Some specific phenomena related with air pollution are as follows: 1. Ozone 2. Greenhouse Effect 3. Acid Rain 4. Smog 5. Pollution by Vehicular Exhaust!
Ozone is normally present in the atmosphere at about 0.05 ppm at sea level. It is produced naturally in the atmosphere by the action of electric discharges on oxygen. Ultraviolet radiation in the wave length of 260 mm is absorbed by oxygen molecules, causing them to react in this manner.
Radiation from the sun probably causes high concentrations of ozone in the outer portions of the earth’s atmosphere.
Ozone plays a key role in the photo-chemical formation of atmospheric pollutants as well as being end product in the intricate complex of reactions that take place. The ozone layer provides a cover to the earth’s surface and protects the earth from ultraviolet rays.
Until 1974, atmospheric scientists were proceeding in their research on possible impacts of nitrogen oxide (jet engine exhausts) on the ozone layer. But later it was found that a new threat to the ozone layer has emerged from synthetic chemicals called chlorofluorocarbon. This is a simple compound of chlorine, fluorine and carbon. The chlorine can destroy ozone.
Recent studies indicate that if the production of chlorofluorocarbon continues at the present rate, the compound will enter the stratosphere in quantities capable of seriously depleting the ozone layer, normally termed as ‘holes in ozone layer’.
The use of chlorofluorocarbon is increasing because of the demand of ‘personal care products’ such as deodorants, hair sprays, shaving creams and countless other consumer cosmetic products as well as in refrigeration. The ozone layer serves as a shield, protecting the troposphere and earth’s surface from most of the ultraviolet radiation found in the sun’s rays.
If these ultraviolet rays reach the earth’s surface in full intensity, not only all exposed bacteria would be destroyed, but plants and animal tissues would also be severely damaged. In this protective role, the presence of the ozone layer is essential in man’s environment.
2. Greenhouse Effect:
The temperature at the surface of the earth is maintained by the energy balance between the sun’s rays that strike the planet and the heat that is radiated back into space. Some of the heat is absorbed and retained by the earth or objects on the surface.
Much of this does not pass through the air envelope to outer space but is absorbed by the carbon dioxide and water vapours in the atmosphere and adds to the heat that is already present.
Thus, carbon dioxide acts like the glass of a greenhouse, and on a global scale, tends to warm the air in the low levels of the atmosphere. This is called the greenhouse effect, which is also responsible for the increase in temperature over the earth’s surface.
Scientists are of the opinion that carbon dioxide will increase global temperature significantly.
There is no contrasting opinion about the fact that due to industrial and other forms of air pollution the carbon dioxide content is increasing. Volcanic eruptions are also responsible for the increase in carbon dioxide.
Volcanic activity during historical period as well as in present times has thrown out dust and ash which spreads throughout the entire atmosphere and lasts for years.
There is evidence that the temperature of the entire earth has risen slightly during recent decades. Between 1885 and 1940, global warming was a worldwide phenomenon. There was an increase in mean annual air temperature of about 0.5°C. The greatest increase was in the northern hemisphere between 40°N and 70°N lat, where it was an increase of 1.6°C in the average winter temperature.
After 1940 this process of global warming had become much slower, but with the increase in carbon dioxide content in the atmosphere from combustion, this has once again on the move. This will result in the receding of glaciers and melting of snow and raise the sea level. That may destroy not only the coastal regions but also other parts of the earth.
With the increase in temperature, scientists have predicted that the process of melting of ice caps of Antarctic, Greenland, etc., may lead to a rise in the sea level. Some of them have pointed out that there can be a rise of four feet in every ten years. There is a crucial relationship between the presence of the polar ice cap and global temperatures and if it happens its impact on climatic conditions of the world will be drastic and will change the entire pattern of our ecosystem.
3. Acid Rain:
Acid rain is the outcome of sulfur dioxide (SO2) gas, released into the air by the combustion of fossil fuels. This readily forms sulfuric acid (H2SO4) in the minute water films of suspended droplets in the air over cities. The washout of sulfuric acid by precipitation results in rain water with an abnormally high content of the sulfate ion, a condition known as acid rain. Nitric acid, formed by reactions involving the pollutant nitrogen oxides, may also contribute to the acidity of rain water.
