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Monday, May 7, 2012
Friday, May 4, 2012
Ozone hole
What is the ozone layer?
Ozone layer is a layer in the outer atmosphere (stratosphere) that contains the gas ozone, which acts as a shield in the atmosphere absorbing much of the harmful ultraviolet (UV) rays from the Sun, preventing them from reaching Earth's surface. It is a protective layer made up of oxygen, with a molecular formula of O3, lying 15 to 35 kilometres above Earth's surface, protecting life on Earth from the Sun's harmful ultraviolet rays. In the past century, the ozone layer is getting thinner and thinner. In 1987, a hole as big as United States developed over continent of Antarctica comes to be known as "Antarctic Ozone Hole". In 2000, the hole was at its largest, 3 times the size of United States.Urgent measures are needed to combat increasing ozone depletion as more harmful ultraviolet rays from the Sun can now reach the Earth.
Who is responsible for all these disastrous effects on Earth?
No doubt, it is humans. They have created CFC. CFCs developed in 1930s are man-made substances that contain chlorine, which are used as solvents in paints, propellants in aerosol sprays, coolants in refrigerators and air-conditioners, manufacturing of foam packaging and cleaning of electronic parts. They are compounds of carbon, chlorine and fluorine, with two main examples having molecular formulas of CFCl3 and CF2Cl2.
CFCs are harmless to the environment at ground level. However, once released, they rise up in the atmosphere and carried to the stratosphere. Under intense sunlight, they are broken down by UV rays from the Sun. CFCs decompose and release chlorine atoms which interact with ozone and convert it to oxygen. Chlorine atoms can destroy ozone molecules. One chlorine atom can destroy about 100000 ozone molecules, leading to ozone depletion. CFCs are very stable gases and remain for a very long time, from 100 to 20 000 years in the atmosphere, also, being difficult to destroy. Hence, though ozone layer can replace itself as ozone is continually created in the atmosphere as a result of the reaction of the Sun’s rays with oxygen in the atmosphere, CFCs, are being released at a faster rate than the rate at which ozone replaces itself.
How would ozone depletion affect the earth?
Ozone layer depletion reduces the protective ability environment from harmful ultraviolet rays of the Sun, which means greater exposure to harmful ultraviolet rays from the Sun. UV rays react with exhaust fumes, dust and smoke to produce smog (thick layer of pollutants) which worsens air quality in cities and weakens structures exposed to UV radiation. Too much ultraviolet radiation kills plankton, which is the basic food for marine life, leading to decline in fish supply. Aquatic/ marine creatures will die as the food chain is broken. Some aquatic species may be endangered. Increased exposure to UV rays also destroys or harms plant life because chlorophyll level reduces and plant tissues get damaged.This leads to poor harvest, decline in food supply and eventually starvation, e.g. rice. As plant growth is adversely affected, forest resources may decline and wildlife cum natural habitats become threatened.
There are also straightforward effects on human health. Increased exposure to harmful UV rays can cause skin cancer (melanoma) and eye cataracts (blurred vision) in people, which if left untreated, it can lead to blindness. Also, it weakens people's immune system, thus reducing resistance to illnesses and diseases.
Ozone layer is a layer in the outer atmosphere (stratosphere) that contains the gas ozone, which acts as a shield in the atmosphere absorbing much of the harmful ultraviolet (UV) rays from the Sun, preventing them from reaching Earth's surface. It is a protective layer made up of oxygen, with a molecular formula of O3, lying 15 to 35 kilometres above Earth's surface, protecting life on Earth from the Sun's harmful ultraviolet rays. In the past century, the ozone layer is getting thinner and thinner. In 1987, a hole as big as United States developed over continent of Antarctica comes to be known as "Antarctic Ozone Hole". In 2000, the hole was at its largest, 3 times the size of United States.Urgent measures are needed to combat increasing ozone depletion as more harmful ultraviolet rays from the Sun can now reach the Earth.
Who is responsible for all these disastrous effects on Earth?
No doubt, it is humans. They have created CFC. CFCs developed in 1930s are man-made substances that contain chlorine, which are used as solvents in paints, propellants in aerosol sprays, coolants in refrigerators and air-conditioners, manufacturing of foam packaging and cleaning of electronic parts. They are compounds of carbon, chlorine and fluorine, with two main examples having molecular formulas of CFCl3 and CF2Cl2.
