Pollution types
CAUSES, EFFECTS AND CONTROL MEASURES
OF POLLUTION
Air Pollution
History of air pollution: The origin of air pollution
on the earth can be traced from the times
when man started using firewood as a means
of cooking and heating. Hippocrates has mentioned
air pollution in 400 BC. With the discovery
and increasing use of coal, air pollution
became more pronounced especially in urban
areas. It was recognized as a problem 700 years
ago in London in the form of smoke pollution,
which prompted King Edward I to make the first
antipollution law to restrict people from using
coal for domestic heating in the year 1273. In
the year 1300 another Act banning the use of
coal was passed. Defying the law led to imposition
of capital punishment. In spite of this air
pollution became a serious problem in London
during the industrial revolution due to the use
of coal in industries. The earliest recorded major
disaster was the ‘London Smog’ that occurred
in 1952 that resulted in more than 4000 deaths
due to the accumulation of air pollutants over
the city for five days.
In Europe, around the middle of the 19th century,
a black form of the Peppered moth was
noticed in industrial areas. Usually the normal
Peppered moth is well camouflaged on a clean
lichen covered tree. However the peppered pattern
was easily spotted and picked up by birds
on the smoke blackened bark of trees in the
industrial area, while the black form remained
well camouflaged. Thus while the peppered
patterned moths were successful in surviving in
clean non-industrial areas, the black coloured
moths were successful in industrial areas. With
the spread of industrialization, it has been observed
that the black forms are not only see in
Peppered moth, but also in many other moths.
This is a classic case of pollution leading to adaptation.
Air pollution began to increase in the beginning
of the twentieth century with the development
of the transportation systems and large-scale use
of petrol and diesel.
The severe air quality problems
due to the formation of photochemical
smog from the combustion residues of diesel
and petrol engines were felt for the first time in
Los Angeles. Pollution due to auto-exhaust remains
a serious environmental issue in many
developed and developing countries including
India.
The Air Pollution Control Act in India was passed
in 1981 and the Motor Vehicle Act for controlling
the air pollution, very recently. These laws
are intended to prevent air from being polluted.
The greatest industrial disaster leading to serious
air pollution took place in Bhopal where
extremely poisonous methyl isocyanide gas was
accidentally released from the Union Carbide’s
pesticide manufacturing plant on the night of
December 3rd 1984. The effects of this disaster
on human health and the soil are felt even today.
Structure of the atmosphere
The atmosphere is normally composed of 79
percent nitrogen, 20 percent oxygen and one
percent as a mixture of carbon dioxide, water
vapour and trace amounts of several other gases
such as neon, helium, methane, krypton, hydrogen
and xenon.
The general structure of the
atmosphere has several important features that
have relevance to environmental problems. The
atmosphere is divided into several layers.
The innermost layer the troposphere extends 17
kilometers above sea level at the equator and
about 8 kilometers over the poles. It contains
about 75 percent of the mass of the earth’s air.
The fragility of this layer is obvious from the fact
that if the earth were an apple this particular
layer would be no thicker than an apple’s skin.
Temperature declines with altitude in the troposphere.
At the top of the troposphere temperatures
abruptly begin to rise. This boundary
where this temperature reversal occurs is called
the tropopause.
The tropopause marks the end of the troposphere
and the beginning of the stratosphere,
the second layer of the atmosphere. The stratosphere
extends from 17 to 48 kilometers above
the earth’s surface. While the composition of
the stratosphere is similar to that of the troposphere
it has two major differences. The volume
of water vapour here is about 1000 times
less while the volume of ozone is about 1000
times greater. The presence of ozone in the
stratosphere prevents about 99 percent of the
sun’s harmful ultraviolet radiation from reaching
the earth’s surface thus protecting humans
from cancer and damage to the immune system.
This layer does not have clouds and hence
airplanes fly in this layer as it creates less turbulence.
Temperature rises with altitude in the
stratosphere until there is another reversal. This
point is called the stratopause and it marks the
end of the stratosphere and the beginning of
the atmosphere’s next layer, the mesosphere.
In the mesosphere the temperature decreases
with altitude falling up to –110 oC at the top.
Above this is a layer where ionization of the
gases is a major phenomenon, thus increasing
the temperature. This layer is called the thermosphere.
Only the lower troposphere is routinely
involved in our weather and hence air
pollution. The other layers are not significant in
determining the level of air pollution.
Types and sources of Air Pollution
What is air pollution?
Air pollution occurs due to the presence of undesirable
solid or gaseous particles in the air in
quantities that are harmful to human health and
the environment. Air may get polluted by natural causes such as volcanoes, which release ash,
dust, sulphur and other gases, or by forest fires
that are occasionally naturally caused by lightning.
However, unlike pollutants from human
activity, naturally occurring pollutants tend to
remain in the atmosphere for a short time and
do not lead to permanent atmospheric change.
Pollutants that are emitted directly from identifiable
sources are produced both by natural
events (for example, dust storms and volcanic
eruptions) and human activities (emission from
vehicles, industries, etc.). These are called primary
pollutants. There are five primary pollutants
that together contribute about 90 percent
of the global air pollution. These are carbon
oxides (CO and CO2), nitrogen oxides, sulfur
oxides, volatile organic compounds (mostly hydrocarbons)
and suspended particulate matter.
Pollutants that are produced in the atmosphere
when certain chemical reactions take place
among the primary pollutants are called secondary
pollutants. Eg: sulfuric acid, nitric acid, carbonic
acid, etc.
