Acidification and Acidic Rain
The term acidic rain was used for the first time by the British chemist, Angus Smith, in 1958. Precipitation that is more acidic than natural precipitation, which has an average pH of about 5.6. When precipitation has a pH of less than 5.6, it is usually due to the injection of sulphur compounds of nitrogen oxides into the atmosphere. Coal burning, electric power plants, industrial furnaces, and motor vehicles inject large amounts of these chemicals into the atmosphere. In the atmosphere, the chemicals combine with water to form sulphuric acid and nitric acid. These droplets may be transported great distances by wind before they precipitate to the ground. The main gases involved in the production of acid rains are nitrogen oxides, especially sulphur dioxide (SO2).
Acid rain has become a worldwide problem. In Europe, Norway, Sweden, Denmark, Finland, Germany, Great Britain, Belgium, the Netherlands, Poland, Russia, Belarus, Japan, Australia, New Zealand, Singapore, and the industrial regions of the developing countries are the worst affected by acidic rains. Many of the cities of eastern China ï¿½ (Beijing, Zhejiang (Shanghai), Tianjin, etc.) and India (Kolkata, Mumbai, Delhi, Chennai, Ahmadabad, Kanpur, Jamshedpur, Bhilai, etc.) have high concentration of nitrate in their environments. The impact of acid rain on aquatic ecosystems, particularly fresh-water lakes, has been clearly established. Aquatic ecosystems, especially fish are very susceptible to acidification. The most susceptible are the smaller organisms such as mollusks and minnows. High acidity, however, favours the growth of sphagnum mosses and lamentous algae.
The lakes located in areas where the parent rock is igneous and metamorphic containing lots of silicates are most sensitive to acid deposition. In regions where the parent rock is high in the mineral salts such as calcium, magnesium and phosphorous, lakes can better tolerate the acid runo. All over the world, there are more than 1000 lakes that have become too acidic to support life. Most of these lakes are in the Scandinavian countries (Norway, Sweden, Finland, and Denmark), Canada, Adirondack Mountains of New York and New England region of USA. The impact of acidic rains on terrestrial ecosystems is less visible. The acidic-rain however, is the main cause of widespread damage to natural vegetation (forests) in the temperate latitudes. In India, the forests in the vicinity of Bhilai (Durg), Raurkela, Ranchi, Bengaluru (Bangalore), Kanpur, Kolkata, Hardwar, Dehra Dun, Nainital, Shimla, Srinagar (Kashmir Valley) are seriously damaged by acidic rains.
Apart from fauna and flora, the health of human beings is also adversely affected by the acidic rains. It has been established by the experts of medical sciences that acidic rains create respiratory diseases, particularly in children.
Ozone is a molecule formed of three atoms of oxygen instead of usual two. Ozone forms naturally when lightning strikes through air; larger quantities are generated spontaneously in the stratosphere. The gas is colurless, and odorless, except at very high concentration. It is a major ingredient of smog. A diffuse layer of ozone mixed with other gases ï¿½the ozone layer-surrounds the world at a height of about 20 to 40 kilometers Ozone is formed near the ground when pollutants such as unburned petroleum, hydrocarbons and nitrogen oxides from automobiles exhaust and fossil fuel power plants react in sunlight. The chemical reactions are faster on hot sunny days. It
takes several hours after the sun rises for the chemical reactions to reach the level where ozone accumulates. It usually begins to form at about 10 A.M. on each day. It however, does not persist for long periods. Breathing ozone may cause respiratory problems for those who exercise out of doors. Ozone is an irritant to the lungs and air passages. It is especially irritating to those who engage in vigorous exercise. Some individuals are affected almost immediately if they exercise in air elevated ozone levels. They may cough or experience chest pain and shortness of breath. It is best to exercise before 10 A.M. Jogging along major tract thorough fares is also to be avoided. A general rule is: The more and harder you exercise, the greater the intake of air and therefore of ozone.
Harmless synthetic chemicals released into the atmosphere ï¿½primarily chlorofluorocarbons (CFCs) used as cleaning agents, refrigerants, fire-extinguishing fluids, sprayï¿½can propellants, and insulating foamsï¿½are converted by the energy of sunlight into compounds that attack and partially deplete the Earthï¿½s atmospheric ozone. Ozone levels in the stratosphere have decreased by at least three percent over most of the United States since 1969. A four percent drop has been noted over Australia and New Zealand, and a percent decrease has been observed near the North and South Poles. The amount of depletion varies with latitude (and with the season) because of variations in the intensity of sunlight. This decline in ozone alarms scientists because stratospheric ozone intercepts some of the high-energy ultraviolet radiation coming from the Sun. Ultraviolet radiation injures living things by breaking strands of DNA and unfolding protein molecules. Species normally exposed to sunlight have evolved defenses against average amounts of ultraviolet radiation, but increased amounts could overwhelm those defenses. Land plants such as soya beans, and planktons in the uppermost two meters of ocean would be adversely affected.
The first significant data about the depletion of ozone layer was presented by Farman, the leader of the British Antarctic Survey in 1985 who established that an ozone hole has occurred in the stratospheric ozone layer over Antarctica. About 90 per cent of Antarctica is covered by the ozone hole which has expanded also over the ocean areas. Subsequently, Kerr (1988) reported that ozone layer is being depleted over the Arctic atmosphere. This is of specific significance because ozone depletion is in the Northern Hemisphere which has large concentration of population.
Antarctic Ozone Hole
The maximum depletion of ozone is at the South Pole which is known as Antarctic Ozone Hole. The ozone hole is a loss of stratospheric ozone over Antarctica. The hole app ears in September when sunlight first reaches the region and ends in October when the general circulation brings al summer warming over Antarctica. In the winter season over Antarctica, a very large mass of extremely cold, dry air keeps out warmer air surrounding the continent. This cold air gets even colder during the winter months when there is no sunlight. In parts of Antarctica, during the winter season, temperature drops as low as -84o C. In the extreme cold, moisture condenses into ice crystals and nitric acid crystals also form.
These crystals form very high thin clouds called polar stratospheric clouds. The cloud crystals play a very important role in the chemistry of the CFCs and ozone depletion. The nitrogen oxide crystals drop out of the stratosphere leaving behind the chlorine and bromine compounds and the ice crystals. The ice crystal provides a place for accelerated chemical reactions. The chemical processes are more rapid where there is surface on which the reaction can take place. Ice particles are good surfaces and are some 10 times as the surface of water droplets. This partially explains the speed with which ozone depletion process takes place in the Antarctic spring. It also explains why the process is less effective in low latitudes. The chemical process begins when sunlight appears in the spring. The warming increases the rate of chemical reactions and chlorine destroys ozone at a rapid rate. The depletion, first begins near the Antarctic circle, where sunlight begins to peneterate the stratosphere. It may begin here by mid-August spring over the South Pole occurs in September and October.
During this time, the ozone level drops until there is no more ozone the clouds evaporate. There may be a total loss of up to 60 per cent of the ozone in the center of the Antrarctic hole. At some attitudes, it is 90 per cent. The eventually, air from surrounding regions flows into the area and ozone levels recover. Polar stratospheric clouds disappear with the spring warmup. The same process takes place elsewhere in the atmosphere, but at higher attitudes and slower rates.
Written by princy