Links for Ozone Resources

1
Temperature Inversion
Earths Surface
2
The Atmosphere

• Regions of the Atmosphere
• Troposphere - below 10 km (T gt 217 K)
• Stratosphere - 10 - 50 km
• Mesosphere - 50 - 90 km
• Thermosphere - above 90 km
• The absorption of UV by ozone in the stratosphere
  causes a rise in temperature with altitude. At 50
  km, the temperature reaches 271 K. O3av 10
  ppm.
• For ? gt 330 nm, penetration to Earths surface is
  possible. For 330 nm gt ? gt 200 nm, the UV
  penetrates to 50 km.

3
Penetration of UV
4
Atmospheric Chemistry

• The troposphere is the thinnest layer of the
  atmosphere, yet it contains 80 of the total
  mass of air and practically all of the
  atmospheres water vapor.
• Above the troposphere is the stratosphere, which
  consists of N2, O2 O3. Here the air temperature
  rises with altitude as a result of exothermic
  reactions triggered by solar UV O2
  ? lt 240 nm O? O ?
• O ? O2 O3 heat

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Related Web Sites for Ozone

• ozone hole tour
• http//www.epa.gov/ozone/science/hole/0808.htm
  http//www.epa.gov/ozone/science/process.html
• http//www.nas.nasa.gov/Services/Education/Resourc
  es/TeacherWork/Ozone/Antarctic.hole.html

6
Ozone in the Stratosphere

• In the stratosphere (20-50 km), oxygen absorbs UV
  light of the right energy (? 250 nm) to form
  ozone O2 O ? O ?
• O ? O2 M O3 M
• When O3 absorbs UV radiation of wavelength 210 -
  300 nm, it undergoes photodissociation and splits
  up again O3 UV light O2(g) O ?(g)
• In this way, O3 produces screening of UV-B
  (200-300 nm)
• In the stratosphere, O3 is constantly created and
  destroyed 3O2(g) O3(g) A steady-state
  O3 is reached.
• Ozone in the stratosphere undergoes
  photodissociation by absorbing UV. The free O?(g)
  atom further reacts with another O3 molecule
  O?(g) O3(g) O2(g) O2(g)
• This has the effect of reducing O3stratosphere.
  However, O3 is being made at the same time. Under
  natural equilibrium the rate of ozone production
  is equal to the rate of destroying O3, thus
  maintaining a fairly constant O3.

Ozone Production
7
Ozone Science

• Collision frequencies are low in the upper
  atmosphere, the fraction of collisions that
  results in reaction must be high, as in free
  radical reactions (with very low Ea).
• In the stratosphere, O2 molecules dissociate upon
  absorption of wavelength 135-176 nm 250 nm.
• O2(g) UV(250 nm) 2 O(g)
  O(g) O2(g) M
  O3(g) M /\H -100 kJ/mol O(g)
  O3(g) 2 O2(g) /\H -
  390 kJ/mol
• The two highly exothermic reactions require a
  third body M (possibly a N2 molecule) to absorb
  some of the energy released and prevent the
  dissociation of ozone. Because of the low
  concentration of third bodies, photochemical
  reactions in the stratosphere are very slow. The
  energy acquired by M warms the stratosphere. The
  O3 formed absorbs UV radiation at wavelength 220
  - 330 nm and causes the temperature rise observed
  at the tropopause

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3O2 2O3

• O2 UV 2 O
• O O2 M M O3
• O3 UV O2 O

The heat produced by photo- chemical reactions in
the stratosphere accounts for a temperature
inversion at the tropopause.

The balance between the rates of formation and
destruction results in a steady state
concentration of O3 in the stratosphere. If the
thickness of ozone layer decreases, UV penetrate
through the stratosphere to reach O2 in the
troposphere the rates of photolysis of O2 and
formation of ozone increase. If the ozone layer
becomes thicker, and the rates of photolysis of
oxygen and formation of O3 drop.
Ozone absorbs strongly at wavelengths of 220-330
nm.
Ozone filters out UV-B (300 nm)
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10
Chlorofluorocarbons in the Stratosphere
CCl3F has a lifetime in the atmosphere of 75
years. This gives it plenty of time to be
transported up into the stratosphere CFCs are
not removed either by rainfall or by dissolution
in the ocean.
Once in the stratosphere, the UV radiation from
the sun is intense enough to dissociate CFCs,
giving Cl free radicals which are very effective
in destroying ozone.
In Oct 1992, the ozone hole was almost 3 times as
large as the area of the U.S.
11
Free Radical Chain Reactions involved with CFCs

• O3 has fallen over the Antarctica Arctic for
  decades.
• CCl3F(g) UV CCl2F(g) Cl (g)
  (1) CCl2F2(g) UV
  CClF2 (g) Cl (g) (2)
• The Cl (g ) from the initiation steps readily
  depletes O3 via a sequence of reactions (3) and
  (4). Cl (g)
  O3(g) ClO(g) O2(g)
  (3) ClO(g) O?(g) Cl(g)
  O2(g) (4)
• O3(g) O?(g) 2O2(g)
  Ozone destruction
• The depletion of O3 by reaction (3) is
  considerably faster than that by reaction O?(g)
  O3(g) O2(g) O2(g). This has the
  effect of upsetting the balance in the natural
  production and destruction of ozone.
• Since the reactive species Cl? consumed in (3) is
  actually regenerated in (4), the presence of one
  Cl? effectively destroys many O3 molecules.
  (100,000).

