METO 737

Lesson 9

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.

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

Column amount of CF2Cl2 from 1985 to 1996

Mixng ratio of CFCl3 as a function of altitude

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.

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

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.

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.

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%.

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.

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

Contributions to the Equivalent Effective Stratospheric Chlorine

Increase in UV dose at 40 N for three different scenarios of CFC production