CFCs are harmless to the environment at ground level. However, once released, they rise up in the atmosphere and carried to the stratosphere. Under intense sunlight, they are broken down by UV rays from the Sun. CFCs decompose and release chlorine atoms which interact with ozone and convert it to oxygen. Chlorine atoms can destroy ozone molecules. One chlorine atom can destroy about 100000 ozone molecules, leading to ozone depletion. CFCs are very stable gases and remain for a very long time, from 100 to 20 000 years in the atmosphere, also, being difficult to destroy. Hence, though ozone layer can replace itself as ozone is continually created in the atmosphere as a result of the reaction of the Sun’s rays with oxygen in the atmosphere, CFCs, are being released at a faster rate than the rate at which ozone replaces itself.
How would ozone depletion affect the earth?
Ozone layer depletion reduces the protective ability environment from harmful ultraviolet rays of the Sun, which means greater exposure to harmful ultraviolet rays from the Sun. UV rays react with exhaust fumes, dust and smoke to produce smog (thick layer of pollutants) which worsens air quality in cities and weakens structures exposed to UV radiation. Too much ultraviolet radiation kills plankton, which is the basic food for marine life, leading to decline in fish supply. Aquatic/ marine creatures will die as the food chain is broken. Some aquatic species may be endangered. Increased exposure to UV rays also destroys or harms plant life because chlorophyll level reduces and plant tissues get damaged.This leads to poor harvest, decline in food supply and eventually starvation, e.g. rice. As plant growth is adversely affected, forest resources may decline and wildlife cum natural habitats become threatened.
There are also straightforward effects on human health. Increased exposure to harmful UV rays can cause skin cancer (melanoma) and eye cataracts (blurred vision) in people, which if left untreated, it can lead to blindness. Also, it weakens people's immune system, thus reducing resistance to illnesses and diseases.
Sulfur dioxide (3)
How is sulfur dioxide emission reduced?
Firstly, cars could use cleaner fuels. In Singapore, oil fuels used in factories are forbidden to contain more than 2% of sulfur. Also, the amount of sulfur allowed in diesel fuel for vehicles has been steadily reduced in recent years. In 1996, diesel fuel could contain up to 0.5% sulfur by weight but in 1999, the allowed amount of sulfur was reduced to 0.05%. In Singapore, power stations are now burning more natural gas while buses and taxis are increasingly using compressed natural gas (CNG) instead of diesel. Natural gas is mainly methane and contains no sulfur. The products of combustion are carbon dioxide and water which are non-polluting.
Furthermore, powdered limestone (calcium carbonate) is also added to the hot gases produced from the burning of fuels at a coal or oil burning power station.
The heat decomposes the limestone to give calcium oxide, as represented in the equation below:
CaCO3 --> CaO (s) + CO2 (g)
The calcium oxide then removes the sulfur dioxide, tuning it into calcium sulfite:
CaO (s) + SO2 (s) --> CaSO3 (s)
Limestone is used because of its inexpensive price. About 95% of the sulfur dioxide is removed by these reactions. The calcium sulfite, CaSO3, is reacted with air to convert it into unreactive calcium CaSO4, and then it is dumped.
Firstly, cars could use cleaner fuels. In Singapore, oil fuels used in factories are forbidden to contain more than 2% of sulfur. Also, the amount of sulfur allowed in diesel fuel for vehicles has been steadily reduced in recent years. In 1996, diesel fuel could contain up to 0.5% sulfur by weight but in 1999, the allowed amount of sulfur was reduced to 0.05%. In Singapore, power stations are now burning more natural gas while buses and taxis are increasingly using compressed natural gas (CNG) instead of diesel. Natural gas is mainly methane and contains no sulfur. The products of combustion are carbon dioxide and water which are non-polluting.
Furthermore, powdered limestone (calcium carbonate) is also added to the hot gases produced from the burning of fuels at a coal or oil burning power station.
The heat decomposes the limestone to give calcium oxide, as represented in the equation below:
CaCO3 --> CaO (s) + CO2 (g)
The calcium oxide then removes the sulfur dioxide, tuning it into calcium sulfite:
CaO (s) + SO2 (s) --> CaSO3 (s)
Limestone is used because of its inexpensive price. About 95% of the sulfur dioxide is removed by these reactions. The calcium sulfite, CaSO3, is reacted with air to convert it into unreactive calcium CaSO4, and then it is dumped.
Acid Rain
What is acid rain?
Acid rain is defined as rainwater with a pH of less than 5, or any rainfall that has an acidity level beyonf what is expected in a non-polluted rainfall. To give a better understanding on what this number "5" entail, I would give you the pH of some other substances. Pure water has a pH of 7 while rainwater, a pH of 5.6 due to the slightly acidic carbon dioxide in the air.
