It has long been known that air pollution can affect the local climate, particularly the rainfall. In the recent years there has been considerable debate about the potential impact of air pollution on global climate. Let us examine some of these climatic effects of air pollution.
Greenhouse Effect :
During the past two centuries, carbon dioxide in the atmosphere has increased dramatically. The probable cause is the burning of fossil fuels and burning of forests by farmers.
Scientists have been worrying that the growing burden of carbon dioxide and other gases may change the earth's climate. In normal circumstances, much of the solar radiation that penetrates the earths' atmosphere is re-radiated as heat from the earth's surface, and dissipates into space. But an increase of carbon dioxide allows most solar radiations to penetrate the atmosphere, but prevents part of the heat re-radiated by the land and water bodies from escaping into space. As carbon dioxide accumulates, enough heat may be trapped to gradually warm the atmosphere.
Other gases that contribute to the greenhouse effect are methane and chlorofluorocarbons. Methane is attributed to release- from livestock manure, additional rice fields, and digestion of termites proliferating on dead wood left by worldwide clearing of forests.
Most emissions from industrial plants and factories fall in the areas nearby. Some are carried away by winds to be washed from the sky by snow, rain, or mist. But some chemicals, such as chlorofluorocarbons, do not dissolve or recombine at lower levels. They rise to higher levels and contaminate the stratosphere. Chlorofluorocarbons are used as refrigerants and in some places, still used as 'spray-can propellants1. They add to the greenhouse effect as they drift upwards.
Researchers estimate that the expected rise in atmospheric temperature. Would be 2°C by the end of the century if the greenhouse gases continue to increase at current rates. Higher global temperature would cause glaciers to melt, and also lead to an expansion of the oceans because warm water occupies larger volume than cold water. If the levels of greenhouse gases in the atmosphere double, sea level will rise; estimates of the average rise vary between 0.5 to 2.5 metres. This is expected to occur gradually in the next century affecting coastal areas where about a billion people, a quarter of the world's population now live.
Acid Rain :
Sulphur dioxide released from coal burning power plants, industrial boilers and smelters, is normally carried away and gets oxidised in the atmosphere. But when it gets absorbed in suspended particulate matter in the air such as dust, fly ash, etc. and comes in contact with moisture droplets, it turns into sulphuric.acid. The same thing can happen when it comes in contact with the moist tissue in the lungs. Similarly, nitrogen dioxide gas from motor vehicle exhaust is converted into nitric acid. Unless they are neutralised in reaction with alkaline compounds in the atmosphere, these strong acids eventually return to the earth as 'acid rain'.
Rainfall in unpolluted areas is usually slightly acidic with a pH between 5.5 and 6.5 because water and carbon dioxide combine in the atmosphere to form a weak acid, carbonic acid. The pH of acid rain is often less than 4, even 3 to 3.5 is not unusual.
Acid rain corrodes metals and limestone, causing extensive damage. It damages vegetation and wildlife, etches car finish and erodes buildings and bridges. Moreover, acid rain is seldom localised. These pollutants can travel hundreds, even thousands of kilometers to rain far away with no respect for state or* national boundaries.
The overall effect of acid, rains on ecosystems is not well understood, but it has the potential for serious damage. Sweden's forests have suffered a reduction in growth rate, starting in the 1950s, thought to be duet& acid rains. The fish like salmon and trout have disappeared from Swedish lakes and streams where the pH has fallen much below 5. The origin of acid rain in Sweden has been traced to Western Europe, especially the United Kingdom. Acid rain has also taken its toll on the conifer forests of West Germany. As acids leach nutrients from their leaves and soil, spruce and fir trees are dying. Aluminum and heavy metals such as cadmium, are present in many soils but generally remain immobile. In Germany, they have become mobile in the presence of acids and are sucked up by trees into their systems causing more damage to already weakened trees. Acid rains can also affect soil microorganisms, especially those responsible for nitrogen fixation. They can influence the behavior of bacterial and fungal pathogens.
Corrosive and Soiling Effects
Buildings, bridges and other man-made structures can be soiled and damaged extensively by pollution. The accumulation of dirt and organisms that utilize polluted products can alter the appearance of a building. But most damage is caused by acids present both in the air and in rain-water. Stone work may get corroded. A case in point is the marble Parthenon in Athens. Sulphur oxides in the smog over Athens chemically transform marble into gypsum, causing it to crack and flake off. This has caused more erosion of the frieze panels of the Parthenon in the past twenty four years than had occurred iti twenty four centuries. The Westminster Abbey in London, and the ancient Roman Colloseum have also suffered such damage due to acid deposition.
In India, acid rain threatens our famous monument, the Taj Mahal. Crude-oil refineries situated in nearby Mathura, spew out vast quantities of sulphur dioxide into the atmosphere. This sulphur dioxide is carried by winds to Agra, among other places, where it gets adsorbed by water droplets, and rains down on the Taj Mahal. The Taj Mahal, made only of marble slabs, is susceptible to corrosion. The damage to this majestic monument, once it takes place, will be irreversible. Therefore, there is a need to monitor both the quality of the sulphur dioxide emissions from the refinery as well as the wind direction in the area. It is essential, in any case, that suitable measures should be taken for the removal of sulphur dioxide from refinery emissions.
