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

Air pollution is exerting increasing impact on the environment.Humans have been altering the chemistry of the atmosphere on a global scale through their activities.This may already be modifying the climate.These changes not only threaten the health of human beings and other organisms,but also create many environmental problems such as acid rain,diminish the protective ozone layer in the upper atmosphere and contribute to the potential of global climatic changes.All these are seriously upsetting the balance of the global ecosystem.

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 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 "holes".Today,scientists have discovered holes 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.

The cause of ozone depletion

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

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(ranging from 40 to 150 years).These chemicals may drift up to the upper atmosphere and are involved in chlorine-releasing reactions.The chlorine radical(CI.) 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(CIO.).When CIO.reacts with a free oxygen radical,another CI. radical is released.This Cl. radical can than attack another ozone molecule.It is reported that one chlorofluorocarbon molecule can destroy more than 10 to the power 4 ozone molecules.

The effects of ozone depletion

The destruction of the ozone layer results in a significant increase in ultraviolet radiation reaching the Earth's surface.It may induce various adverse effects on humans:

-Ultraviolet radiation 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 funtion 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 productivity 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.

Global Warming

Global warming refers to the gradual increase in the average temperature

in the atmosphere as a resultt of the accumulation of greenhouse gases.The increase in concentration of greenhouse gases in the atmosphere slows down the heat loss from the Earth to the Space.As a result,the atmosphere is warming up.This leads to changes in climate and other subsequent effects on the Earth.

The effects of global warming

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 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.

-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 places would have increments in rainfall and some would have decrements.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.Ecosystems such as polar seas,coral reefs and mountains are considered to be more vulnerable to changes 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.

-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.

Acid rain

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 reactions and it occurs in many different places globally.Acid rain 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.Normally,rain water is slightly acidic(pH5.7)owing to the presence of carbon dioxide in the air.Acid rain typically has a pH from 4.0 to5.4.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 1,600 times that of normal precipitation.

The causes of acid rain

The major processes of the formation of acid rain are as follows:

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(SO3).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(H2SO4).Apart from forming H2SO4,SO2 can directly dissolves in rain water to form sulphurous acid(H2SO3).

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(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).

The effects of acid rain

Acid rain is usually regarded as a 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 as follows:

-Acid rain causes damage to buildings,statues and monuments.Marble and various metals 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 increase the acidity of soil.Soil has 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 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.

-Acid rain can induce the release of aluminium ions from soil particles.Once released,the water-soluble aluminium ions may reach a concentration that is poisonous to plants.This can damage the roots of trees.Once the aluminium ions are washed into the water bodies,they can also kill aquatic organisms.

-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 chains and food webs.

What we can do?

In order to overcome these problem,we have several solutions.We can plant more trees to absorb carbon dioxide.We can take public transport rather than using our own cars as well.Also we can prevent to use aerosol sprays which contain CFC compounds.