Acid rain is classified as regional air pollution problem, and it causes major damages to our environment. Not only does it kill plants, since acid rain makes soil more acidic and hurts leaves of plants, which in turn lead to destruction of sensitive forests, it also affects the respiratory systems in human and other animals. In addition, it acidifies the lake water with toxic effects especially to fish, and corrodes the exposed structures, electrical relays, equipment and ornamental materials. The hydrogen ions from the acid rain dissolve the limestone (CaCO3) and thus cause damage to marble structures and galvanized iron sheets. Furthermore, air pollutants from one country can produce acid rain in another. For example, China burns large amounts of coal, and gases from this burning are carried to Japan and other countries where acid rained are formed, such as North Korea and South Korea and Japan. Let us now look at another case study.
Case Study:
Acid rain not only kills fish in rivers and lakes (since the increased acidity reduces ability of aquatic life to take in oxygen, salts and nutrients for survival), it also dirties and corrodes buildings, dissolves minerals like aluminum which is toxic to plants in the soil and also make plants lose their leaves, limiting photosynthesis and die. For example, toxic gases are blown by wind from industrialised Northwest Europe to Norway and Sweden, resulting in death of salmon and trout in Norwegian and Swedish lakes and rivers; whereas in the United States of America, the Sphinx of Egypt and the Statue of Liberty have been severely damaged by acid rain or corroded.
How is acid rain formed?
There are mainly two main pollutants, which are namely sulfur dioxide and nitrogen dioxide.
Sulfur dioxide in the air reacts with oxygen to form sulfuric acid, which is represented in the following equation:
2SO2 (g) + O2 (g) +2H2O (l) --> 2H2SO4 (aq)
The sulfuric acid dissolves in rainwater, making the rainwater slightly acidic and giving it the ability to corrode substances.
Nitrogen dioxide, on the other hand, forms nitric reaction, although undergoing a similar reaction in the air:
4NO2 (g) + O2 (g) +2H2O (l) --> 4HNO3 (aq)
Acid rain with a pH of about 4 is about 40 times more acidic than unpolluted rainwater, although only with a mere difference of 1.6 in the pH value. Some acid even have a pH of less than 1.5! This is very VERY acidic.
How to reduce the effects of acid rain?
People could burn fuels which contain little or no sulfur. This is why natural gas is preferred instead of coal or oil for power stations. For example, in Singapore, more and more public buses are changing from fuel to natural gas. Furthermore, people use catalytic converters in cars and and also reduces acidic gases in power stations. Lastly, the acids in lakes and soil could be neutralise by using slaked lime, which is also known as calcium hydroxide. Slaked lime is added to soil while some countries add calcium carbonate powder to lakes. Indeed, Norway and Sweden have successfully done this to help restore lakes and streams. However, what is worth mentioning is that, it is not suitable to add alkali to the affected areas, since we do not know when would the affected area be "neutralized". Thus, if we add in excess, the area would turn alkali instead of becoming neutralized. It is hence decided to use insoluble base, slaked lime, since even if added in excess, it would not change the pH of the place to above 7 since no reaction would take place once all the acids have reacted.
Sulfur dioxide in the air reacts with oxygen to form sulfuric acid, which is represented in the following equation:
2SO2 (g) + O2 (g) +2H2O (l) --> 2H2SO4 (aq)
The sulfuric acid dissolves in rainwater, making the rainwater slightly acidic and giving it the ability to corrode substances.
Nitrogen dioxide, on the other hand, forms nitric reaction, although undergoing a similar reaction in the air:
4NO2 (g) + O2 (g) +2H2O (l) --> 4HNO3 (aq)
Acid rain with a pH of about 4 is about 40 times more acidic than unpolluted rainwater, although only with a mere difference of 1.6 in the pH value. Some acid even have a pH of less than 1.5! This is very VERY acidic.
How to reduce the effects of acid rain?
People could burn fuels which contain little or no sulfur. This is why natural gas is preferred instead of coal or oil for power stations. For example, in Singapore, more and more public buses are changing from fuel to natural gas. Furthermore, people use catalytic converters in cars and and also reduces acidic gases in power stations. Lastly, the acids in lakes and soil could be neutralise by using slaked lime, which is also known as calcium hydroxide. Slaked lime is added to soil while some countries add calcium carbonate powder to lakes. Indeed, Norway and Sweden have successfully done this to help restore lakes and streams. However, what is worth mentioning is that, it is not suitable to add alkali to the affected areas, since we do not know when would the affected area be "neutralized". Thus, if we add in excess, the area would turn alkali instead of becoming neutralized. It is hence decided to use insoluble base, slaked lime, since even if added in excess, it would not change the pH of the place to above 7 since no reaction would take place once all the acids have reacted.
