Understanding Environmental Issues: Acid Rain & Ozone Layer Depletion

1. Week 10, Lesson 2 Chapter 19 – Environmental Issues

2. Acid Rain • Natural rainfall is normally slightly acidic because water dissolves carbon dioxide from the air to produce carbonic acid (H2CO3). • During the last 20 years, many countries, have been experiencing rain that is more acidic than normal. • This is known as acid rain. • Acid rain occurs when acidic oxides of sulfur and nitrogen dissolve in rain water to form acids.

3. Oxides of Sulfur • It is the sulfur oxides produced by human activity that cause environmental problems. • This is because of their high concentration in relation to the small area they are coming from. • Sulfur is found in fossil fuels, when these fuels are burnt, sulfur dioxide is produced. • Industrially, the majority of sulfur oxide emissions are due to the burning of fossil fuels, particularly coal.

4. Reactions of Sulfur Oxides • S(s) + O2(g)  SO2(g) In the presence of sunlight, sulfur dioxide is further oxidised to sulfut trioxide: • 2SO2(g) + O2(g)  2SO3(g) These gases react readily with rain water to form sulfurous acid and sulfuric acid: • SO2(g) + H2O(l)  H2SO3(aq) H+(aq) + HSO3-(aq) • SO3(g) + H2O(l)  H2SO4(aq) H+(aq) + HSO4-(aq)

5. Oxides of Nitrogen • Oxides of nitrogen, NO2 and NO, are known as NOx. • They are formed naturally by volcanoes, lightning flashes and bacterial action in the soil. • However, those produced by human activity and concentrated over small area lead to environmental problems. • NO forms whenever nitrogen and oxygen are mixed at very high temperatures. • High temperatures are generated in internal combustion and jet engines. • This is the main source of NO.

6. Reactions of Nitrous Oxides So at high temperatures: • N2(g) + O2(g)  2NO(g) At lower temperatures, when released into the atmosphere, NO reacts readily with oxygen to form the brown poisonous gas nitrogen(IV) oxide (NO2): • 2NO(g) + O2(g)  2NO2(g) NO2 is an acidic oxide that dissolves readily in rain water to form a mixture of nitrous acid and nitric acid: • 2NO2(g) + H2O(l)  H+(aq) + NO3-(aq) + HNO2(aq)

7. Effects of Acid Rain • Building materials made of marble or limestone (mostly calcium carbonate), for example, react with acid rain, causing them to slowly wear away: CaCO3(s) + H2SO4(aq)  CaSO4(s) + CO2(g) + H2O(l) • CaSO4(s) is slightly water soluble so some of it will wash away. • The remain CaSO4 is bulky and changes the appearance of the stonework. • Acid rain also increases the rate of corrosion of steel structures.

8. Effects of Acid Rain cont… • Acid rain increases soil acidity and stunts plant growth. • In waterways, aquatic life is particularly sensitive to increased levels of acidity. • In humans, acid rain has been linked to irritation of mucous membranes and to respiratory illness, particularly in the old and very young.

9. Efforts to Reduce Acid Rain • Using low sulfur fuels • Removing sulfur from coal before combustion • Removing SO2 from flue gases of power plants • Improving the design of combustion engines to reduce NOx emissions.

10. Depletion of the Ozone Layer • Scientists first became concerned about possible damage to the ozone layer in 1971, with proposals to develop supersonic aircraft, which would fly at very high altitudes. • It was suggested that the jet engines would produce NO from the reaction between nitrogen and oxygen in the air drawn into the engines. • The nitrogen(II) oxide produced in the reaction in the stratosphere could then act as a catalyst to break down the ozone. NO(g) + O3(g)  NO2(g) + O2(g) NO2(g) + O(g)  NO(g) + O2(g) • The net effect of these reactions is: O3(g) + O(g)  2O2(g) • The nitrogen(II) oxide is then free for further reactions with ozone.

11. Depletion of the Ozone Layer cont… • CFC’s are long lived because they are unreactive in the lower atmosphere. • Eventually they move up into the stratosphere, where they can be broken down by ultraviolet light, releasing free halogen atoms, such as chlorine. • These atoms have a destructive effect on ozone, similar to nitrogen oxide. • For example: CCl2F2(g)  CClF2(g) + Cl(g) • Then: Cl(g) + O3(g)  ClO(g) + O2(g) • And: ClO(g) + O(g)  O2(g) + Cl(g) • The net result is: O3(g) + O(g)  2O2(g)

12. Alternatives to Ozone-Depleting Chemicals • Individual government regulations have been designed to reduce the production of ozone-depleting chemicals and to find alternatives. • For example, in many countries aerosol cans can be CFC’s as a propellant. • Alternative chemicals are also available for the production of foam plastics. • The CFC’s used in refrigerators and air-conditioning systems can be recycled. • However, it is thought that an 85% reduction in CFC and halon production worldwide would be needed just to stabilise the ozone depletion at current levels.

