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The Atmosphere

The Atmosphere. Chemical Storylines. A1 What’s in the air?. It is a relatively thin layer of gas (about 100km thick) The two most chemically important regions are the STRATOSPHERE and the TROPOSPHERE 90% of all molecules in the atmosphere are in the bottom 15km ( the troposphere ).

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The Atmosphere

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  1. TheAtmosphere Chemical Storylines

  2. A1 What’s in the air? • It is a relatively thin layer of gas (about 100km thick) • The two most chemically important regions are the STRATOSPHERE and the TROPOSPHERE • 90% of all molecules in the atmosphere are in the bottom 15km (the troposphere)

  3. Mixing is easy in the troposphere due to convection currents • Mixing is much more difficult in the stratosphere due to the reverse temperature gradient • Horizontal mixing in the stratosphere, however, is rapid • Concentrations of some substances are very small so they are measured in parts per million by volume (ppm) • Assignment 1 • Some of the gases in the atmosphere are produced by human activity, these will eventually mix by diffusion. • Atmospheric pollution is therefore a global problem; in this topic we will look at two of the biggest problems; • Depletion of the ozone layer in the stratosphere • Global warming and greenhouse gases in the troposphere

  4. A2 Screening the Sun • The sun radiates a wide spectrum of energy. • Part of this corresponds to the energy required to break bonds. • This includes molecules such as DNA. • This can damage genes and lead to skin cancer and also damage proteins of connective tissue leading to wrinkles. • CI 6.2 ”Radiation and Matter” • The most damaging region is the ultra violet region – high frequency and high energy. • Some chemicals absorb this radiation e.g. glass and manufactured chemicals - sunscreens. • The best sunscreen of all is the atmosphere itself. • A2.1 “What substances can act as sunscreens?” • A2.2 “Investigating sunscreens”

  5. Atmospheric gases in the stratosphere absorb ultra violet (uv) radiation very well (“strongly”). • Ozone (O3) is particularly good at this. • In the stratosphere the uv radiation breaks covalent bonds in molecules to give fragments called radicals • Above the stratosphere the radiation is of such high energy that it can remove electrons from molecules, producing ions • This area is called the ionosphere. • Assignment 2 • A2.3 “Effect of atmosphere on Sun’s radiation”

  6. A3 Ozone: A vital sunscreen • Ozone is only present in the atmosphere in tiny amounts, mixed amongst other gases • If it was all collected together on the earth’s surface it would form a layer 3mm thick! • Protects us in stratosphere but is harmful in troposphere (see DF) • It is a very reactive gas and a powerful oxidising agent (causes oxidation, gets reduced) • If ozone is so reactive, why hasn’t it all run out? …there must be some reactions making it as well….. • CI6.3 “Radiation and Radicals”

  7. How is ozone formed? • Step 1: O2 + hν O + O dioxygen molecule oxygen atoms (Bond Energy = +498 kJmol-1) (RADICALS) • This process is called PHOTODISSOCIATION. • This occurs when molecules in the stratosphere absorb uv radiation of the correct frequency (hv) • It is also caused by electrical discharges or in photochemical smog (see DF) • The O atoms (radicals) produced are very very reactive. • There are three possibilities of what they will do next……

  8. O + O2 O3 ΔH = -100 kJmol-1 • This is how ozone is made in the stratosphere • Other reactions which the O radicals might do are… O + O O2 ΔH = -498 kJmol-1 O + O3 2O2 ΔH = -390 kJmol-1 • When ozone ABSORBS radiation (in the region 10.1 x 1014 to 14.0 x 1014 Hz) we have another photodissociation; O3 + hv O2 + O • This is the vital reaction that protects us from the harmful u.v. radiation • Assignments 3 + 4 • A3.1 “Photodissociation of bromine ” • A3.2 “Bromine and hexane”

  9. Ozone - here today and gone tomorrow • These reactions would eventually reach a STEADY STATE where ozone is made as quickly as it is used up. • Chemists can use this, plus knowledge about rates of the reactions, to calculate what the conc. of ozone in the atmosphere should be • CI10.1 “Rate of reaction” • CI10.2 “Temperature and rate” • A3.3 “How do conc and temp affect rate?” • Measured amounts are a lot lower than the calculated amounts. • Ozone must be being used up by reactions with some of the other radicals in the stratosphere….