The stratospheric pool of ozone is continually being produced and destroyed. Production takes place when molecular oxygen O2 is split by ultraviolet (uv) solar radiation and the resulting oxygen atoms, 'O', attach themselves to other O2 molecules :
O2 -> O + O
O+O2 -> O3 (Ozone)
The net result of destruction is
2O3 -> 3O2
Ozone can impair vision and breathing when it occurs in smog. But, in the upper atmosphere, 12 to 30 miles above the ground, it protects life on earth by intercepting the sun's damaging uv radiations. During the past ten years, however, this protective layer of ozone has become thinner over the South Pole. From 1979 to the present, the hole has deepened within which ozone concentration has fallen by almost 40%.
Some scientists believe that the ozone was attacked by chlorine released by chlorofluorocarbons, widely used as industrial chemicals. When chlorofluorocarbons, which drift upwards, reach the stratosphere, they react destructively with ozone. Ozone can also be destroyed by nitric oxides from emissions of supersonic jets. It is also possible that ozone was pushed aside by the upwellings of air from lower levels of .the atmosphere. Whatever the cause, the potential effects could be serious. If the ozone that protects us from excessive uv radiations, continues to disappear, it would result in an increase in melanomas and skin cancer incidents as well as a reduction in crop yield.
EFFECTS OF POLLUTION ON BIOLOGICAL SYSTEMS
Plants are not immune to the pollution we produce. In fact, being the only organisms capable of producing food and thus sustaining life on earth, they are liable to pass on whatever pollution they pick up. Crop yield may be greatly affected with serious consequences to the human race.
Photochemical smog has a deleterious effect on plants. As described earlier, in the presence of sunlight, various pollutants combine to form ozone and peroxyacetyl nitrate (PAN). Ozone is extremely damaging to plants. It enters leaves through the stomata which are used for normal gas exchange, and alters the permeability of the membranes of the stomata. This causes nutrient—and electrolyte—imbalances resulting in the death of the cells. In effect, ozone increases respiration of leaves, killing the plant by depleting its food. Chronic exposure to ozone may weaken plants and make them more susceptible to disease, or age them prematurely, reducing crop yields without signs of outward injury.
PAN, the other component of photochemical smog, is phyto-toxic. It blocks the process of photosynthesis killing the plant by shutting down food production.
Sulphur dioxide has a potential for serious damage to plants by contributing to acid rains. As said earlier, these rains leach nutrients from soil and foliage, and affect soil organisms responsible for nitrogen fixation. Acids enhance the uptake by plants, of toxic heavy metals from soil. This has seriously effected the existing conifer forests in Europe and Western United States.
Though many pollutants are absorbed by animal tissues from the contaminated food eaten. Animals can be directly affected by pollutants in the air. The following discussion will concentrate on the effects of air pollutants on man, since the effects on animals is much the same as on man.
An average adult breathes approximately 14,000 litres of air per day. This air is cleared by various filtering devices in the nasal passage and lungs. Large particles breathed in, are trapped by hair in the nose. Soft spiral bones in the nasal passage break the air into narrow whirling streams from which, smaller particles are forced against the sticky walls of the passage. Other impurities are caught in the trachea and bronchi by a thin layer of mucous which is propelled like an undulating blanket towards the throat, where both mucous and impurities are spat out. In the lungs, the tiny air sacs or alveoli are protected by scavenger cells that engulf many of the smaller particles that penetrate this system of defence.
These filtering mechanisms work well enough in the normal atmospheric conditions. But, in many cities the atmosphere is so charged with pollutants that these mechanisms are put under intolerable strain. Moreover, many of the most dangerous pollutants are attached to dust particles of so small size that they evade the filtering mechanisms altogether, while others in the gaseous form eventually pass into the blood. One of the most serious effects of certain pollutants is to slow down the rate at which oxygen is passed from the alveoli into the blood stream. This puts an additional load, not only on the lungs, but also on the heart.
Ozone and PAN, both components of photochemical smog, 'irritate eyes, impair vision, make breathing difficult, and aggravate asthama. SO2 in the air, obstructs breathing and irritates eyes. But, its effect is far greater when it gets adsorbed on moist tissues in the lungs. It forms sulphuric acid which burns into the lungs and causes respiratory ailments. Nitric acid, similarly formed, also causes respiratory ailments in a like manner. Both NO2 and SO2 contribute to acid rain. Acid rain can have a very adverse effect on aquatic life. Certain fish cannot tolerate even small fluctuations in pH, with the result that acid rain over streams and lakes has greatly affected fish populations. As mentioned earlier, in Swedish streams, salmon and trout have disappeared where the pH has fallen below 5.
Lead is poured into the atmosphere through automobile exhaust. When inhaled, it accumulates in bones and other tissues, threatening to cause irreversible brain and kidney damage. Young children are most vulnerable because their nervous systems are still developing. Excessive absorption of lead can decrease a child's intelligence, shorten his or her attention span, cause learning disabilities or cause hyperactivity. Elevated blood lead levels in adult males have been linked to high blood pressures indicating that many heart attacks may be brought on by lead in the systems.
Carbon monoxide (CO), is encountered in auto exhaust fumes and incomplete burning of any organic material. The gas is colourless and odourless, but very poisonous to living things. CO reduces the oxygen carrying capacity of the blood by combining with haemoglobin to form carboxyhaemoglobin. And thus it brines about asphyxiation. In minute quantities, it can cause breathlessness and tiredness.