2. Air pollution by human
activities
a) Industrial chimney waste
A number of industries are sources of air pollution.
Petroleum refineries are the major sources of gaseous pollutants (e.g., SO2, NOx etc.).
Industrial processors such as metallurgical plants and smelters, chemical plants, petroleum
refineries, pulp and paper mills, sugar mills, cotton mills and synthetic rubber
manufacturing plants are responsible for about one fifth of air pollution.
Cement factories emit plenty of dust in the air, and poses potential health hazard.
Stone crusher also create pollution.
The SPM (suspended particulate matter) levels in such stone crushing areas are five time
the industrial safety limits.
Chemical manufacturing industries emit acid vapours in air.
3. 2. Thermal Power Station
The coal consumption of thermal power stations of Pakistan (e.g Sahiwal thermal
power station) releasing several million tonnes pollutants.
The environmental impact of Sahiwal coal power plant includes issues such as
damage to agricultural fertile land, pollution of canal water and air pollution.
Coal burning in thermal power station discharge roughly 20 harmful toxic
elements, (including arsenic, mercury and lead), fly ash, SO2 and other gases (CO,
NO2), aldehydes and hydrocarbons.
4. 3. Automobiles
The transportation industry includes railroads, trains, ships, aircrafts, trucks, buses,
tractors etc.
These also contribute the same type of pollutants as cars.
The vehicular exhausts are toxic.
It is a considerable source of air pollution.
The sources of emission in the automobiles are:
Exhaust system
Fuel tank and carburetor
Crankcase
The exhaust produces many air pollutants including unburnt hydrocarbons, CO,
Nox and lead oxides.
There are also traces of aldehydes, esters, ethers, peroxides and ketones.
These are chemically active and combine to form smog in the presence of light.
5. Petrol has a volatile nature.
Its evaporation from fuel tank goes on constantly and results in emission of
hydrocarbons.
The evaporation through carburetor occurs when engine is stopped and heat
builds up and as much as 12 to 40 ml of fuel (petrol/ diesel) is lost during each
long stop. It causes emission of hydrocarbons.
Some gas vapours escapes between walls and the piston.
These vapours enter the crankcase and then discharges into the atmosphere.
Thus, total hydrocarbon emission of an engine reaches up to 25%.
Other sources of air pollution are from agriculture.
It is responsible for pesticides, dust from agricultural practices and field burning
and the construction industry.
6. General Effects of Air pollutants on plants
Air pollutants have a negative impact on plant growth, primarily through interfering with
resource accumulation.
As leaves are in close contact with the atmosphere, many air pollutants, such as O3 and
NOx, affect the metabolic function of the leaves and interfere with net carbon fixation by
the plant canopy.
Air pollutants that are first deposited on the soil, such as heavy metals, first affect the
functioning of roots and interfere with soil resource capture by the plant.
These reductions in resource capture (production of carbohydrate through
photosynthesis, mineral nutrient uptake and water uptake from the soil) will affect plant
growth through changes in resource allocation to the various plant structures.
At the ecosystem level, air pollution can shift the competitive balance among the species
present and may lead to changes in the composition of the plant community.
In agroecosystems, these changes causes reduced economic yield.
7. Damage To Leaf Structure By Air pollutants
Causes
Damage to Leaf structure by Air pollutants can be explained by contaminants, for
example, ground-level ozone truly harms leaves by causing chlorosis.
Damage to Leaf structure by Air pollutants causes damage to chlorophyll, and
without chlorophyll, a plant can't produce food or energy.
The Air pollutant also causes delayed flowering, root damage and stomata
damage.
8. Specific Effects of Air pollutants (SO2 and
H2S ) on plants
Concentration of SO2 as low as 0.3 ppm may damage plants.
Lichens are particularly sensitive to SO2 and in polluted regions, one could not find
lichens growing on tree trunks.
The lower quantities of sulphur dioxide suppress the overall vegetative as well as
reproductive growth and yield.
Its high atmospheric concentration produces various injuries to leaves such as
interveinal chlorosis and blade damage, necrosis of leaves and cellular collapse.
However, moderate SO2 pollution results in chlorosis of leaves without cellular
collapse.
Pine tree are more susceptible than broad leaved trees and react by partial
defoliation and reduced growth.
Plants exposure to hydrogen sulphide results in leaf lesions, mottling, defoliation
and reduced growth.
9. Effect of NO2 and PAN on plants
Nitrogen dioxide causes bifacial necrosis in plant leaves.
It causes collapse of leaves, enhancement of green colour, followed by chorosis
and extensive leaf drop.
Ultimately there occur an increase in fruit drop and decrease in fruit crop.
Photochemical smog (peroxyacyl nitrate; PANs) severly injure many forms of plant
life.
It destroy the cells of leaves, damaging the chloroplasts and interfering with the
plant’s metabolic processes.
PAN is known to block Hill reaction of photosynthesis.
It results in bronzing of abaxial leaf surface. It occurs due to plasmolysis and
collapse of mesophyll cells around substomatal chambers.
Epidermal and guard cells are not injured by PAN.
10. Influence of ozone and hydrocarbons on
plants
In plants, O3 enter through stomata and produces visible damage to leaves.
It results in decrease in yield and quality of plant products. Thus O3 causes necrotic
flecking of upper surface of leaf, general chlorosis and bronzing, premature
senescence of plants, premature dropping of leaves, reduced growth of shoots and
roots, suppression of nodulation, reduction in seed set and yield.
Ozone causes shrinkage of nuclei and cytoplasm of mesophyll cells. These become
granular and results in increased intercellular space.
At 0.02 ppm, O3 damages tobacco, tomato, bean, pine and other plants. It causes
several widespread diseases of plants like weather fleck of tobacco, leaf tip burn of
carnations, tip burn of onions, bronzing of beans, speckle-leaf of potato and brown
leaf of grapes.
The hydrocarbon ethylene causes yellowing and necrosis of leaves and chlorosis of
floral buds, inhibition of terminal growth. It also decreases the amount of
chlorophyll and carotenoids. Hydrocarbon aniline causes appearance of bands on
leaves.
Weather Fleck of tobacco
Speckle-leaf of potato
11. Influence of Fluorocarbons, hydrogen
chloride and ammonia on Plants
Fluoride burn the tip of plant leaves.
Its low amounts impair plant growth.
It results in excessive dropping of bloom and young fruits.
It develops small, partially or completely seedless fruits and premature formation
of soft red flesh splitting of peach.
Hydrogen chloride causes plasmolysis and collapse of epidermal cells of leaves. It
results in abaxial glazing of leaves.
Ammonia in plants causes bleaching of leaves, rusty spots on leaves and flowers,
reduction of root and shoot growth and browing and softening of fruits.
Soft red flesh splitting of peach
