CH: Air Pollution
Air pollutants can broadly classify into two types
1)
Primary
pollutants: Pollutants
that are emitted directly from either natural or from human activities are
called primary pollutants.
➤ About 90% of global air pollution is
constituted by five natural pollutants. These are
i.
Carbon
Oxides (CO and CO2)
ii.
Nitrogen
Oxides (NO2, NO3)
iii.
Sulphur
Oxides (SO, SO2)
iv.
Hydrocarbons
v.
Particulate
matter
2)
Secondary
Pollutants: When primary
pollutants reacting with each other or from basic components of air, forms a
new pollutant called secondary pollutants. For example:
I.
Sulphuric
acid
II.
Nitric
acid
III.
Carbonic
acid
IV.
Ozon
V.
PAN (Peroxyacetyl
nitrate) (PAN is formed by the interaction of nitrogen oxides and hydrocarbons)
etc.
v Particulate
Material: thousands of
different solid or liquid particles suspended in air, includes soil particles,
lead, asbestos, sea salt and sulphuric acid droplets
➤ Particles less than 10 µm float and move freely
with the air current. Particles less than 0.02 µm form Persistent aerosols
➤According to the Central Pollution Control Board (CPCB), particulate size 2.5 µm or less in diameter are responsible for causing the greatest harm to human health.
v Carbon monoxide (CO): Sources include incomplete combustion of
organic materials like gasoline(automobiles), oil, wood and tobacco(smoking),
Thermal Power Plants, Fertilise factories
➤ It is produced as a result of incomplete
combustion of carbon
➤ It is colourless and odourless gas
➤ Effect of CO are Cardiac attack, asthma,
bronchitis, irritation of eyes, Throat, lung cancer
v Toxic Elements in air:
i.
Lead
(pb)
ii.
Arsenic
(As)
iii.
Mercury
(Hg)
iv. Cadmium (Cd)
v Ozone: it occurs naturally in Stratosphere and absorbs harmful Ultra Violate
(UV) rays of the sun. However, at the ground level, it is a pollutant (Greenhouse
gas) with highly toxic effects.
1.
Tropospheric
Ozone:
➤ Man made pollutant in the lower atmosphere
➤ Secondary air pollutant
➤ The majority of tropospheric ozone formation
occurs when Nitrogen Oxides (NOx), carbon Monoxide (CO) and Volatile Organic
Compounds (VOCs), react in the atmosphere in the presence of sunlight,
specifically the UV Spectrum
2.
Stratospheric
Ozone:
➤ Essential component that screens out UV
radiation in the upper atmosphere
➤ Man-made pollutants (Ex: CFCs) can destroy it.
v Ozone Layer Depletion:
➤ About 90% of total ozone content of our
atmosphere occurs in the Stratosphere at altitude between 15 to 50 km.
➤ The ozone layer acts as a filter for
Ultra-Violet (UV) radiation from the sun.
➤ Depletion of Stratospheric ozone would be
harmful to life on this earth
➤ Hence ozone layer is termed as ozone umbrella
➤ Primary reason for ozone depletion is CFC
(Chlorofluoro Carbon) or Freons (CFCl3)
v Effect of Ozone Depletion:
➤ Higher level of UV-radiations hitting the earth
➤ Eye contracts
➤ Skin cancer
➤ Weakened immunity
➤ May disrupt ecosystem
➤ May damage crops and forests
v Recovery of Ozone Layer:
➤ Montreal Protocol (1987)
➩ Reduction
of CFCs
Acid Rain:
➤ If moisture is present in the atmosphere,
Sulphur dioxide (SO2) is converted into Sulphuric acid (H2SO4).
➤ Acid rain is caused by a chemical reaction that
begins when compounds like Sulphur Oxide and Nitrogen Oxides are released into
the air. These substances can rise very high into atmosphere where they mix and
react with Water, Oxygen, and other Chemicals to form more acidic pollutants
known as acid rain.
