition)


and oxygen combine to make oxides of nitrogen.

+ 2O2 ? 2NO2

(rain water) + O2 (Oxygen in air) ? HNO3 (Acid Deposition)




Acid rain has many ecological effects, but none is greater than its
impact on lakes, streams, wetlands, and other aquatic environments. Acid rain either falls directly
on aquatic bodies or gets run off the forests, roads and fields to flow into
streams, rivers and lakes makes waters acidic, which lower the overall pH of water
body. The
aquatic plants and animals need a particular pH level of about 4.8 to survive.
If the pH level falls below that, the conditions become hostile for the
survival of aquatic life.

     Acid rain
also damages forests, especially those at higher elevations. It robs the soil
of essential nutrients and releases aluminium in the soil, which makes it hard
for trees to take up water. It makes trees vulnerable to disease, extreme
weather, and insects by destroying their leaves, damaging the bark and
arresting their growth. 

     Moreover, acid rain can have destructive
effect on many objects, including buildings, statues, monuments, and
cars. The chemicals found in acid rain can cause paint to peel and stone
statues to begin to appear old and worn down, which reduces their value and

     For example, acid rain will eventually
dissolve a marble statue. Acid rain often contains sulphuric acid, H2SO4, which will react with
the calcium carbonate, CaCO3, to
produce relatively insoluble calcium sulphate, CaSO4, carbon
dioxide, CO2, and water, H2O.

CaCO3 (s) + H2SO4
(s) + CO2 (g) + H2O(l)

If large quantities
of water are present, the calcium sulphate will be removed from the surface of
the statue. In time, the quantities of calcium carbonate converted by acid
rains into calcium sulphate, water, and carbon dioxide will become quite





One of the most fundamental solutions is to utilize fuels that burn more
cleanly, or to burn coal more efficiently. This will greatly reduce the amount
of acids released in the atmosphere. Besides fossil fuels, there is a wide
range of alternative energy sources that can generate electrical
power. For example, wind energy, geothermal energy, solar energy, hydropower, and nuclear power. Fuel cells, natural gas,
and batteries can also substitute use of fossil fuel as cleaner energy sources.
As of today, all energy sources have environmental and economic costs
as well as benefits. The only solution is using sustainable
energy that can protect the future.


     On the
other hand, individuals can help to prevent acid rain by conserving energy. A small step can be taken by the individual
by turning off the lights, computers, and other electrical appliances, when not
in use. The less electricity
people use in their homes, the fewer chemicals power plants will emit. While purchasing, individual can buy the
appliances, which consume less energy.
Vehicles are also major fossil fuel users, so drivers can reduce emissions by
using public transportation, carpooling, biking, or simply walking wherever
possible to nearby places instead of
driving. This can help us save fuel and gas, as well as reduce the adverse


     Lastly, there are some things that people can
do to repair the damage caused by acid rain to lakes and rivers. Limestone or
lime can be added to acidic surface waters to balances the acidity. This
process, called liming. Liming tends to be expensive, and has to be done
repeatedly to keep the water from returning to its acidic condition.
Furthermore, it only offers   a short-term solution at the expense of
solving the broader challenges of SO2 and NOx emissions and risks to
human health.