Air pollution, photochemical smog

Air pollution
The major problem that the atmosphere is being subjected to is pollution. In India, about 100 million tons of pollutants are being added to the atmosphere annually. This figure is likely to go up in future. Polluted air is harmful to man and the biosphere on the whole, as well. This problem needs to be tackled urgently. Since all of us contribute to it directly or indirectly, we must study air pollution in detail and discuss the mean
s of controlling and preventing it.

Air pollution can be defined as 'the change of composition of air by the addition of harmful substances like the industrial and automobile gases and particulate matter.'

Sources of Air Pollution

Most of the sources of air pollution are related to man's activities as a result of the modern lifestyle. Added to this are also natural causes like the volcanoes, anaerobic decomposition of organic matter, atmospheric reactions, etc.

Burning of Fossil Fuels

Fossil fuels include petroleum and coal. Burning of coal produces a lot of smoke and dust whereas burning of petrol mainly produces sulphur dioxide. In addition to these, the pollutants include carbon monoxide (CO), carbon dioxide (CO2), nitrogen oxides, hydrocarbons, particulate matter and traces of metals.

Automobiles

Petrol on combustion produces carbon monoxide, hydrocarbons, nitrogen oxides, aldehydes, sulphur compounds, organic acids and ammonia and carbon particles. Incomplete combustion of petrol produces a hydrocarbon, 3,4 benzpyrene. There is more pollution during acceleration and deceleration than during constant speed.

Industries

Fertiliser Plants

They produce oxides, sulphur, nitrogen, hydrocarbons, particulate matter and fluorine.

Thermal Plants

Since they are coal based the pollutants are fly ash, soot and sulphur dioxide.

Textile Industries

They produce cotton dust, nitrogen oxides, chlorine, naphtha vapours, smoke and sulphur dioxide.

Steel Plants

They produce carbon monoxide, carbon dioxide, sulphur dioxide, phenol, fluorine, cyanide, particulate matter, etc.

Volcanic eruptions release oxides of nitrogen that pollute the atmosphere.

Decomposition of organic matter under anaerobic conditions produces methane which on being oxidised in the atmosphere produces carbon monoxide. Decomposition of these matter also produces foul smelling gases.

Photochemical oxidation of marine organic matter and biological oxidation by marine organisms produce lot of carbon monoxide on the surface of the oceans which enters the atmosphere.

Major Pollutants

There are six main categories of air pollutants:

  • oxides of carbon
  • sulphur dioxide
  • oxides of nitrogen
  • hydrocarbon
  • inorganic particulate matter and aerosols
  • organic particulate matter

Harmful Effects of the Pollutants in Air

The various categories of air pollutants and their harmful effects are summarised in the given table:

PollutantSource/CauseEffect
Carbon monoxideAutomobile exhaust, photochemical reactions in the atmosphere, biological oxidation by marine organisms, etc.Affects the respiratory activity as haemoglobin has more affinity for Co than for oxygen. Thus, CO combines with HB and thus reduces the oxygen-carrying capacity of blood. This results in blurred vision, headache, unconsciousness and death due to asphyxiation (lack of oxygen).
Carbon di oxideCarbon Burning of fossil fuels,depletion of forests (that remove excess carbon dioxide and help in maintaining the oxygen-carbon dioxide ratio).Global warming as it is one of the greenhouse gases.
Sulphur dioxideIndustries, burning of fossil fuels, forest fires, electric generation plants, smelting plants, industnal boilers, petroleum refineries and volcanic eruptions.Respiratory problems, severe headache,reduced productivity of plants, yellowing and reduced storage time for paper, yellowing and damage to limestone and marble, damage to leather, increased rate of corrosion of iron, steel, zinc and aluminium.
Hydrocarbons Polynuclear Aromatic Compounds(PAC) and Polynuclear Aromatic Hydrocarbons(PAH)Automobile exhaust and industries,leaking fuel tanks, leaching from toxic waste dumping sites and coal tar lining of some water supply pipes.Carcinogenic (may cause leukemia)
Chlorofluoro carbons (CFCs)Refngerators, air conditioners, foam shaving cream, spray cans and cleaning solvents.Destroy ozone layer which then permits harmful UV rays to enter the atmosphere.
Nitrogen OxidesAutomobile exhausts, burning of fossil fuels, forest fires,electric generation plants, smelting plants, industnal boilers, petroleum refineries and volcanic eruptionsForms photochemical smog, at higher concentrations causes leaf damage or affects the photosynthetic activities of plants and causes respiratory problems in mammals.
PAN - peroxylacetyl -nitratePhotochemical reactions of hydrocarbons and nitrogen oxides.Irritation of eye, throat and respiratory tract, damage to clothes, paint and rubber articles, damage to leaves and stomatal tissue in plants.
Particulate matter Lead halides (lead pollution)Combustion of leaded gasoline productsToxic effect in man.
Asbestos particlesMining activitiesAsbestosis - a cancerous disease of the lungs
Silicon dioxideStone cutting, pottery, glass manufacturing and cement industries.Silicosis, a cancerous disease.
Biological matter like the pollen grainsFlowersAllergy
Fungal spores, bacteria, virus, etcMicrobesInfectious deseases

