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- 1. may contains Air pollutants (refer to table) Air pollution is the condition in which air contains a high concentration of chemicals that may harm living things or damage non-living things. Clean, dry air The Atmosphere Carbon cycle (Box A) Carbon cycle is the mechanism that maintains the level of carbon dioxide in the atmosphere. The Atmosphere and Environment Pollutants Sources (how they are produced) Harmful effects How to reduce the effect? Carbon monoxide (CO) Incomplete combustion of fossil fuels in power plants, factories or combustion engines of vehicles due to lack of oxygen • Reacts irreversibly with haemoglobin in blood to form carboxyhaemoglobin which reduces the ability of haemoglobin to transport oxygen to rest of the body • Results in breathing difficulties, fatigue and even death. • Catalytic converters (Box E) Sulfur dioxide (SO2) • Combustion of fossil fuels which contain sulfur impurities in motor vehicles, power stations and factories • S (s) + O2 (g) → SO2 (g) • Volcanic eruptions • Irritate the eyes and lungs and cause breathing difficulties • Reacts with water in atmosphere to form acid rain (Box B) which corrodes metal and limestone structures and harms aquatic life and plants. • Liming (adding calcium carbonate to neutralise and hence restore lakes and rivers polluted by acid rain) – expensive and effects are temporary. • Flue gas desulfurisation (Box F) Oxides of nitrogen (NO and NO2) • Internal combustion of engines • Lightning activity *temperature must be high for oxides of nitrogen to be formed. N2 (g) + O2(g) → 2NO (g) 2NO(g) + O2 (g) → 2NO2 (g) • Irritate the eyes and lungs and cause breathing difficulties • Reacts with water in atmosphere to form acid rain (Box B) which corrodes metal and limestone structures and harms aquatic life and plants. • Leaches nutrients from the soil and causes plants to wither and die. • Liming (adding calcium carbonate to neutralise and hence restore lakes and rivers polluted by acid rain) – expensive and effects are temporary. • Catalytic converters (Box E) Unburnt hydrocarbons Released in vehicle exhaust fumes and chemical plants • Health: cause cancer • Environment: react with nitrogen oxides to form ozone which can damage crops • Catalytic converters (Box E) Methane Colourless and odourless gas Decay of plant and animal matter • It is a greenhouse gas which causes global warming (Box C) and thus cause melting of large quantities of ice in north and south poles, resulting in floods that destroys crops and buildings. Ozone (O3) Pale blue pungent gas • Health: forms photochemical smog which irritates the eyes and lungs and causes breathing difficulties • Environment: damages crops, reduce visibility for traffic Chlorofluorocarbons (CFCs) – compound that contains carbon, fluorine and chlorine. Propellants in aerosols, as well as coolants in refrigerators and air conditioners, release CFCs into the atmosphere • Cause ozone depletion (Box D) which results in increases UV radiation on earth. This causes skin cancer, genetic mutations, eye damage and death to marine life. • Ban the use of CFCs. Note: Even if the use of CFCs is totally stopped at once, the depletion of ozone layer will continue for many years due to the CFCs already present in the atmosphere. • Use alternative materials such as HFCs HFCs (hydrofluorocarbons) do not have C-Cl bonds which can be broken easily by UV, so HFCs do not release Cl atoms to break down ozone in atmosphere. Composition of clean dry air (dry means no water or water vapour) Gas Composition by volume / % Nitrogen (N2) 78 Oxygen (O2) 21 Other gases 1 • Noble gas (mostly 0.97 argon) • Carbon dioxide 0.03 How is air separated into its constituent gases? 1 2