➤ Generally, ‘clean’ rain is slightly acidic as
it dissolves varying amount of Carbon Dioxides. If pH of rain is less than 5.6,
it is called acid rain (the lowest pH value of rain is 5.6, when it is ‘clean’)
Benzene:
➤ Benzene is a natural constituent of crude oil
and is one of the elementary petrochemicals.
➤ It is important component of gasoline
➤ Benzene increase the risk of cancer and other
illness of human
➤ Benzene is causes of bone marrow failure.
Smog:
➤ Smog is mixture of Smoke and Fog
➤ Smog are two types
1.
Classical
Smog:
➤ Classical smog occurs in cool humid climate.
➤ It is combustion of coal and fog
➤ Fog is mainly a mixture of SO2, SO3
and humidity.
➤ It is generally harmful in early morning and
becomes worst after sunrise.
➤ London Smog is a Classical Smog.
2.
Photo
Chemical Smog:
➤ Photo Chemical Smog occurs in warm, dry and
sunny climate.
➤ The main component of the Photo Chemical Smog result
from the reaction of sunlight on unsaturated Hydrocarbons and Oxides of
Nitrogen produced by automobiles factories.
➤ Photo Chemical Smog has high concentration of
oxiding agents and is, therefore, called as Oxiding Smog.
➤ Photochemical Smog is also known as Los Angeles Smog
v Green House Gases:
i.
Water
Vapour (H2O)
ii.
Carbon
dioxide (CO2)
iii.
Methane
(CH4)
iv.
Nitrogen dioxide
(N2O)
v.
Ozone (O3)
vi.
Chlorofluorocarbon
(CFCs)
vii.
Hydrofluorocarbon
(HFCs)
viii.
Perfluorocarbons
(PFCs)
ix.
Sulphur
hexafluoride (SF6)
Green House Effect and Global Warming: IF a large amount of Carbon Dioxide present in the atmosphere, the temperature of the earth rises beyond this (15oC). Thus, much of the earth’s heat is retained and causes a warming effect. This phenomenon is known as the Greenhouse Effect. Hence the temperature of the earth rises. This is known as Global Warming.
Importance of Natural Greenhouse Effect: Without this phenomenon, average temperature
might have been around -17oC and at such low temperature life would
not be able to exist.
Control Devices for Particulate Matters:
1.
Gravitational
Settling Chamber:
➤ Applicable for particle size more than 50 µm
2.
Centrifugal
Gas Collectors
i.
Cyclone
Collectors
ii.
Dynamic
Precipitator
1.
Wet
Scrubbers (or Collector)
i.
Spray Tower
ii.
Wet
Cyclone Scrubbers
iii.
Venturi
Scrubbers
2.
Electrostatic
Precipitator (ESP)
➤ Very small particles also, wet or dry, can be
trapped easily (size about 1 µm)
➤ Usual range of efficiency is 95 to 95%
➤ Can be operate at high temperature up to 300 to
450oC
v
Lapse rate: In the
troposphere, the temperature of surrounding layer normally decreases with
increase in the altitude. This rate of change of temperature is called lapse
rate.
1. Environmental Lapse Rate / Ambient Lapse Rate (ELR): The ELR can be determined by sending up a ballon equipped with a thermometer and self-recording mechanism. The lapse rate so obtained is known as Prevailing lapse rate, or Ambient Lapse Rare or The Environmental Lapse Rate (ELR).
2. Adiabatic Lapse Rate: When a parcel of air which is hotter and higher than the surrounding air is released, the naturally it tends to rise up until it reaches a level at which its own temperature and density becomes equal to that of surrounding air. This rate of decrease of temperature with height is called Adiabatic Lapse Rate (ALR).
Super Adiabatic Lapse Rate:
➤ When the Ambient Lapse Rate (ELR) exceeds the
Adiabatic Lapse Rate (ALR) than it is called Super Adiabatic Lapse Rate
➤ The atmosphere is highly unstable
Sub-Adiabatic Lapse Rate:
➤ When the Ambient Lapse Rate (ELR) is less the
Adiabatic Lapse Rate (ALR), then it is
termed as Sub-adiabatic Lapse rate.
➤ The atmosphere is Stable
Neutral:
