Chemistry of the Stratosphere Stratospheric Ozone

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Chemistry of the Stratosphere Stratospheric
Ozone

• Ozone plays a vital role in the atmosphere as the
  principal absorber of 240 -320 nm radiation. It
  acts as an important filter of highly energetic
  radiation capable of causing severe damage to
  living cells.
• Ozone Layer implies localized region of the
  atmosphere where ozone concentrations are high
• Ozone concentrations are highest in the
  stratosphere but also present in the troposphere
• present as a trace component
• even at maximum concentrations of 10 ppmV

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Stratospheric Ozone

• Total ozone concentration is small but of great
  importance in the stratosphere
• if compressed to 1 atm at 0 ºC, the ozone
  thickness measures only 3 mm.
• O3 thickness measured in Dobson Units
• G.M.B. Dobson 1920s-1930s
• 1 D.U. 0.01 mm thickness at S.T.P.
• Ozone columns usually have thicknesses of 300
  D.U.

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Formation and Destruction of Ozone

• Photochemical formation and destruction of ozone
• Each day, 350,000 tonnes (3.5 x 108 kg) of O3
  are made and destroyed

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Formation and Destruction of O3
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Formation and Destruction of O3

• Note
• Energy difference accounted for by the excitation
  energies of O2 ( 90 kJ/mol) and O (188 kJ/mol)
• Reactions are energy releasing ( h? ? K.E. )
• Photochemical production O3 at night O3 at
  end of day

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Analysis of Ozone Concentrations

• Steady state concentrations of ozone
• (rate of production rate of destruction)

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Analysis of Ozone Concentrations
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Analysis of Ozone Concentrations
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Analysis of Ozone Concentrations
Usually fair to assume that k1O2 ltlt k3O3
(especially in the lower stratosphere) Since the
solar flux in the far UV is much smaller than the
solar flux in the near UV
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O3 subject to large variations since the solar
flux varies throughout the day and seasonally.
But take the time averaged flux and look at O3
versus altitude ... The measured ozone profile
has the same shape as the calculated profile but
the calculated values are about 2 x too high
!!! There must be other ozone destruction
mechanisms!
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Catalytic Destruction of Ozone

• O3 is metastable and can be destroyed by
  catalytic chain reactions. Ozone is converted to
  O2 by a chain carrier X (free radical) which is
  regenerated in the process

(reaction 4 of Chapman mechanism) important
additional sink of O3!
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Catalytic Destruction of Ozone

• Since formation of O3 is unaltered (determined
  only by solar flux) additional sink lowers the
  steady state concentration of O3
• X is a catalyst - A single X is removed by some
  side reaction (termination reaction)
• Many possible candidates for X, but 4 5 major
  ones

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Chlorine and Bromine

• one possible termination reaction
• Cl HO2? HCl O2
• Major sources of Cl and Br are CFCs and Halons
  (fire extinguishers)

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Nitric Oxide

• NO produced abundantly in the troposphere but
  undergoes oxidation to NO2 and HNO3 and is then
  rained out
• N2O much less reactive (can reach stratosphere)

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Nitric Oxide
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Nitric Oxide

• importance of NO/NO2 cycle depends on altitude
• above 25 km, net effect is O3 loss
• 50 of all O3 destroyed in upper atmosphere
• below 23 km, net effect is to protect O3 from
  destruction by taking it out of circulation

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Additional Reactions in Stratospheric Chemistry

• Reactions comprising the Chapman mechanism and
  the catalytic cycles involving Cl/Br, NO, OH, and
  H do not account for all of the chemistry of the
  stratosphere

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Temporary Reservoirs of Active Species

• Catalytically active species can be converted
  into substances which reduce their concentrations
  but from which they can be regenerated
• estimated that at any given time, ½ of
  atmospheric nitrogen is tied up as HNO3

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Temporary Reservoirs of Active Species

• NO2 regenerated by photolysis
• HCl is a temporary reservoir for Cl which can be
  regenerated via

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Interaction Between Cycles

• The catalytic cycles involving Cl/Br, NO, OH, and
  H do not occur independently of each other
• X from one cycle can react with XO from another
• affects both NO/NO2 and OH/HO2 cycles

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Null Cycles

• Null cycles result in no net chemical change but
  usually involve the conversion of sunlight to
  kinetic energy
• also can be viewed as a pseudo-equilibrium
• affects both NO/NO2 and OH/HO2 cycles

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Initiation and Termination Reactions

• Net rates of the catalytic cycles depend on the
  rates of radical initiation and termination
• Most initiation reactions are photochemical and
  their rates depend upon the intensity of sunlight
  (approach zero in darkness)

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Initiation and Termination Reactions

• Termination reactions remove radical chain
  carriers from the system

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