Carbon monoxide is a colourless, odorless and
toxic gas produced when organic materials such
as natural gas, coal or wood are incompletely
burnt. Vehicular exhausts are the single largest
source of carbon monoxide. The number of vehicles
has been increasing over the years all over
the world. Vehicles are also poorly maintained
and several have inadequate pollution control
equipment resulting in release of greater
amounts of carbon monoxide.
Carbon monoxide
is however not a persistent pollutant. Natural
processes can convert carbon monoxide to
other compounds that are not harmful. Therefore
the air can be cleared of its carbon monoxide
if no new carbon monoxide is introduced
into the atmosphere.
Sulfur oxides are produced when sulfur containing
fossil fuels are burnt.
Nitrogen oxides are found in vehicular exhausts.
Nitrogen oxides are significant, as they are involved
in the production of secondary air pollutants
such as ozone.
Hydrocarbons are a group of compounds consisting
of carbon and hydrogen atoms. They either
evaporate from fuel supplies or are
remnants of fuel that did not burn completely.
Hydrocarbons are washed out of the air when
it rains and run into surface water. They cause
an oily film on the surface and do not as such
cause a serious issue until they react to form
secondary pollutants. Using higher oxygen concentrations
in the fuel-air mixture and using
valves to prevent the escape of gases, fitting of
catalytic converters in automobiles, are some of
the modifications that can reduce the release
of hydrocarbons into the atmosphere.
Particulates are small pieces of solid material (for
example, smoke particles from fires, bits of asbestos,
dust particles and ash from industries)
dispersed into the atmosphere. The effects of
particulates range from soot to the carcinogenic
(cancer causing) effects of asbestos, dust particles
and ash from industrial plants that are dispersed
into the atmosphere. Repeated exposure
to particulates can cause them to accumulate
in the lungs and interfere with the ability of the
lungs to exchange gases.
Lead is a major air pollutant that remains largely
unmonitored and is emitted by vehicles.
High
lead levels have been reported in the ambient
air in metropolitan cities. Leaded petrol is the
primary source of airborne lead emissions in Indian
cities.
Pollutants are also found indoors from infiltration
of polluted outside air and from various
chemicals used or produced inside buildings.
Both indoor and outdoor air pollution are equally
harmful.
What happens to pollutants in the atmosphere?
Once pollutants enter the troposphere they are
transported downwind, diluted by the large volume
of air, transformed through either physical
or chemical changes or are removed from the
atmosphere by rain during which they are attached
to water vapour that subsequently forms
rain or snow that falls to the earth’s surface.
The atmosphere normally disperses pollutants
by mixing them in the very large volume of air
that covers the earth. This dilutes the pollutants
to acceptable levels.
The rate of dispersion however
varies in relation to the following aspects:
Topography
Normally as the earth’s surface becomes
warmed by sunlight the layer of air in contact
with the ground is also heated by convection.
This warmer air is less dense than the cold air
above it, so it rises. Thus pollutants produced in
the surface layer are effectively dispersed.
However on a still evening, the process is reversed.
An hour or two before sunset after a
sunny day, the ground starts to lose heat and
the air near the ground begins to cool rapidly.
Due to the absence of wind, a static layer of
cold air is produced as the ground cools. This in
turn induces condensation of fog. The morning
sun cannot initially penetrate this fog layer. Thecold air
being dense cannot rise and is trapped
by the warm air above. It cannot move out of
the area due to the surrounding hills. The topographic
features resemble a closed chemical reactor
in which the pollutants are trapped. This
condition often continues through the cool night
and reaches its maximum intensity before sunrise.
When the morning sun warms the ground
the air near the ground also warms up and rises
within an hour or two. This may be broken up
by strong winds. In cold regions this situation
can persist for several days. Such a situation is
known as smog (smoke + fog).The most well known example is that of the
‘London Smog’ that occurred in 1952. The city
used large quantities of sulphur containing coal
for domestic heating that released smoke, along
with smoke from thermal power plants and other industrial establishments. This used to lead
to the generation of high levels of smoke containing
sulphur oxides. Due to a sudden adverse
meteorological condition air pollutants like
smoke and sulphur oxides started to build-up in
the atmosphere. The white fog accumulated
over the city turned black forming a ‘pea-soup’
smog with almost zero visibility. Within two days
of the formation of this smog, people started
suffering from acute pulmonary disorders which
caused irritation of bronchi, cough, nasal discharges,
sore throat, vomiting and burning sensations
in the eyes. This event lead to several
deaths.
Meteorological conditions:
The velocity of the wind affects the dispersal of
pollutants. Strong winds mix polluted air more
rapidly with the surrounding air diluting the
pollutants rapidly. When wind velocity is low
mixing takes place and the concentration of
pollutants remains high When sulphur dioxide and nitrogen oxides are
transported by prevailing winds they form secondary
pollutants such as nitric acid vapour,
droplets of sulfuric acid and particles of sulphate
and nitrate salts. These chemicals descend on
the earth’s surface in two forms: wet (as acidic
rain, snow, fog and cloud vapour) and dry (as
acidic particles). The resulting mixture is called
acid deposition, commonly called acid rain.
Acid deposition has many harmful effects especially
when the pH falls below 5.1 for terrestrial
systems and below 5.5 for aquatic systems. It
contributes to human respiratory diseases such
as bronchitis and asthma, which can cause premature
death. It also damages statues, buildings,
metals and car finishes. Acid deposition
can damage tree foliage directly but the most
serious effect is weakening of trees so they become
more susceptible to other types of damage.
The nitric acid and the nitrate salts in acid
deposition can lead to excessive soil nitrogen
levels. This can over stimulate growth of other
plants and intensify depletion of other important
soil nutrients such as calcium and magnesium,
which in turn can reduce tree growth and
vigour.
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