  10. What are these radicals? Chlorine (Cl) and Bromine (Br) atoms • Small amounts of chloromethane (CH3Cl) and bromomethane (CH3Br) are released by oceans and burning vegetation • Most react in the troposphere but some reaches the stratosphere • Once in the stratosphere, their molecules are split up by solar radiation… CH3Cl + hv CH3● + Cl ● (both species have an unpaired electron; they are radicals) • Similar reactions will occur for other chlorine compounds produced in human activities such as CFCs

  11. Chlorine reacts with ozone as follows… Cl + O3 ClO + O2 (radical) (new radical) then.. ClO + O Cl + O2 (regenerated) Overall: Cl + O3 + ClO + O ClO + O2 + Cl + O2 O3 + O O2 + O2 This is a catalytic cycle with Cl acting as the catalyst In this way a single Cl atom can remove about 1 million ozone molecules

  12. The reaction between Cl and ozone would not matter if it was slower than the reaction of O3 with O • However, the reaction between Cl and O3 is 1500 times faster • Cl radicals are present in much lower concentrations than O atoms so this should reduce the rate… • …nevertheless, they still have a very big impact on ozone destruction • To make things worse, they are regenerated (catalytic cycle) • As a result, a single Cl atom can remove about 1 million ozone molecules

  13. Other radicals can also destroy ozone in this catalytic cycle… Hydroxyl radicals (HO●) • Water in the stratosphere… H2O + O 2HO● Nitrogen monoxide (NO) • This is made in car engines from N2 and also from N2O released by bacteria in the soil and oceans • NO and NO2 are radicals already but they are both unusually stable • CI10.6 • A3.4

  14. In general… X + O3 XO + O2 (radical) (new radical) then.. XO + O X + O2 (regenerated) Catalytic cycle with X acting as the catalyst Overall: X + O3 + XO + O XO + O2 + X + O2 O3 + O O2 + O2 • Ass 6 • A3.5

  15. A4 The CFC story CFCs: very handy compounds In 1930 Thomas Midgley inhaled CCl2F2 (dichlorofluoromethane) and used it to blow out a candle. This demonstrated that it was neither toxic nor flammable It was invented to replace ammonia as a refrigerant (toxic and smelly) CCl2F2 is an example of a chlorofluorocarbon (CFC) They are very unreactive, have low flammabilities and toxicities and have a variety of different boiling points.

  16. Their main uses have been as; refrigerants and in air conditioning propellants in aerosols, blowing agents in expanded plastics such as polystyrene cleaning solvents The problem with CFCs is that they are now known to be too unreactive… They have plenty of time to reach the stratosphere. Scientists have now shown that, once in the stratosphere, the uv radiation causes them to photodissociate to form Cl radicals (see next page) These then cause ozone depletion The chemical industry has had the job of finding suitable replacements.

  17. The impact of CFCs on ozone was first predicted in 1974 • The difficulty was that the predictions were long term ones… • …and they could only be tested in the atmosphere itself • In 1985 studies of the atmosphere 18km above the Antarctic discovered a direct link between concentrations of ClO and ozone… … ozone concs begin to fall sharply. As ClO concs begin to rise sharply… Travelling towards S pole

  18. Chlorine reservoirs Other molecules in the stratosphere, react with chlorine CH4 + Cl• CH3• + HCl NO2 + ClO ClONO2 HCl and ClONO2 (chlorine nitrate) are chlorine reservoir molecules because they ‘store’ the chlorine Some of these will be carried down into the troposphere, Most however, stay in the stratosphere…

  19. Why does the hole develop over the poles? Decreases in ozone concentration are most dramatic in the Antarctic spring. This is due to two changes occurring during the winter… 1. Very low temperatures (below -80ºC) 2. A vortex of air forms The low temps cause clouds to form made of solid particles of ice rich in nitric acid polar stratospheric clouds (see below) The vortex isolates the air from the rest of the atmosphere HCl and ClONO2 (chlorine reservoir molecules)are adsorbed onto the particles in the clouds and react… ClONO2 + HCl HNO3 + Cl2