Monday, April 30, 2012
Nitrogen Oxides (2)
What are the effects of nitrogen oxides?
High concentrations of NO2 can produce an abnormally high accumulation of fluid in lung tissue. For exposures ranging from several minutes to one hour, a level of 50 – 100 ppm NO2 causes inflammation of lung tissue for a period of 6 – 8 weeks, after which time the subject normally recovers. Exposure of the subject to 150 – 200 ppm of NO2 causes bronchititis fibrosa obliterans , a conditions fatal within 3 – 5 weeks after exposure. Death generally results within 2 – 10 days after exposure to 500 ppm or more of NO2.
NO2 also causes extensive damage to plants through its secondary products such as peroxy acyl nitrate formed in smog. Exposure of plants to several parts per million of NO2 in the laboratory causes leaf spotting and break down of plant tissue. It also causes fading of dyes and inks used in some textiles. Much of the damage to materials caused by NOx, such as stress – corrosion cracking of electrical apparatus, comes from secondary nitrates and nitric acid.
How to control nitrogen oxides' emission?
It is possible to lower nitrogen oxides by carrying out the combustion in two stages, the first of which is rich in fuel and the second of which is rich in air. In this way the fuel is burned completely, but the temperature is never as high as it would be for a stoichiometric mixture. This two-stage approach is being incorporated in power plants; it has been tried in cars but with less success.
The other method that is done to reduce emissions is to remove the pollutant from the exhaust gases. In automobiles, this is done by the use of a threeway catalytic converter.
In order to deal with both NO and unburned gases the converter has two chambers in succession. In the reduction chamber, NO is reduced to N2 by hydrogen, which is generated at the surface of a rhodium catalyst by the action of water on unburned fuel molecules.
HC +H2O→H2 + CO
2NO+ 2H2 →N2 + 2H2O
In the oxidation chamber, air added, and the CO and unburned hydrocarbons are oxidised to CO2 and H2O at the surface of platinum/palladium catalyst.
2CO + O2 →2CO2
HC + 2O2 → CO2 + 2H2O
Nitrogen Oxides (1)
How are nitrogen oxides formed?
In this post, I am going to discuss two of the nitrogen oxides that are important in the study of air pollution, namely, nitrogen monoxide (NO) and nitrogen dioxide (NO2). The most abundant oxide is nitrous oxide. This is however chemically rather unreactive and is formed from the natural biological processes in the soil. Nitrogen monoxide first undergoes photochemical reaction. The formed atomic oxygen reacts with another molecule of N2O to give NO. The formed nitric oxide reacts with ozone, thereby causing ozone depletion.
The following equations show how nitrogen and oxygen in the air combine to form nitrogen monoxide:
N2O+O→2NO
The nitrogen monoxide then reacts with more oxygen to become nitrogen dioxide:
NO+O2 → 2NO2
Nitrogen monoxide is formed by the combustion of nitrogen-containing compounds (including fossil fuels). Thus all high temperature processes produce NO, which is then oxidised to NO2 in the ambient air. In the natural world, these reactions occurs in lightning and forest fires. What is nitric oxide used for in the natural world then? Actually, it is an important source of nitrogen for growing plants.
In contrast to nitric oxide, nitrogen dioxide is very reactive and significant species in the atmosphere. The principal reactions among NO,NO2, and HNO3 are indicated below:
In conclusion, nitric oxide and nitrogen dioxide are important constituents of polluted air. These oxides collectively designated as NOx, enter the atmosphere mainly from combustion of fossil fuels in both stationary and mobile sources.
In this post, I am going to discuss two of the nitrogen oxides that are important in the study of air pollution, namely, nitrogen monoxide (NO) and nitrogen dioxide (NO2). The most abundant oxide is nitrous oxide. This is however chemically rather unreactive and is formed from the natural biological processes in the soil. Nitrogen monoxide first undergoes photochemical reaction. The formed atomic oxygen reacts with another molecule of N2O to give NO. The formed nitric oxide reacts with ozone, thereby causing ozone depletion.