The degree of acidity is measured in terms of pH value of rain water. The pH of rain water, as measured in the laboratory, is 5.65 owing to the dissolution of atmospheric CO2 in it.
Rain, having a pH value of less than 5.65, the value resulting from the equilibrium between atmospheric CO2 and pure distilled rain water at 25°C, can be termed acid rain.
In recent years, it has been pointed out by water chemists that pH value of rain in north-western Europe and USA is going down to 3 and 5, which is an indication of acidity in rain water. The pH numbers are on a logarithmic scale; these values mean that rain in these areas has now often 100 to 1,000 times more acid than before. After studying rain water for 20 years the US scientists have reported average pH value of about 4.
They have also observed values as low as 2.1 in rain water at certain localities. The acid rain is now not only the problem of industrial developed world but is also seen in countries like India. The observations of Khemani and his associates (1989) have been summarised in Table 7.3.
Some of the adverse environmental effects of acid rain may include excessive leaching of nutrients from plants, foliage and the soil, disturbance of balance of predators and prey in aquatic ecosystems, various metabolic disturbances to organisms, acidification of lakes and streams and the corrosion of structures.
Average pH Values of Rain Water over Cities in India:
High pH Values
Medium pH Values
Low pH Values
Very low pH
Values (< 4.5)
‘Smog’ is the name given for ‘smoke fog’. The term was first used by H.A. Des Voeux in 1911 in a report on smoke fog deaths. During the autumn of 1909, in Glasgow, Scotland, 1,063 deaths were attributed to smoke and fog. In 1930, in Meuse valley of Belgium, a large number of people fell ill and many were killed by smog.
The ‘black fog’ that settled on London in December 1952 caused 3,500 to 4,000 deaths. It was accompanied by thermal inversion over most of the British Isles. In Pennsylvania smog killed 20 people and about 40 per cent population became ill. The residents of the Kanto Plains area of Yokohama, Japan, were afflicted with an accumulation of smog during winters of 1945 and 1946.
Similar type of attack was reported in New Orleans, Louisiana, in 1959. In recent years, with the increase in atmospheric pollution, the instances of smog resulting in serious injury or deaths have also become common.
The most irritating and injurious components of smog are the products of reactions in the atmosphere between oxygen, ozone and emission pollutants. The mixture of these products is called photochemical smog. These are reactions in which oxygen, ozone, nitrogen oxides and hydrocarbons produce those compounds which are toxic and irritating.
The constituents of smog are quite toxic and are responsible for respiratory and cardiac difficulties. Eye irritation is the most common symptom of smog injury. Similarly, bronchial asthma, chronic bronchitis and pulmonary emphysema are associated with smog. It is also injurious to animals and plants and one of the main causes of environmental degradation.
5. Pollution by Vehicular Exhaust:
The automobile is man’s greatest achievement in minimising distances. The number of automobiles is increasing day by day and has become a cause of air pollution and degradation of the environment. The automobile, with its internal combustion engine, emits poisonous gases, which are harmful to human health and is the most serious pollution problem of this technological age.
Exhaust emissions from diesel engines include carbon monoxide, hydrocarbons, sulfur oxides, organic acids, etc. The two primary pollutants are carbon monoxide and nitrogen dioxide, both of which are extremely poisonous gases and are present in the atmosphere of urban areas, especially in metropolitan areas.
A third form of pollutant is constituted by the unburned hydrocarbons of gasoline. Lead, is also a very toxic compound and its main source in the environment is believed to be from leaded gasoline used as fuel for internal combustion engines. The lead usually added to gasoline is the organic compound tetraethyl lead (TEL), which is extremely poisonous. The presence of lead in the atmosphere is a threat to the environment as well as for all living organisms.
It becomes clear from the above analysis that the problem of air pollution is increasing with the growth and expansion of industries and automobiles. It is high time that all of us should know the harmful effects of air pollution and also evolve technology to control it.