Effects of Air Pollution

Global Warming

Most of the solar radiation entering the earth's atmosphere is reflected back into the space. However some of the heat is absorbed by the gases like the carbon dioxide. This serves to keep the earth warm much like the greenhouses. Greenhouses are glasshouses which maintain a temperature higher than the surroundings for the plants to grow and yield better. The other gases that contribute to this are water vapour, methane, chlorofluorocarbons (CFCs) and nitrous oxide. These gases are called the greenhouse gases.

While greenhouse effect is a necessary and natural phenomenon. Every year teh temperature are going up due to pollution and the levels of these greenhouse gases is also going up. This is called global warming. According to estimates, at the current rate of increase, the average global temperature will go up by 3oC to 8oC in the next 100 years.

This will have the following effects:

  • Climate of different regions
  • Distribution of plants and animals
  • Disturbance in agriculture and food production
  • Melting of snow caps and resultant increase in sea levels. This will submerge parts of coastal cities of Calcutta, New York, London and other major cities.

Formation of Photochemical Smog

When pollutants like hydrocarbons and nitrogen oxides combine in the presence of sunlight, smog is formed. This is a mixture of gases and since it is formed by photochemical reactions, it is called the photochemical smog. The word 'smog' is derived from the two words-smoke and fog.

It forms a yellowish brown haze especially during winter and hampers visibility. It also causes many respiratory disorders and allergies as it contains polluting gases.

Formation of Acid Rain

Sulphur dioxide and nitrogen oxides react with water in the atmosphere producing sulphuric acid and nitric acid. These acids come down along with the rain. This phenomenon is called acid rain. The pH of acid rain varies from 3-6. The composition of acid rain is sulphuric acid, nitric acid and weak carbonic acid.

It has the following adverse affects on the environment:

  • Causes respiratory and skin disorders.
  • Affects productivity of plants by damaging the leaves.
  • Enters the soil and affects the soil pH and other conditions.
  • Enters the ground and river waters which causes harm to the aquatic life.
  • Causes damage to marble and thus damages buildings and monuments like the Taj Mahal

Aerosol Formation

Aerosol is formed by the dispersion of solid or liquid matter in the atmosphere. There are natural aerosols also in the atmosphere. However, polluting aerosols are formed by the pollutant particulate matter like carbon particles.

If the aerosols form a thick layer in the troposphere, they affect the weather conditions by blocking the solar radiation. Aerosols are also deposited on the leaves and affect the photosynthesis. Aerosols disperse the organic metallic pollutants far and wide.

Depletion of Ozone

The stratosphere of the atmosphere has ozone (O3). Ozone is known to absorb the Ultraviolet (UV) rays present in the sun's radiation. The UV rays are believed to cause skin cancer and mutations. Thus, the ozone protects us from the harmful effects of the UV rays.

However, hydrocarbons such as the chlorofluorocarbons (CFCs) destroy the ozone molecules which deplete the ozone layer. Ozone holes have been detected in the atmosphere which permit the UV rays to reach the earth's surface. The harmful effects of the UV rays are visible in the countries such as Australia and New Zealand where the rate of skin cancer is higher than the other regions of the world.

Control of Air Pollution

Air pollution can be controlled by different methods depending on the source and the pollutant. The different methods are:

One of the major causes of air pollution are the automobiles. The fuels being used should be lead-free as this will reduce the level of lead in the atmosphere. The carburetor should be cleaned regularly and good quality fuel should be used. This reduces the smoke emission from the exhaust pipes of the vehicles. Efforts to introduce vehicles running on alternate sources (for example solar energy) of energy should be made. These methods will go a long way in reducing the occurrence of photochemical smog.

The industrial pollution is best controlled at source. The polluting gases should be passed through filters and other devices such as cyclone collectors, scrubbers, precipitators, etc. so that the particulate matter is removed before the waste gases are released out. The toxic gases should be detoxified.

The domestic and industrial smoke producing units should have long chimneys to take the polluting gases far above and then disperse over a larger area. They should also invest in solar cookers or bio gas.