12
Depletion of Ozone layer

• In the stratosphere, photodissociation of NOx
  CH4 occurs to form free radicals, which act to
  destroy O3. CH4 CH3 H
  N2O
  N2 O
• H, NO, O other free radicals destroy O3.
  H O3 OH O2
  HO O O2
  H
  NO O3 NO2 O2
• The rotting of wood produces chloromethane, a
  source of 25 of the chlorine in the
  stratosphere. Hydrolysis of chloromethane
  produces HCl, which reacts with OH to form an
  effective O3 killer, Cl.
• HO HCl H2O Cl Initiation step
  In the stratosphere, chlorine
  compounds are photolysed to give Cl

CO
NOx
CxHy
N2O is sufficiently inert to reach the
stratosphere where NO is generated. O N2O
2NO
13
Action of Cl on ozone

• Cl O3 ClO O2 (kills O3 2000 times
  faster than NO) ClO O Cl O2
  The 2
  reactions constitute a chain reaction.
  Chain
  length Propagation rate/Termination rate
  With a
  chain length of 5000, one Cl destroys 5000 O3
  molecules.
  
• O O3 2O2
  ClO O3 O2
  ClO2
• ClO NO2 M ClNO3 M
• ClNO3 is a temporary reservoir for chlorine. It
  can photolyze to give Cl. CFCs diffuse into the
  stratosphere where photolysis occurs to produce
  Cl which can attack the ozone layer.
• There is a large decrease in O3 over Antarctic
  during the spring (Aug-Oct), which is described
  as the ozone hole

CO
Propagation steps
NOx
CxHy
O3 easily donates a O(g) to ClO and Cl
Termination steps
14
Ozone Hole (FAQ)
CO

• Over the Antarctic, the temperature of the
  stratosphere is low enough for clouds to form and
  reaction at the surface of water droplets
  increases the rate of decomposition of chlorine
  reservoirs of ClNO3 to give ClO
• In the spring, warming of of clouds facilitate
  release of ClO
• Ozone depletion is largely confined to the
  Antarctic because the air mass there, called the
  polar vortex, rotates in isolation from the rest
  of the global air mass.
• In 1990, the levels of ozone-depleting gases are
  50 times higher than expected over the Arctic.
  The hole is increasing more slowly than that over
  the Antarctic because the temperature in the
  Arctic is higher and the ozone-depleting
  chemicals are less effective.
• The C-H bond is susceptible to attack by HO in
  the troposphere. Completely fluorinated alkanes
  do not react with HO or O3 they are not
  involved in O3 depletion or smog formation. Their
  very long lifetime makes it an important
  greenhouse gases.

NOx
CxHy
15
Photochemical Smog
Air pollutants are trapped near the ground in a
temperature inversion
16

Photochemical smog formation

• HO? attacks NOx and a mixture of HNO2 HNO3 is
  formed, leading to acid rain.
  HO? NO2? HNO3
  
  HO? ?NO M HNO2 M
• Oxidants react with hydrocarbons to form organic
  free radicals, e.g. O3 RH R? RCO2?
  RCHO R? O2 ROO? (strongly
  oxidising radical)
• A variety of compounds are formed, including
  ketones aldehydes which condense to form
  aerosols.
• Chain propagation occurs HOO? ?NO HO?
  NO2
• Chain termination occurs when peroxide radicals
  react with NO2 to form PAN peroxybenzoyl
  nitrate
• CH3CO-OO? NO2 CH3COOONO2 (PAN)
  

17
Secondary Pollutants during the day
Variation in concentration of atmospheric
pollutants with time
18
Ozone Levels Throughout the Day
The ozone level in the stratosphere is 20-50 ppb
In the afternoon, O3 drops as it
oxidizes hydrocarbons to photochemical smog
CxHy
Stage 1 alert (US)
O3 causes breathing difficulties at this level

Ozone is formed in the stratosphere from O2
through the absorption of UV of wavelength lt 242
nm (O3 absorbs UV of ? lt 290 nm)

NO2 is a substance which can be photolysed in the
range of visible-UV radiation (? gt 300 nm) that
penetrates to the Earths surface. (NO2 is a
secondary pollutant derived from NO.)
19
Ozone in the troposphere