The following equations show how nitrogen and oxygen in the air combine to form nitrogen monoxide:
N2O+O→2NO
The nitrogen monoxide then reacts with more oxygen to become nitrogen dioxide:
NO+O2 → 2NO2
Nitrogen monoxide is formed by the combustion of nitrogen-containing compounds (including fossil fuels). Thus all high temperature processes produce NO, which is then oxidised to NO2 in the ambient air. In the natural world, these reactions occurs in lightning and forest fires. What is nitric oxide used for in the natural world then? Actually, it is an important source of nitrogen for growing plants.
In contrast to nitric oxide, nitrogen dioxide is very reactive and significant species in the atmosphere. The principal reactions among NO,NO2, and HNO3 are indicated below:
In conclusion, nitric oxide and nitrogen dioxide are important constituents of polluted air. These oxides collectively designated as NOx, enter the atmosphere mainly from combustion of fossil fuels in both stationary and mobile sources.
Sulfur Dioxide (2)
What produces sulfur dioxide?
The two natural sources of SO2 are volcanic eruptions and sulfur containing geothermal sources like geysers and hot springs. Emission rates of sulfur dioxide from an active volcano range from 20 tonnes per day to 10 million tonnes per day according to the style of volcanic activity and type and volume of magma involved.
For example, the large explosive eruption of Mount Pinatubo on 15 June 1991 expelled 3-5 km3 of dacite magma and injected about 20 million metric tons of sulfur dioxide into the stratosphere. Together with the increased stratospheric chlorine levels from human-made chlorofluorocarbon (CFC) pollution, the sulfate aerosols dispersed from the volcanic eruption destroyed ozone and led to some of the lowest ozone levels ever observed in the atmosphere.
At Kilauea Volcano, the recent effusive eruption of about 0.0005km3/day (500,000 m3) of basalt magma releases about 2,000 tonnes of sulfur dioxide into the lower troposphere. Downwind from the vent, acid rain and air pollution is a persistent health problem in Hawaii when the volcano is erupting. These large explosive eruptions would also inject a tremendous volume of sulfur aerosols into the stratosphere can lead to lower surface temperatures and promote depletion of the Earth's ozone layer.
There are also industrial production of SO2, including the following few.
Burning of fuels
Large amounts of coal and petroleum are burnt around the world in power stations to generate electricity and in industries to provide energy. Both types of fuels contain sulfur as an impurity, although coal contains a higher concentration of it. Diesel fuel, which is used in vehicles, also contains a little sulfur. It is estimated that 70% of worldwide atmospheric SO2 comes from power stations.
When these fuels are burnt, the sulfur undergoes oxidization to become sulfur dioxide. The chemical equation is as follows:
S (s) + O2 (g) --> SO2 (g)
*Sulfur is from fuel, while oxygen is from the air.
Oil refining:
Sulfur and hydrogen sulfide are constituents of crude oil and H2S is released as a gas during catalytic cracking. Since H2S is considerably more toxic than SO2 it is burned to produce SO2 before release to the ambient air.
Pulp and paper manufacture:
The sulfite process for wood pulping uses hot H2SO3 and thus emits SO2 in air. The kraft pulping process produces H2S, which is then burned to produce SO2.
The two natural sources of SO2 are volcanic eruptions and sulfur containing geothermal sources like geysers and hot springs. Emission rates of sulfur dioxide from an active volcano range from 20 tonnes per day to 10 million tonnes per day according to the style of volcanic activity and type and volume of magma involved.
For example, the large explosive eruption of Mount Pinatubo on 15 June 1991 expelled 3-5 km3 of dacite magma and injected about 20 million metric tons of sulfur dioxide into the stratosphere. Together with the increased stratospheric chlorine levels from human-made chlorofluorocarbon (CFC) pollution, the sulfate aerosols dispersed from the volcanic eruption destroyed ozone and led to some of the lowest ozone levels ever observed in the atmosphere.
There are also industrial production of SO2, including the following few.
Burning of fuels
When these fuels are burnt, the sulfur undergoes oxidization to become sulfur dioxide. The chemical equation is as follows:
S (s) + O2 (g) --> SO2 (g)
*Sulfur is from fuel, while oxygen is from the air.
Oil refining:
Sulfur and hydrogen sulfide are constituents of crude oil and H2S is released as a gas during catalytic cracking. Since H2S is considerably more toxic than SO2 it is burned to produce SO2 before release to the ambient air.
Pulp and paper manufacture:
The sulfite process for wood pulping uses hot H2SO3 and thus emits SO2 in air. The kraft pulping process produces H2S, which is then burned to produce SO2.
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