  20. The HNO3 stays dissolved in the ice Cl2 is released as a gas but stays trapped in the vortex When sunlight returns in spring, the vortex breaks up Cl2 molecules undergo photolysis to form Cl atoms These react rapidly with the ozone (see A3) Ass 7

  21. A5 what is the state of the ozone layer now? Ozone is a vital sunscreen. It absorbs harmful u.v. radiation. Ozone depletion leads to; Skin cancer Eye cataracts Death of plankton Effects on food chains Changes in temperature and weather 1987 Montreal Protocol agreed restrictions on CFCs and similar bromine compounds (halons) Since then it has been amended based on further research

  22. CFC replacements Short term: hydrochlorofluorocarbons (HCFCs) e.g. CHClF2 C-H bond is broken in troposphere (often by OH radicals) However, some still gets to stratosphere and forms Cl radicals Will be phased out by 2020 in developed countries and 2040 in rest of world Longer term: hydrofluorocarbons (HFCs) e.g. CH3CF3 No ozone depleting effect even if reaches stratosphere No perfect solution as both are greenhouse gases! Predictions are that ozone layer may just be beginning to recover but won’t be completely recovered until 2060-2070

  23. A6 The Greenhouse Effect Now turn our attention to the bottom 15km of the atmosphere… …the troposphere Here methane (CH4) is less helpful. It is made by methanogenic bacteria through anaerobic respiration (in the absence of oxygen) It is therefore made wherever carbohydrate breaks down (or decays) anaerobically… Marshes and compost heaps Rice paddy fields Biogas digesters Digestive tracts (a cow releases 0.5 m3 of methane per day !!) Methane’s concentration in the troposphere is now 2.5 times what it was in pre-industrial times Methane is good in the stratosphere but bad in the troposphere To see why, we need to look at how the sun keeps the Earth warm…

  24. Radiation in, radiation out Hot objects emit electromagnetic radiation The sun (6000 K) radiates i.r., visible and u.v. light The Earth is much less hot (about 285 K) It still emits radiation but only lower energy i.r. radiation. Ass 8 The end result is that a steady state is reached and the Earth’s temperature remains constant…

  25. It is a delicate balance that can be easily disturbed if the amounts of certain gases in the atmosphere change. Methane is a greenhouse gas… A greenhouse gas will absorb i.r. radiation but not u.v. or visible radiation. They will let the sun’s radiation IN, but will absorb some of the Earth’s i.r. radiation that would otherwise go into space. As a result the atmosphere gets warmer which makes the Earth warmer. This is the greenhouse effect. Once the energy is absorbed, two things can happen; Some i.r. is re-emittedby the molecules This occurs in all directions; some towards Earth, some into space i.r. increases the vibrational energy of the molecules Bonds vibrate more This vibration can be transferred to other molecules in the air (e.g. O2 and N2) by collisions They move faster, so have more kinetic energy So temperature of the air is raised

  26. Do other gases have a greenhouse effect? Carbon dioxide and several other substances in the troposphere are also greenhouse gases Some have a greater effect than others depending on… How good it is at absorbing i.r. Its concentration Its lifetime in the troposphere One way of comparing these gases is by determining their global warming potential This compares everything to CO2 which is given a value of 1 A6.1 Ass 9 The greenhouse effect is good for you The greenhouse effect keeps the average temperature high enough to support life. Moon – no atmosphere – v. hot days, v. cold nights Venus – 90% CO2– huge greenhouse effect (about 450ºC)

  27. A7 What happens if concentrations of greenhouse gases increase? 1880-1940 average temperature rose by 0.25°C Then 1940-1970 fell by 0.2°C So why are we worried? During 1970s CO2 levels rose significantly Very difficult to make predictions due to huge number of variable factors… Concentrations of gases All possible chemical reactions occurring and their rates Changes in solar radiation Changes in human activities Interactions between the atmosphere and the oceans Feed all this data into powerful computers to generate models of how the climate might change