The pollution by sulphur dioxide is mainly due to coal-based industries. Alternate non-sulphur containing fuel must be used. It is also possible to remove the sulphur from the fuel before use.

There are many plant species like the neem (Azadirachta indica), bel (Aegle marmelos), gulmohur (Delonix regia), etc. that clean the atmosphere. More trees of such types should be planted.

For effective control and prevention of air pollution it is important to educate people and create public awareness about the ill-effects of air pollution.

The following are some methods that may be adopted to control pollution on a large scale:

Combustion

Pollutants in the form of organic gases or vapours can be burnt to convert them into water vapour and relatively less harmful products, such as carbon dioxide.

Absorption

The gaseous effluents may be made to pass through scrubbers or absorbers. These contain a suitable liquid absorbent, which removes or modifies one or more of the pollutants present in the gaseous effluents making it comparitively harmless.

Adsorption

The gaseous effluents are passed through porous solid adsorbents kept in suitable containers. The organic and inorganic constituents of the effluent gases are trapped at the interphase of the solid adsorbent. Adsorbents hold (molecules of a gas or liquid or solute) to its surface, causing a thin film to form.

Methods to Control Particulate Emissions

Particulate emissions may be controlled by using mechanical devices that generally work on the basis of the following:

Gravity

In this process, the particles settle down by gravitational force. Sudden changes in the direction of the gas flow causes the particles to separate out due to greater momentum.

Fabric Filters

The gases containing dust are passed through a porous medium. which is usually woven fabrics. The particles present in the gas are trapped and collected in the filters. The gases freed from the particles are then discharged.

bag filter equipment to control particulate emissions

A Typical Bag Filter

Wet Scrubbers

Wet scrubbers are used in chemical, mining and metallurgical industries to trap sulphur dioxide, ammonia, metal fumes, etc.

Electrostatic Precipitators

When a gas or an air stream containing aerosols in the form of dust, fumes or mist, is passed between two electrodes, then, the aerosol particles get precipitated on the electrode.

electrostatic precipitator to control particulate emission

Electrostatic Precipitator

The following practices also help in controlling air pollution:
  • Better designed equipment and smokeless fuels must be used in hearths in industries and at home.
  • Automobiles should be properly maintained and must adhere to emission - control standards. (Bharat II or Euro II)
  • More trees should be planted along road-side.
  • Renewable energy sources, such as wind, solar energy, ocean currents, must be tapped to fulfil energy needs.
  • Tall chimneys should be installed for vertical dispersion of pollutants.
Photochemical smog

Photochemical smog occurs due to the action of sunlight on air pollutants, generating photochemical reactions. The principal constituents of the photochemical smog are gaseous hydrocarbons (leading to ozone formation) and oxides of nitrogen (NOx) that are a part of automobile exhaust.)

The reaction of sunlight with hydrocarbons and NO2 results in a variety of chemical products. One of these is ozone, made up of 3 atoms of oxygen (O3), which is an irritating noxious gas. Other undesirable chemicals such as aldehydes also result from this photochemical reaction. This type of smog is oxidizing in character because of the presence of O3, NO2 and some photochemical oxidants.

Note: While Ozone formed during photochemical reaction in the lowest region of the atmosphere is an air pollutant, in the upper atmosphere, the natural existence of the ozone layer helps protect living organisms from harmful U.V. rays from the Sun.

Because sunlight is essential to this type of smog, the concentration of ozone and other measurable chemicals it is maximum around mid day of summer months and falls of considerably at night.

Photochemical smog causes eyes irritation and coughs due to the presence of ozone and can lead to respiratory problems and reduced physical (athletic) ability.

Formation of photochemical smog

When pollutants like hydrocarbons and nitrogen oxides combine in the presence of sunlight, smog is formed. This is a mixture of gases and since it is formed by photochemical reactions, it is called the photochemical smog. The word 'smog' is derived from the two words - smoke and fog.

It forms a yellowish brown haze especially during winter and hampers visibility. It also is a cause of many respiratory disorders and allergies as it contains polluting gases.

Photochemical smog is mainly composed of ozone (O3), peroxyacetyl nitrate (PAN) and NOx. It is also known as brown air where solar radiation is intense. In seasons of lesser solar radiation or areas, smog formation is incomplete and the air is referred to as grey air.

causes of photochemical smog

A simplified set of photochemical reactions involved in smog formation is as follows:

Reactions occuring inside engine:

Reactions occuring in atomsphere:

Smog ozone may damage plant as well as animal life. Several species of plants are very susceptible to PAN in smog. PAN damages choloroplasts, which results in reduction of photosynthetic effeciency and growth of plants.

effect of peroxyacetyl nitrate on leaves

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