Stratospheric Ozone Depletion

1
Stratospheric Ozone Depletion

• Catalytic Processes and the Ozone Hole over
  Antarctica
• (Lecture 1)

2
Review on Atmospheric Layering
3
Review of Atmospheric Layering
4
Review UV Filtering
5
Variation of O3 Density

• The maximum density of ozone occurs at 25km over
  tropical areas, 21km over mid-latitudes, 18km
  over subarctic regions.
• The O3 density is at a maximum when the product
  of UV-C intensity and O2 conc is at a maximum.
• Most O3 occurs between 15-35 km, the Ozone
  Layer.

6
Review UV Filtering
7
Recent Skin Cancer Data
8
UV Index
9
Ozone Hole Over Antarctica
10
Ozone Hole Over Antarctica
11
Ozone Hole Seasonal Variation
12
Catalytic Processes- Ozone Destruction

• The X species are catalysts for O3 destruction in
  stratosphere.
• We are inadvertently increasing X conc by release
  of certain gases (especially Cl-containing) at
  ground level.

13
Review of Effect of Catalyst on Reaction Rates
14
Free Radicals

• In mechanisms I and II, X is a species with an
  odd unpaired electron.
• X can be NO?, Cl?, OH?, Br? H? (not O?)
• Cl? is the one of most concern!
• Read Box 2-1 Box 2-2
• Do 2-6 2-8

15
Free Radical Mechanism I

• Cl? O3 ? ClO? O2 Eact2 kJ/mol
• ClO? O? ? Cl? O2
• O3 O? ?? 2 O2 Eact 18.4kJ/mol
• Occurs in mid- and upper stratosphere.
• Cl? source can be natural or anthropogenic.

16
Free Radical Mechanism I

• Catalytic destruction of O3 can even occur in a
  clean atmosphere (Involves NO?)
• N2O rises from troposphere reacts with O.
• N2O O ? 2NO? (or SSTs)
• Then via Mechanism I
• NO? O3 ?? NO2? O2
• NO2? O? ? NO? O2
• O3 O? ? 2O2

17
Free Radical Mechanism I

• In the upper stratosphere, OH? is the dominant
  free radical catalyzing O3 destruction.
• CH4 or H2O react with excited oxygen, O
• CH4 O ? OH? CH3? (Do 2-9)
• Then via Mechanism I
• OH? O3 ? HOO? O2
• HOO? O? ? OH? O2
• O3 O? ?? 2O2

18
Activation Energies for Steps 1 2 of Mechanism
I with Different X
19
Relative Rates for O3 Decomposition Reactions via
Mechanism I Follow

• NO/NO2 gt uncatalyzed rxn ? Cl/ClO gt OH/HO2 gt
  H/OH
• Above data shows that smaller Eacts will not
  always mean a faster reaction. Rates also depend
  on conc of reactants.
• For same reason, step 2 of Mechanism I is the
  rate determining step!

20
Free Radical Mechanism II

• Mechanism I requires atomic O to complete the
  cycle XO O ? X O2
• In lower stratosphere, atomic O conc is low
  because UV-C doesnt penetrate that low.
• Hence, most O3 loss in lower stratosphere occurs
  via Mechanism II
• X O3 ? XO O2
• X O3? XO O2
• XO XO ?? X X O2
• 2O3?3O2

21
Rates of O3 Formation and Destruction

• Rate of O3 formation depends upon O2, O3,
  UV-light intensity at a given altitude.
• Rate of O3 destruction is more complex. It
  depends upon O3 x sunlight Intensity x X.
  Each Cl? can destroy as many as 10,000 O3
  molecules. (Do problem 2-10)
• When above opposing rates are equal, then a
  steady state is reached and O3 is constant.

22
Additional Reactions in Stratospheric Chemistry

• Temporary reservoirs of active species
• Interaction between catalytic cycles
• Null (do-nothing) cycles
• Initiation and termination reactions

23
Temporary Reservoirs

• Catalytically active species (NOx ClOx ) can be
  temporarily converted into other substances
• NO2? OH? ? HNO3
• Cl? CH4 ? HCl CH3?
• ClO? NO2? ?? ClONO2 ( sunlight)
• A sample regeneration reaction would be
• HCl OH? ? Cl? H2O

24
Interaction Between Cycles

• The different catalytic cycles all involve pairs
  of catalysts, X and XO? These can interact, e.g.
• NO? HOO? ? NO2? OH?
• Above would affect conc of reactants in both the
  NO/NO2 OH/HO2 cycle

25
Null Cycles

• See Problem 2-7
• NO2? h?(?lt400 nm)?? NO? O?
• O? O2 M ? O3 M
• O3 NO? ?? NO2? O2
• When these are added, all species drop out!
• Sunlight is converted into kinetic energy.

26
Initiation Termination Reactions

• Initiation Reactions are usually Photochemical
• HNO3 h? ? NO2? OH?
• O3 h? ? O2 O?
• CH3Cl h? ? CH3? Cl?
• N2O O? ? 2NO?
• Last two reactions are principally biological in
  origin.

27
Initiation Termination Reactions

• Termination reactions remove radicals from the
  system.
• (Examples)
• 2ClO? ? ClOOCl
• 2HOO?? H2O2 O2
• (Do RQ 11 18)