  28. In 1988 the Intergovernmental panel on Climate Change (IPCC) was set up This led, in 1997 to the Kyoto Protocol In this 169 countries agreed to proposed limits on the emissions of greenhouse gases It came into effect in 2004 Records suggest that the 11 years from 1995-2006 were among the 12 warmest years on record Using modelling studies, the IPCC have said it is 95% certain that the global pattern of warming over the last 50 years cannot be explained without including warming due to human emissions

  29. A8 Keeping the window open The two most significant greenhouse gases are CO2 and H2O, mainly because they are so abundant. Water, however, is different from other greenhouse gases… Usually it’s a liquid and so isn’t a problem but, if the Earth gets warmer… we will get more water vapour… …so greater greenhouse effect - BAD Water as droplets in clouds will block out the sun… …GOOD This makes it difficult to predict what will happen

  30. CO2 and H2O absorb in two ‘bands’ of the Earth’s radiation spectrum Between these two bands is a window where 70% of the Earth’s radiation can escape (as it isn’t absorbed) Gases made by human activity can increase the natural greenhouse effect in two ways: Increasing amounts of gases already present e.g. CO2 from burning fossil fuels. Adding other gases not naturally present e.g. CFCs These absorb radiation in the vital ‘window’ region They have a very large global warming potential and so small amounts have a big effect The role of other greenhouse gases

  31. A9 Focus on carbon dioxide At least half the expected increase in the greenhouse effect due to human activities is likely to be caused by carbon dioxide. We must therefore control the amount of CO2 we produce. CO2 in the atmosphere is about 0.038% We need to be able to detect tiny changes to this… 1) Qualitative(to show that it is present – of no use here) Turns lime water cloudy 2) Quantitative(to show how much is there) Infra red spectroscopy – the more CO2 present, the more i.r. gets absorbed (that’s the whole problem of the greenhouse effect!) Calculations suggest that the increase in CO2 in the atmosphere should be twice what it actually is. Not all the CO2 produced is going into the atmosphere. Where is it going?

  32. Oceans soak up carbon dioxide CO2 is fairly soluble in water. Large amounts of CO2 (g) dissolve in the oceans CO2(g) +aq CO2(aq) This is a REVERSIBLE REACTION (it can occur in both directions) CI 7.1 “Equilibria” Phytoplankton use up most of the CO2 which goes into the sea The concentration of CO2(aq) is therefore kept small and so CO2(g) is encouraged to dissolve

  33. A very small proportion of the CO2(aq) (about 0.4%) reacts with the water… CO2(aq) + H2O(l) HCO3-(aq) + H+(aq) H+ is the species which causes solutions to be acidic. But, reaction is in equilibrium and only 0.4% of the CO2 reacts A solution of CO2 will therefore be only weakly acidic A9.1 Chemical equilibria pH is related to the concentration of H+ ions We can therefore link pH to the amount of CO2 present in solution… …and then relate this to the amount of CO2 present in the air Over last 20 years pH has gone down by about 0.04 pH units C.I. 5.2 A 9.2 hydrogencarbonate ion hydrogen ion

  34. Coping with carbon Oceans do a good job but CO2 is still rising The steep rise in the 20th century is unprecedented In 1750s CO2 conc was 280 ppm… …it is now 383 ppm… …if we don’t take drastic action it may have doubled to 560ppm within your lifetime Climate change models predict this will result in a temperature rise of between 2ºC and 4.5ºC

  35. The link between CO2 and global warming is now well supported by scientific evidence. It is uncertain how these changes will affect the planet However, the IPCC has warned of; Reduction of snow cover Thawing of arctic permafrost Melting of polar sea ice Rising sea levels Increases in extreme weathers such as heat waves More rain in northern latitudes Less rain in tropical regions More intense typhoons and hurricanes A9.3 A10.1 A10.2

  36. This will cause the sea level to rise due to the melting of ice. Some people believe we are heading for disaster from accelerating global warming. Others believe that the Earth will develop ways of compensating for any serious departure from the equilibrium

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