• Such primary pollutants as NOx hydrocarbons
  build up during the morning rush hour.
• By photolysis, nitrogen dioxide produces
  aldehydes, O3 PAN (Peroxyacetyl nitrate)
• Near the Earths surface, hydrocarbons and NOx
  react in sunlight to form ozone, which is rather
  short-lived.
• In the afternoon, ozone acts to remove
  hydrocarbons NOx so that there is a appreciable
  drop in O3 after 1 p.m
• O3 absorbs most of the UV photons having
  wavelengths lt 340 nm. Photochemical reactions
  involving ozone do not occur at Earths surface.
• Photolysis of oxygen gives excited O(g). These
  react with water molecules to give hydroxyl
  radicals. O(g) H2O(g)
  2 OH

20
Ozone in the Troposphere
CO
NOx

• NO2? UV NO? ?O
  O? O2 M O3
  M
  NO? O3 NO2? O2

O3 is formed
O3 is destroyed
NO2 photolyses to O? which reacts with O2 to
form O3 NO? NO? reacts with O3 to reform NO2?
Ozone oxidises hydrocarbons to form
A brown haze of photochemical smog
Due to high intensity and long duration of
sunlight, the ozone concentrations during the
summer months are higher. The residence time of
ozone in the troposphere is 60 hours.
O3 and SO2 have a synergistic effect. Some
compounds present in smog react with ozone to
form carcinogenic or mutagenic chemicals
21
Photochemical smog

• UV, hydrocarbons and nitrogen oxides are 3
  factors for the formation of LA smog.
• In stagnant air, photochemical oxidants turn
  hydrocarbons to a mixture of compounds which form
  an aerosol haze. The oxidants include O3, ?OH,
  RCO-O-O-NO2 (PAN), ROOR
• In London, the presence of sulphur dioxide in
  smog (from high-sulphur fuel) makes it a reducing
  smog.
• NO2 and hydrocarbons accumulate in the morning.
  NO2 absorbs UV and photolyses NO2? NO?
  O?
• O? O2 M O3 M (fast)
  O? NO2? NO? O2
  O3 NO? NO2? O2 (fast)
  
• O3 remains low until NO? falls to a low value.

NO? consumes O3 in the air
22
The Effects of Photochemical Smog

• Health hazard ozone at 0.15 ppm causes coughing
  PAN and aldehydes are eye irritants.
• Material damage ozone causes rubber to
  deteriorate through fission of the double bond.
• Toxicity to plants PAN O3 cause crop damage
• The small particles formed in smog-forming
  reactions polymerize to form aerosol particles,
  lowering visibility

CxHy
CO
NOx
23
Air Pollutants

• Methane is produced in large quantities from the
  anaerobic decomposition of organic matter in
  water, sediments soil.
• Methane is formed from bacterial digestion of
  food in the gut
• Photochemical reaction of methane produces
• CO and O3
• CO2 and water vapor
• Aldehydes and ketones enter the atmosphere from
  industrial sources where they are used as
  solvents and as raw materials, and from
  incinerators spray painting. Methanal ethanal
  are produced by micro-organisms.
• The carbonyl group readily absorbs UV light and
  is photolysed into a ?CHO or RCO? and an alkyl
  radical. CH3CHO
  CH3? ?CHO
  RCOR RCO? R
• The radicals take part in producing photochemical
  smog.

24
Reduction of Pollutants in Industry

• Sulphate ions in acid rain can combine with
  aluminium in compounds to form soluble aluminium
  sulphate, which washes into streams. There it
  interferes with the operation of fish gills, so
  that they become clogged with mucus. The fish die
  from lack of oxygen.
• PFBC cut greatly the emission of nitrogen oxides
  SO2
• Flue gas desulphurisation (FSD) is a technique
  for removing SO2 from the exhaust gases in the
  chimney stacks of power stations. A slurry of
  limestone and lime is used to scrub the flue
  gases. CaCO3(s)
  SO2(g) CaSO3(s) CO2(g)
  CaO(s) SO2(g) CaSO3(s)
  2CaCO3(s)
  O2(g) 4H2O(l) 2CaSO4.2H2O (s)
• Fitted with a low-NOx burner, the emission of NOx
  by 50. The low-NOx burner controls the
  combustion by
• lowering the flame temperature in the combustion
  zone
• reducing the availability of oxygen in the NOx
  fomation zone

Pulverised Fluidised Bed Combustion
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Ways of Removal of Particulates
Sedimentation
27
Pollution in the City

• Atmospheric particles
• reduce visibility by scattering light
• reflect radiation from space prevent it from
  reaching the Earths surface, thus exerting a
  cooling effect on the Earth
• Provide surfaces on which heterogeneous
  atmospheric chemical reactions can occur, thus
  exerting an effect on air pollution - and nuclei
  for the condensation of water vapor in the
  atmosphere, thus influencing the weather.