- 2. Acid rain (Box B) Acid rain is formed when acidic air pollutants such as SO2 and NO2 react with water in the atmosphere. pH of acid rain is pH 4 or less. SO2 (g) + H2O (l) → H2SO3 (aq) (sulfurous acid) In the presence of O2 in air, sulfurous acid is oxidised to sulfuric acid (H2SO4). Oxides of nitrogen also cause acid rain. In the presence of oxygen and water, NO2 is converted to nitric acid. 4NO2 (g) + 2H2O(l) + O2 (g) → 4HNO3 (aq) Note that pH of unpolluted rainwater is about 5.7. This is because CO2 (an acidic oxide) in the air dissolves in rainwater to form carbonic acid (H2CO3) which is a weak acid. These acids cause the acid rain. Global warming (Box C) What is greenhouse effect? Greenhouse gases such as CO2, CH4 and water vapour trap some of the infrared radiation on earth, thus heat energy is retained in the atmosphere. This produces a warming effect called the greenhouse effect. It is crucial for maintaining the proper temperature needed to sustain life on Earth. Without these greenhouse gases, Earth’s surface will be around -40o C and be permanently covered with ice. However, activities such as burning of fossil fuels are causing more CO2 (greenhouse gas) to be produced. The excess CO2 is added to the atmosphere at a higher rate than photosynthesis can remove (refer to carbon cycle). This then lead to global warming. (Global warming is the increase in the Earth’s average temperature due to the build-up of greenhouse gases in the atmosphere). Carbon cycle (Box A) Carbon cycle is the mechanism that maintains the level of carbon dioxide in the atmosphere. (rate of removal of CO2 is balanced by the rate of CO2 production) Removal of carbon dioxide --- photosynthesis and ocean uptake (ocean uptakes serves as massive sinks that trap CO2 which is later used by marine plants.) Production of carbon dioxide --- respiration, combustion and decay and bacterial decomposition Respiration converts glucose in the food we eat into CO2 and water and energy. C6H12O6 (aq) + 6O2 (g) → 6CO2 (g) + 6H2O (l) + Energy Combustion of fuel like methane releases CO2. CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (l) + heat energy Green plants convert CO2 and H2O into glucose and oxygen in sunlight. 6CO2 (g) + 6H2O (l) → C6H12O6 (aq) + 6O2 (g)
- 3. Depletion of ozone (Box D) Ozone layer is important to us. It acts as a giant sunscreen that filters some of the harmful UV radiation from the sun. However, the use of chlorofluorocarbons (CFCs) found in propellants in aerosols, as well as coolants in refrigerators and air conditioners, causes the depletion of ozone layer. This will cause an increase in amount of UV radiation that can cause skin cancer, eye damage or death to marine life. In the presence of UV light, CFCs decompose to form chlorine atoms. CFC UV chlorine atoms Chlorine atoms react with ozone molecules to form chlorine oxide and oxygen. Cl + O3 → ClO + O2 Catalytic converters (Box E) • A catalytic converter helps to remove air pollutants in vehicle exhaust gases. • When the hot exhaust gases pass over the platinum and rhodium catalysts, the harmful pollutants undergo redox reactions and are converted into harmless substances. Carbon monoxide is oxidised to carbon dioxide while nitric oxide is reduced to nitrogen: 2CO(g) + O2(g) → 2CO2(g) 2NO(g) + 2CO(g) → N2(g) + 2CO2(g) Unburnt hydrocarbons are oxidised to carbon dioxide and water: Example: 2C8H18(l) + 25O2(g) → 16CO2(g) + 18H2O(g) Note: Other ways to reduce air pollution caused by motor vehicles. • • • Use lightweight alloys to make car bodies to improve fuel efficiency. Switch to electric or hybrid electric vehicles to reduce exhaust gas emissions. Use alternative fuels such as hydrogen fuel, where products of combustion are harmless.
- 4. Flue gas desulfurisation (Box F) Two ways to minimise the effects of sulfur dioxide pollution: Way 1: • Remove sulfur from fossil fuels before burning. – However, this method is too expensive and difficult. Way 2: • • Remove sulfur dioxide from the waste gases (flue gases) formed when the fossil fuels undergo combustion (cheaper). The process of removing sulfur dioxide from flue gases is called desulfurisation. SO2 reacts with calcium carbonate to form calcium sulfite (CaCO3) and CO2. CaCO3(s) + SO2(g) → CaSO3(s) + CO2(g) The calcium sulfite than oxidized to CaSO4 by O2 in the air. 2CaSO3(s) + O2(g) → 2CaSO4(s) Alternatively, we can also use calcium oxide for desulfurisation. CaO(s) + SO2 (g) → CaSO3 (s)