Stratospheric Ozone Depletion

1
Stratospheric Ozone Depletion

• Catalytic Processes and The Ozone Hole over
  Antarctica
• (Lecture 2)

2
Atomic Chlorine and Bromine as X Catalysts

• Atomic Cl? in the stratosphere is both from
  anthropogenic and natural sources
• CH3Cl UV-C ? Cl? CH3? or
• OH? CH3Cl ? Cl? other products
• Recall that most Cl? is tied up as catalytically
  inactive ClONO2 (about 99 of all Cl? ) HCl.
• Br? (from CH3Br and Halons) also can destroy O3
  via Mech. I
• Br? is a more efficient destroyer of O3 since
  less is tied up in inactive reservoirs.

3
The Ozone Hole and Other Sites of O3 Depletion

• Extensive research explained why O3 hole over
  Antarctica largest in their spring (our Fall SON)
• Special Polar winter weather conditions in the
  lower stratosphere temporarily convert some
  inactive ClONO2 and HCl into active Cl? (How?)
• Answer to above question involves formation of
  Polar Stratospheric Clouds (PSCs).

4
Polar Stratospheric Cloud Formation

• Total darkness at South Pole during sunless
  winter months. Thus, O2 O ? O3 heat does not
  occur. Lower stratosphere gets to 80oC.
• Low temp yields drop in air pressure (PVnRT).
• Earths Rotation, combined with low temp, yields
  a whirling mass of air(vortex) with high wind
  speeds(gt180 mph). Matter cannot penetrate this
  vortex. Air inside vortex remains cold til Oct.
• Gases inside vortex condense to PSCs(I II).

5
Reaction on PSC Ice Crystals

• Molina found that reactions on PSC ice crystals
  occurs on a thin aqueous surface layer
• ClONO2(g) H2O ? HOCl HNO3
• HCl(g) ? H Cl-
• HOCl Cl- ? Cl2(g) OH-
• Once sunlight reappears in early Antarctic
  spring
• Cl2 hv ? 2Cl? (Do 2-11)
• HOCl hv? OH? Cl?
• (O3 destruction results from Cl? buildup)
• Denitrification of lower stratosphere enhances O3
  destruction by 20)
• HCl ClONO2?HNO3 Cl2

6
Denoxification on PSCs

• Removal of gaseous nitrogen oxides(NOx) on PSCs
  also occurs via the following
• NO2 is in gas phase equilibrium with N2O5
• 2 N2O5 ? 4 NO2 O2
• N2O5 is removed from the gas phase via
• N2O5 H2O ? 2 HNO3
• N2O5 HCl ? ClNO2 HNO3
• As the PSCs grow, they settle out of the
  stratosphere, taking HNO3 with them. This
  denitrification further encourages
  denoxification. Why is this important?
• Recall ClO NO2 ? ClONO2 (Inactive Cl)

7
O3 Destruction in Ozone Hole

• Most O3 destruction in ozone hole occurs by Mech
  II with both X and X being Cl?
• 2Cl? 2O3 ? 2ClO? 2O2
• 2ClO? ? 2Cl? O2_ (slow)
• 2O3 ? 3O2 (Do 2-12 2-14)
• Notice rate ? ClO?2 , so O3 destruction is
  appreciable only when ClO? is high.

8
The Recipe for O3 Loss over Antarctica

• Polar winter leads to formation of the polar
  vortex which isolates the air within it.
• Low temperatures inside the vortex form PSCs,
  which persist.
• Heterogeneous reactions take place on PSC
  particles converting inactive Cl and Br
  reservoirs into more active forms.
• Ozone loss occurs rather suddenly when sunlight
  returns to the air inside the vortex. This
  produces Cl? (and Br?) initiates catalytic O3
  destruction cycles.

9
Ozone Hole Dimensions

• The Ozone Hole covers a geographic region a
  little bigger than Antarctica and extends 10 km
  in altitude in the lower stratosphere.

10
Artic Ozone Depletion

• Episodes of partial O3 depletion over parts of
  the arctic have been observed recently
• The stratospheric temp does not fall as low as
  over Antartic.
• PSCs form less frequently.
• Conditions may be changing for the worse.

11
Global Decreases in Stratospheric Ozone
12
Global Decreases in Stratospheric Ozone

• Reactions leading to O3 loss may also occur on
  surfaces of sulfate particles. Steep declines on
  above plot from 1982-84 1992-93 likely due to
  volcanic eruptions injecting SO2.
• In the mid-latitude lower stratosphere, we have
• Cl? O3 ? ClO? O2
• OH? O3 ? HOO?? O2
• ClO? HOO? ? HOCl O2
• HOCl ? OH? Cl??

13
O3 Depletion Outside Antarctica?

• Some modest O3 depletion has occurred, but temp
  does not drop as low and arctic vortex breaks up
  too soon for major depletion.
• There was a worldwide decrease in the steady
  state O3 by several during the 80s, with
  greatest loss over northern mid-latitudes.
• In the mid-latitude lower stratosphere, the most
  important O3 destruction reactions involve Cl?
  and OH? . Do Problem 2-15.

14
Effects of Volcanic Eruptions

• Scientists speculate that O3 destruction
  reactions may take place on cold liquid droplets
  of H2SO4.
• Denitrification occurs via the process
• H2SO4
  Droplets
• N2O5 H2O(droplets) ? 2
  HNO3
• Volcanic Eruptions lead to the direct injection
  of SO2 into the Stratosphere(El Chichonb in 82
  Mt Pinatubo in 92).

15
Effects of Volcanic Eruptions Pinatubo 91
El Chichon 82
16
Effects of Vocanic Eruptions El
Chichon Caldera Pinatubo Data
17
Effects of Volcanic Eruptions

• Volcanic Aerosols include sulfate particles

18
Effects of Volcanic Eruptions

• Notice the decrease in total overhead O3 after
  Pinatubo in 91 and after El Chichon in 82.

19
Chemicals That Cause O3 Destruction

• The recent increase in stratospheric chlorine is
  due to the use and release of CFCs or
  chlorofluorocarbons.
• These anthropogenic chemicals have no atmospheric
  sink. They are not washed out or oxidized. After
  remaining in the troposphere for awhile. They
  diffuse to the stratosphere where they
  photo-decompose to release chlorine.

20
Chemicals That Cause O3 Destruction

• Dashed curve shows the conc of Cl(Br changed to
  equiv Cl) over last 40 years and projected 100
  years.
• Peak at 4 ppb 4x gt natural levels (CH3Cl
  CH3Br)

21
CFCs

• These nontoxic, nonflammable, nonreactive
  chemicals found a lot of uses,such as coolants
  propellants in aerosol sprays. Do problems 2-16
  2-17(add 90 to code)
• Through collisions with other molecules, CFCs
  migrate slowly to stratosphere. (CFC-1160
  yrs. CFC-12105 yrs.)

22
CFC Replacements

• HCFCs will be temporary replacements for CFCs
  through early years of 21st century.
• These all have HCX3 and react with OH?,
• OH? H?CX3 ? C-based radical ?
• 
  CO2HCl
• CHF2Cl (HCFC-22) has15 O3 reducing potential as
  CFC-11
• HFCs are long-term replacements for CFCs
• However, TFA is a concern

23
Halons

• These are Bromine-containing hydrogen-free
  substances. They have no tropospheric sink. They
  are Ozone Depleting Substances. Do problem 2-20
• All ODS destined for phase out in all nations
• Rowland, Molina along with Dutch chemist Crutzen
  were awarded the Nobel Prize in 1995 to honor
  their research in the science underlying ozone
  depletion!