The Atmosphere

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TheAtmosphere

• Chemical Storylines

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A1 Whats 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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• 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

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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

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• 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 Suns radiation

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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 earths
  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 hasnt it all run
  out?
• there must be some reactions making it as
  well..
• CI6.3 Radiation and Radicals

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How is ozone formed?

• Step 1
• O2 h? 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

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• O O2 O3 ?H -100 kJmol-1
• This is how ozone is made in the stratosphere
• Other reactions which the O radicals might do
  are
• O O O2 ?H -498 kJmol-1
• O O3 2O2 ?H -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

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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.

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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

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• 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
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• 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

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• 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

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• 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

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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.

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• 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.

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• 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
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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

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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

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• 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

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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

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• 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 wont be completely
  recovered until 2060-2070

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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 !!)
• Methanes 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

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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 Earths temperature remains constant

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• 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 suns radiation IN, but will
  absorb some of the Earths 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-emitted by 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

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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)

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A7 What happens if concentrations of greenhouse
gases increase?

• 1880-1940 average temperature rose
• by 0.25C
• Then 1940-1970 fell by 0.2C
• 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

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• 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

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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 its a liquid and so isnt 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

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The role of other greenhouse gases

• CO2 and H2O absorb in two bands of
• the Earths radiation spectrum
• Between these two bands is a window
• where 70 of the Earths radiation
• can escape (as it isnt 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

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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
• (thats 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?

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• 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

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• 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
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• 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 dont 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

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• 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

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• 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