6s Elaine Leung 25

6. Explain the causes and effects of ozone depletion, global warming and acid rain. State the role of individual in overcoming the environment.

Acid rain refer to precipitation in the form of dilute acid solutions. It is not a simple or a single phenomenon; its formation involves a sequence of chemical reaction and it occurs in many different places globally. Acid rain is formed by the reaction of various industrial pollutants, such as sulphur dioxide and nitrogen dioxides with naturally occurring oxygen and water vapour, forming acid solutions. These acid solutions precipitate as rain or snow. Normally, rain water is slightly acidic (pH 5.7) owing to the presence of carbon dioxide in the air. Acid rain typically has a pH from 4.0 to 5.4. In some parts of the world, rain water with a pH as low as 2.5 has been recorded, i.e. the acidity is about 1,600times that of normal precipitation.

Sulphur dioxide (SO2) is the pimary cause of acid rain. It is mainly released from the burning of fossil fuels in the 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 (SO3). During this period, SO3 can be transpotted for a considerable distance by wind and then dissolved in rain water to form a dilute solution of sulphuric acid (H2SO4). Apart from forming H2SO4, SO2 can directly dissolves in rain water to form sulphurous acid (H2SO3). The simplied sequence of change from the emitted sulphur dioxide to acids is shown as follow:

2 SO2(g) + O2(g) à 2 SO3(g)

SO3(g) + H2O(l) à H2SO4(aq)

SO2(g) + H2O(l) à H2SO4(aq)

Nitrogen oxides are another cause of acid rain. The buring of fossil fulesin motor vehicles and power stations are mainly responsible for the release of nitrogen oxides into the atmosphere. Once nitrogen monoxide (NO) is formed, it will combine with atmospheric oxygen to form nitrogen dioxide (NO2). Following a series of chemical reactions, NO2 reacts with water and oxygen to form nitric acid (HNO3). A simplified sequence of changes form the emitted nitrogen oxides to nitric acid is shown as follows:

2 NO(g) + O2(g) à 2 NO2(g)

4 NO(g) + 2H2O(l) + O2(g) à 4 HNO3(aq)

Acid rain is usually regarded as a regional problem rather than a global problem. It is because the acidic components retain the atmosphere only for a short time period. However, acid rain is a serious problem in many places. The harmful effects are that the acid rain causes damage to buildings, status and monuments. Mable and various metals (e.g. iron) are corroded by acid rain. Acid rain causes human respiratory diseases such as bronchitis and asthma. Acid rain can damage tree foliage directly and weaken the trees so that they become more susceptible to diseases, insects and drought. Acid rain increases the acidity of soil. Soil ahs a 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 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. Acid rain can induce the release of alumimium ions from soil particles. Once released, the water-soluble aluminum ions may reach a concentration that is poisonous to plant. This can damage the roots of the trees. Once the aluminium ions are washed into the water bodies, they can also kill aquatic organisms (e.g. fish). Acid rain can acidify lakes. With an increased acidity, moderately toxic inorganic mercury compounds in lake-bottom sediment are converted into methylmercury, which is highly toxic and can accumulate in the fatty tissues of animals. The compound can reach high concentrations in food chain and food webs.

Ozone is a highly reactive gas comprising triatomic oxygen (O3). 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 parts per million. The ozone layer is an important protective layer in the upper atmosphere. It provides a thermal blanket for the Earth, and protects life by filtering out harmful ultraviolet radiation from the sun.

Ozone depletion refers to a lowered concentration of ozone in the upper atmosphere. Evidence has been found that the ozone layer can be damaged by air pollutants, leading to the formation of “holds”. Today, scientists have discovered holds in the ozone layer over the Arctic and Antarctica. In other words, there is an increase in ultraviolet radiation reaching the Earth’s surface. This can bring about undesirable effects on organisms.

Recent studies show that the ozone hole has been formed due to chemical effects. Since ozone is formed and broken down continuously in natural processes, the rate of can be speeded up by the presence of ozone depleting pollutants such as chlorofluorocarbon compounds, halons and bromine.

Chlorofluorocarbon (CFC) Compounds are regarded as the major cause of ozone depletion. They are a group pf 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 haoushold cleaning products; and also an solvents for cleaning electronic circuit boards and computer components.

CFC compounds are remarkably inert and have a long lifetime in the atmosphere (ranging from 40-150 years.). These chemicals may drift up to the upper atmosphere and are involved in chlorine-releasing reactions. The chlorine radical (Cl-) is released from CFC compounds and reacts with an ozone molecules in the presences of sunlight. As a result, the ozone molecule is destroyed and forms a chlorine monoxide radical (ClO-). When ClO- reacts with a free oxygen radical, another Cl- radical is released. This Cl- radical can attrck another ozone molecules. It is reported that one chlorofluorocarbon molecule can destroy more that 104 ozone molecules. A simplified sequence of the breakdown of ozone molecules by chlorine radicals is shown below:

Cl- + O3 à ClO- +O2

ClO- +O- à Cl- + O2

The destruction of the ozone layer results in a significant increase in ultraviolet radation reaching the Earth’s surface. It may induce various effects on humans that are ultraviolet radation has direct fects on DNA. It can modify the genetic information in body cells. 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. An increase in exposure to ultraviolet radiation can greatly reduce the yields of crops. Aquatic life, especially phytoplankton, is very sensitive to ultraviolet light. A reduction of its productive in the ocean could disrupt the ecological balance of food webs. It is because phytoplankton is the producers in aquatic habitats. An increase in ultraviolet radiation leads to an increase in the incidence of photochemical smog.

A warmer global climate could have a number of possible effects. Thermal expansion of the oceans, melting of glacial ice, and melting of ice caps at Arctic and Antarctic would elevate sea levels. Flooding would probably occur in costal regions where about one-third of the human population and their economic infrastructure are concentrated. Low-lying cities, industrial areas and arricultural lands would be submerged. 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. The distribution of the climatic regions and ocean currents would be changed, leading to alterations of agricultural regions. Weather extremes are also expected to increase in frequency and severity. Some palaces would have increments in rainfall and some would have sacraments. These would result in flooding or drought in various areas. Damaging hurricanes, typhoons and violent storms would also increase in intensity and occur more frequently. There would be increases in the average temperature of marine and freshwater bodies. These would affect cold-water fish and marine mammals, and would lead to undesirable competition for habitats and food sources. Global warming would also affect biodiversity. Some species would probably become extinct, particularly those that only survive in narrow temperature ranges and those confined to small habitats. Ecosystem such as polar seas, coral reefs and mountains are considered to be more vulnerable to change in climate. A warmer climate could alter disease pattern in unpredictable ways and favour the spread of pathogenic microorganisms. These would increase the spread of epidemics that affect humans and other organisms. Pets could Pests could become more active and grow faster. Consequently, damages of crops and spread of plant diseases would be faster, resulting in food shortage and famines.