13. Photochemical Smog • Photochemical smog is visible as brown haze, particularly on still sunny days. • Nitrogen oxides are important in the formation of photochemical smog. • It is a collection of gases, including ozone, some of which can cause severe health problems. • It is called photochemical smog because it forms from the action of sunlight on a mixture of nitrogen oxides, unburnt hydrocarbons and oxygen.

14. Waste Production by Motor Vehicles • When fossil fuels are burnt in an efficient engine and a large supply of air, the main wastes produced are carbon dioxide and water. • Because fossil fuels contain traces of sulfur, sulfur dioxide is also produced during this combustion process and released into the atmosphere. • When the supply of oxygen is insufficient combustion of fuel is incomplete. • There may not be enough oxygen for all of the fuel to form carbon dioxide, some of the fuel may form carbon monoxide instead. • Some of the hydrocarbons in gasoline escape in the exhaust gases without burning at all.

15. Carbon Monoxide • Carbon monoxide is a highly poisonous gas. • It combines readily with haemoglobin, the oxygen carrier in blood. • When attached to carbon monoxide, haemoglobin cannot transport oxygen around the body, which leads to oxygen starvation of tissues. • In concentrations of 10ppm CO can cause drowsiness, dizzines and headaches in some people. • At 200ppm, it can lead to death. • Average CO in cities is 7ppm, at some busy intersections it is 120ppm.

16. Oxides of Nitrogen • Oxides of nitrogen are also released by motor vehicles. • These are not produced by the combustion of the fuel itself. • A side reaction occurs because one of the important features of this type of engine is that the gases in its cylinders reach temperatures as high as 2000 degrees. • At these temperatures, some of the nitrogen and oxygen gases in the air entering the combustion chamber combine to produce nitrogen(II) oxide.

17. Oxides of Nitrogen cont… • As the combustion products are released into the atmosphere, they experience a sudden temperature drop. • At the lower temperature, some of the nitrogen(II) oxide reacts further with oxygen to produce NO2.

18. Exhaust Fumes • In summary, the exhaust gases from a car engine include: • Unreacted nitrogen gas • Carbon Dioxide, Water and Sulfur Dioxide gases from complete burning of fuel • Carbon Monoxide and unburnt hydrocarbons from the incomplete burning of fuel • Nitrogen(II) oxide from the high temperature reaction between nitrogen and oxygen in the combustion chamber of the engine.

19. Formation of Photochemical Smog • The formation of photochemical smog tends to be favoured by windless sunny days and when temperature inversions occur. • Normally warm air near the Earth’s surface will rise and carry any smog with it, dispersing it into the atmosphere. • Temperature inversions begin on calm, clear nights with the Earth’s surface cools rapidly as heat radiates away from it. • The air near the ground becomes colder than the air higher up and this denser, colder layer becomes trapped by warmer air above it. • Any smog formed during the day in this bottom layer is also trapped and this gives more time for the reactions between the sunlight and the gases to produce smog to increase the concentration of the pollutants.

20. Enhanced Greenhouse Effect • Greenhouse gases have enabled the Earth to maintain its average temperature over time because of the balance created between the radiation received from the Sun and the heat radiated back into space. • The greenhouse effect is not the problem, the enhanced greenhouse effect is. • This describes an imbalance that has arisen with the levels of greenhouse gases in the atmosphere, in particular carbon dioxide, methane and CFC’s increasing too quickly. • This increase has upset the balance and has caused global warming to occur too rapidly for living things to adjust.

21. Causes of the Enhanced Greenhouse Effect • Carbon Dioxide: since the late 18th and early 19th centuries the concentration has risen from 274ppm to 365ppm. It is increasing by about 0.5% each year. • Methane: the concentration is rising 0.4% per year. • Chlorofluorocarbons and other gases: the concentration of CFC’s is increasing 5-7% each year.

22. Consequences of Global Warming • Earth’s mean temp has risen 0.6 degrees since the 1890’s. • Warming seems to be continuing at about 0.3% per decade, however, this rate may increase if no action is taken. • Scientists predict that the temperature could rise 2.5-5 degrees in the next 30 years. Consequence of this could include: • Rising Sea Levels • Climatic Changes in different parts of the World • Ecological Changes

23. Planning for the Future • There is much discussion in a number of countries about ways of slowing down the increase of greenhouse gases. • These include: • Reducing the consumption of fossil fuel by conserving energy measures and developing renewable energy sources • Slowing the rate at which forests are being cleared, and planting more forests, which would also slow the carbon dioxide levels. • Phasing out the use of CFC’s.

24. Kyoto Protocol • The protocol was negotiated in Kyoto, Japan in 1997 and is an international agreement designed to limit greenhouse gas emissions. • By signing the protocol, countries agreed to continue to develop and refine rules and conditions laid down in the agreement but did not consent to be bound by it. • By 2001 the protocol was finalised. • Countries were asked to ratify or confirm their intentions to abide by the agreement once it took force in 2005. • Australia acknowledged many positive elements within the protocol, however, did not ratify it for a number of reasons, including the lack of a clear pathway for action by developing countries.