7. Explain the causes and effects of ozone depletion, global warming and acid rain. State the role of individual in overcoming these problems.

Acid rain refers to precipitation in the form of dilute acid solutions. It is formed by the reactions of various industrial pollutants, such as sulphur dioxide and nitrogen oxides with naturally occurring oxygen and water vapour, forming acid solutions. These acid solutions precipitate as rain or snow.

Causes of acid rain

1. Sulphur dioxide is the primary cause of acid rain. It is mainly released from the burning of fossil fuels in power stations, heavy industries and motor vehicles. In the atmosphere, SO2 reacts with oxygen in the air. Within about 43 days, it will be converted into sulphur trioxide. During this period, SO3 can be transported for a considerable distance by wind and then dissolved in rain water to form a dilute solution of sulphuric acid. Apart from forming H2SO4, SO2 can directly dissolves in rain water to form sulphurous acid.

2. Nitrogen oxides are another cause of acid rain. The burning of fossil fuels in motor vehicles and power stations are mainly responsible for the release of nitrogen oxides into the atmosphere. Once nitrogen monoxide is formed, it will combine with atmospheric oxygen to form nitrogen dioxide. Following a series of chemical reactions, NO2 react with water and oxygen to form nitric acid.

Effects of acid rain

Acid rain is usually regarded as regional problem rather than a global problem. It is because the acidic components remain in the atmosphere only for a short time period. However, acid rain is a serious problem in many places. The harmful effects are so follows:

1. Acid rain causes damage to buildings, statues and monuments. Marble and various metals are corroded by acid rain.

2. Acid rain causes human respiratory diseases such as bronchitis and asthma.

3. Acid rain can damage tree foliage directly and weaken the trees so that they become more susceptible to diseases, insects and drought.

4. Acid rain increases the acidity of soil. Soil has natural capacity to neutralize some inputs of acids. This normal soil-buffering capacity will be depleted in the presence of acid rain. When the soil Ph is below 4.5, most valuable nutrients are rapidly lost and bacterial activities will be greatly reduced. This affects the soil fertility and in turn, affects the growth of forest trees and crops.

Ozone depletion

Ozone is a highly reactive gas comprising triatomic oxygen. It is formed by the combination of oxygen in the presence of ultraviolet radiation in the atmosphere. It exists as a natural component of the atmosphere and is kept in a constant concentration of approximately 0.01 part per million. Ozone depletion refers to a lowered concentration of ozone in the upper atmosphere.

The cause of ozone depletion

Chlorofluorocarbon compounds are regarded as the major cause of ozone depletion. They are a group of organic compounds composed of chlorine, fluorine and carbon. They have been widely used in daily life as cooling agents in refrigerators and air-conditioners; as blowing agents in the manufacture of foam plastic for insulation and packaging; as propellants for aerosol sprays such as hair mousses and household cleaning products; and also solvents for cleaning electronic circuit boards and computer components.

CFC compounds are remarkably inert and have a long lifetime in the atmosphere. These chemicals may drift up to the upper atmosphere and are involved in chlorine-releasing reactions. The chlorine radical is released from CFC compounds and reacts with an ozone molecule in the presence of sunlight. As a result, the ozone molecule is destroyed and forms a chlorine monoxide radical. When ClO reacts with free oxygen radical, another Cl radical is released. This Cl radical can then attack another ozone molecule.

Effects of ozone depletion

1. Ultraviolet radiation has direct facts on DNA. It can modify the genetic information in body cells.

2. An increase in the incidence of skin cancer, cataract and lung diseases, as well as a reduction in the function of the immune system is probably due to the increase in ultraviolet radiation reaching the Earth.

3. An increase in exposure to ultraviolet radiation can greatly reduce the yields of crops.

Global Warming

It refers to the gradual increase in the average temperature in the atmosphere as a result of the accumulation of greenhouse gases.

Cause of greenhouse effect

Carbon dioxide is present in a small amount in the lower atmosphere. It allows the incoming solar radiation to pass through. However, it absorbs the infra-red radiation which is emitted from the Earth to the Space. In other words, part of the energy is trapped in atmosphere by carbon dioxide. This phenomenon is known as the greenhouse effect.

The effects of global warming

1. Thermal expansion of the oceans, melting of glacial ice, melting of ice caps at Arctic and Antarctic would elevate sea levels. Flooding would probably occur in coastal regions where about one-third of the human population and their economic infrastructure are concentrated. Low-lying cities, industrial areas and agricultural lands would be submerged.

2. Most of the major rainforests in the world are low-lying and at the margin of water level. The global climate would be affected if they are removed due to submergence.

3. The distribution of the climate regions and ocean currents would be changed, leading to alterations of agricultural regions.