METO 737
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Transcript of METO 737
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METO 737
Lesson 9
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Fluorinated Hydrocarbons
• Developed in 1930 by the General Motors Research laboratories in seqrch for a non-toxic, non-inflammable, refrigerant.
• Up to then refrigerators had used SO2 and NH3
• CF2Cl2 (CFC_12) is a typical member of the ‘Freon’ family.
• It is methane with all of the hydrogen atoms replaced by halogen atoms.
• In 1973 Lovelock and collaborators noted that the Freons were present in the troposphere, and calculations showed that the amount in the troposphere was close to the total amount produced.
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Fluorinated Hydrocarbons
• These calculations showed that the lifetime of the Freons in the troposphere was about 100 years.
• Rowland and Molina, 1974, showed that the Freons could only be destroyed in the stratosphere by photodissociation
CF2Cl2 + hν → CF2Cl + Cl• The amount of chlorine released was much
more than the Shuttle released.• It was soon realized that this was a significant
threat to the ozone layer
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Column amount of CF2Cl2 from 1985 to 1996
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Mixng ratio of CFCl3 as a function of altitude
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Fluorinated Hydrocarbons
• Other Freons are CFC-11, CFC-113 sh
• Other halocarbons are carbon tetrachloride CCl4, methyl chloroform CH3CCl3.
• Vertical profiles of the Freons have a constant mixing ratio in the troposphere, only falling off above 20 km.
• This confirms their destruction in the stratosphere.
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Chlorine Chemistry
• The major loss for ozone above 20 km is the catalytic chain:
Cl + O3 → ClO + O2
ClO + O → Cl + O2
O + O3 → O2 + O2
• Below 20 km other chains are more efficient:Cl + O3 → ClO + O2
OH + O3 → HO2 + O2
ClO + HO2 → HOCl + O2
HOCl + hν → OH + ClO3 + O3 → O2 + O2+ O2
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Chlorine Chemistry
• And:
Cl + O3 → ClO + O2
ClO + NO → Cl + NO2
NO2 + O → NO + O2
O3 + O → O2 + O2
• Note that both of these chains involve the different HOx, CLx, and NOx families.
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Bromine compounds
• Bromine compound can also influence stratospheric ozone.
• Source gases are mainly methyl bromide and the brominated CFC’s, known as the halons.
• Halons are used primarily as fire retardants in fire extinguishers. Methyl bromide has natural sources, but is also manufactured and used in soil fumigation.
• The lifetime of methyl bromide is considerably shorter in the troposphere than the halons, as OH can attack the hydrogen atom and break the molecule apart.
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Bromine compounds
• The concern is that bromine can destroy ozone (odd Oxygen) with a very high efficiency, so small amounts of bromine can have a disproportionate effect on the recombination of odd oxygen:
BrO + ClO → Br + CL + O2
Br + O3 → BrO + O2
Cl + O3 → ClO + O2
O3 + O3 → O2 + O2 + O2
• For a mixing ratio of 2.0x10-11 of Br, the effect of chlorine is increased by 5-10%.
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Bromine compounds
• The following reactions also occur
BrO + BrO → Br + Br + O2
BrO + ClO → Br + OClO
BrO + Cl0 → BrCl + O2
• The branching into the three reactions are 0.45, 0.43, and 0.12, (room temperature)
• Reaction 2 is the only known source of OClO. It has been observed. BrCl is a temporary reservoir for Br and Cl, as it is rapidly photolysed during the day.
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Ozone Depletion Potentials
• The net efficiency for the depletion of ozone relative to that for CFCl3 is known as the ozone depletion potential. It depends on the lifetime and the release rate.
• The effect can be lessened by shortening the lifetime in the troposphere.
• Substitutes have been developed wich either contain a hydrogen atom, hydro-chloro-fluorcarbons (HCFC) or the hydro-fluorcarbons (HFC).
• An example of an HCFC is CF3CHCl2 (ODP=0.013) replacing CFCl2 (ODP=1.0) . The H is removed in the troposphere by the OH radical.
• An example of an HFC is CF3CH2F (ODP=0.0) for CF2Cl2 (ODP=0.9)
• Eventual replacement will remove the chlorine e.g. CF4
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Contributions to the Equivalent Effective Stratospheric Chlorine
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Increase in UV dose at 40 N for three different scenarios of CFC production