3. ACID RAIN
Some pollutants molecules released into the atmosphere, like Nitrogen Oxide (NOx) and sulfur
dioxide (SO2),are acidic and create acids when mixed with rain water.
These substances released into the air in large quantities by industrial processes and vehicle
tailpipes, increase the acidity of rain water to a pH 5 this also known as acid rain.
4. WHY DO WE HAVE ACID RAIN
Unlike in deposition, anthropogenic source emissions of SO2, NOx and VOC’s, do not
vary from season to season.
The basic components of acid rain are SO2, NOx, VOC’s (volatile organic compounds)
and several others. Most of the sulphur present in the atmosphere of the Northern
Hemisphere is from anthropogenic sources. Coal and lignite power stations
contribute to a large amount of this pollution.
Other pollutants include particulates, hydrocarbons and carbon monoxide.
Trifluoroacetic acid is an atmospheric breakdown product of the
chlorofluorocarbon replacements HCFC-123, HCFC-124, and HFC-134a.
Trifluoroacetic acid partitions into the various aqueous phases that occur
throughout the environment. HFC’s and HCFC’s have greater reactivity and
therefore lower atmospheric lifetimes than their predecessors, the CFC’s. Because
of this heightened reactivity and reduced tropospheric residence time, the HFC’s
and HCFC’s are less likely to be transported to the stratosphere where they might
mediate the photochemical destruction of ozone. Therefore, the HFC’s and HCFC’s
are likely to cause less environmental damage than the CFC’s.
Ammonia is another determinator of acid rain.
5. HOW OCCUR ACID RAIN
NITROGEN
In the photochemical relationship between nitric oxide, ozone, and nitrogen dioxide, the
concentration of these chemical species is directly affected by the intensity of sunlight. These
chemicals are also known to react photo chemically with hydrocarbons and other atmospheric
chemicals to form photochemical "smog." NO2 reacts faster with OH to form acid nitrate than
does sulphur. In polluted air, NOx can react with organic matter to produce peroxyacetyl nitrate
(PAN), a pollutant which can be transported long distances before it is eventually converted to
acid nitrate. Reactions between NOx and H2O2 are very slow. HO2 reacts with NO and then
nitrous oxide reacts with hydroxyl.
HO2 + NO OH + NO2
NO2 + OH HNO3
6. SULPHUR:
Most airborne acid sulphate appears to be formed in cloud droplets. SO2 dissolves to
form HSO3
- which then reacts with hydrogen peroxide (H2O2) to form acid sulphate.
H2O2 is the most efficient oxidant in the conversion of dissolved SO2 to H2SO4. This
reaction is a product of photochemistry. The lower the pH the faster the reaction
proceeds. The oxidation of dissolved SO2 is rapid even at a pH value below 5.
PRECIPITATION:
Inputs of trifluoroacetic acid into natural water systems occur through wet and dry
deposition, directly from the vapour stage and from runoff from the surrounding
watershed.
7. EFFECTS
SOILS AND VEGETATION
• Nitrogen is the growth depleting factor in most ecosystems. Inputs of
nitrogen are usually taken up by vegetation and soils. Hence, soils are quite
resistant to acidification. After the acid rain enters the soil, it causes
nutrients such as calcium and magnesium to be leached from the soil. This
deprives the plants of their basic nutrients as well as causes harm to nearby
water bodies and to the ground water.
• Sulphur affects plants in a fatal manner by entering through the plant cell.
Sulphur dioxide comes in contact with the chlorophyll of the cell and the
other constituents of the cells and is converted there into corrosive sulfuric
acid which immediately destroys the tissues in its vicinity.
8. EFFECTS
FORESTS
Trees in forests have been found to be affected by pollutants in the air. The main
causes of this degradation are SO2, NOx, H2SO4 and HNO3. Pollutants can also be
absorbed from the soil. This causes the tree to be affected from its roots upward.
Infection of the roots is the easiest way to kill a tree.
9. EFFECTS
WATER
Excess deposition of nitrogen can lead to increased amounts of nitrate which aid in the
acidification of lake waters. Acidic deposition affects aquatic life. Acidification may
eliminate sensitive algae species and decrease phosphorous and inorganic carbon
concentrations. It can also cause damage to fish populations. Heavy metals removed
from the soil during rains could cause death to aquatic life. Fish absorb polluted water
through their gills and this can harmful effects on them such as the amount of oxygen
taken up by the blood is reduced and the blood circulation is affected.
10. EFFECTS
HUMAN HEALTH
Effects on human health are usually seen through the food chain by bioaccumulation
and by water contamination. The chemicals that get deposited in the soil and water
are consumed either directly by humans or by way of the food chain. In this way they
affect human beings. The acid in the water may corrode copper and lead water pipes
contaminating the drinking water.
11. EFFECTS
WILDLIFE
The damage from acid rain to terrestrial wildlife is basically through the food chain.
Accumulated heavy materials cause great damage through bio-magnification.
BUILDINGS
A large variety of materials are affected by acid rain. These extend from sandstone
and limestone to metals such as zinc, aluminium, copper, plastics, paper, textiles,
electrical contacts e.t.c.
12. REMEDIES
Technical solutions
Many coal-firing power station use flue-gas desulfurization (FGD) to remove sulfur-
containing gases from their stack gases. For a typical coal-fired power station, FGD
will remove 95% or more of the SO2 in the flue gases. An example of FGD is the wet
scrubber which is commonly used. A wet scrubber is basically a reaction tower
equipped with a fan that extracts hot smoke stack gases from a power plant into the
tower. Lime or limestone in slurry form is also injected into the tower to mix with the
stack gases and combine with the sulfur dioxide present. The calcium carbonate of
the limestone produces pH-neutral calcium sulfate that is physically removed from
the scrubber. That is, the scrubber turns sulfur pollution into industrial sulfates.
Emissions trading
In this regulatory scheme, every current polluting facility is given or may purchase on
an open market an emissions allowance for each unit of a designated pollutant it
emits. Operators can then install pollution control equipment, and sell portions of
their emissions allowances they no longer need for their own operations, thereby
recovering some of the capital cost of their investment in such equipment. The
intention is to give operators economic incentives to install pollution controls.
