Chapter 3 Stratospheric Ozone Chemistry

1
Chapter 3Stratospheric Ozone Chemistry

• CH350/EV350
• Spring 2008

2
Profile of Ozone (O3) with altitude

• Good ozone in stratosphere
• Bad ozone in troposphere
• Compare mixing ratio and ozone density with
  altitude

3
UV light from sun

• Small percentage of total radiation, but UV
  radiation is important
• UV-A 315-400 nm, not harmful in short term
• UV-B 280-325 nm, can be harmful to plants and
  animals
• UV-C lt280 nm, rapidly damages living organisms

4
Solar flux vs. wavelengthO2 O3 absorption vs.
wavelength

• O2 absorbs most light below 175 nm
• O3 absorbs between 200 and 300 nm
• O2 O3 protects organisms from all UV-C some
  UV-B

5
Measuring O3

• O3 not stable in containers
• Very difficult to take air sample like a water
  sample
• O3 present at low concentrations
• Take instrument to the air
• Instruments need to operate at cold temperatures
• Ground based
• DIAL-differential absorption lidar
• 308 nm light directed into air, O3 absorbs light,
  particles scatter light back to detector, the
  amount of light absorbed indicates O3
  concentration
• Dobson ozone spectrometer
• Measures solar UV at several wavelengths, some of
  which are absorbed by O3 and others that are not
• Gives total O3 in air column
• 300 Dobson units (DU) contains the equivalent of
  3 mm thick layer at 273 K and 1 atm.
• Satellite and airplane based instruments that
  work on similar principles

6
Synthesis and noncatalytic decomposition of ozone

• 3.1 O2 hn (l lt 240 nm) ? O O
• 3.2 O O2 ? O3 heat
• O O2 ? O3
• O3 M ? O3 M
• 3.3 O3 hn (l lt 320 nm) ? O2 O
• 3.4 O3 O ? O2 O2

7
Ozone layer profile

• Upper stratosphere high energy light
• Not much O2 due to rxn 3.1
• O2 lifetime 1 hr
• O3 production limited by the amount of O2

• 3.1 O2 hn (l lt 240 nm) ? O O
• 3.2 O O2 ? O3 heat
• O O2 ? O3
• O3 M ? O3 M
• 3.3 O3 hn (l lt 320 nm) ? O2 O
• 3.4 O3 O ? O2 O2

8
Ozone layer profile

• Lower stratosphere little high energy light
• Not much O generated by rxn 3.1
• O2 lifetime 5 yr
• O3 production limited by the amount of O atoms

• 3.1 O2 hn (l lt 240 nm) ? O O
• 3.2 O O2 ? O3 heat
• O O2 ? O3
• O3 M ? O3 M
• 3.3 O3 hn (l lt 320 nm) ? O2 O
• 3.4 O3 O ? O2 O2

9
Ozone layer profile

• Oxygen chemistry only accounts for 20 of O and
  O3 removal (rxn 3.4)

• 3.1 O2 hn (l lt 240 nm) ? O O
• 3.2 O O2 ? O3 heat
• O O2 ? O3
• O3 M ? O3 M
• 3.3 O3 hn (l lt 320 nm) ? O2 O
• 3.4 O3 O ? O2 O2

10
Natural Catalytic Destruction of Ozone

• X O3 ? XO O2
• XO O ? X O2
• Net O3 O ? 2O2
• X .H, .OH, .NO, .NO2

• 3.1 O2 hn (l lt 240 nm) ? O O
• 3.2 O O2 ? O3 heat
• O O2 ? O3
• O3 M ? O3 M
• 3.3 O3 hn (l lt 320 nm) ? O2 O
• 3.4 O3 O ? O2 O2

11
HOx Catalytic Destruction of Ozone
CH4 Oxygen radicals ? CO2 H2O H2O O ?
2.OH H2O hn ? .H .OH .OH O3 ? .OOH
O2 .OOH O ? .OH O2 Net O3 O ? 2O2 .H O3
? .OH O2 .OH O ? .H O2 Net O3 O ? 2O2

• CH4 is natural, but human activities do increase
  its concentration in the atmosphere
• CH4 doesnt precipitate in troposphere like water
  and some migrates to stratosphere
• These mechanisms are responsible for 70 of O3
  destruction at 50 km

12
NOx Catalytic Destruction of Ozone
Supersonic jets produce 160 kg NOx per hour N2O
O ? 2.NO (lt30 km) N2 hn(llt126 nm) ? N N
(gt30 km) N O2 ? .NO O .NO O3 ? .NO2
O2 .NO2 O3 ? .NO3 O2 .NO2 O ? .NO O2 .NO3
O ? .NO2 O2 Net O3 O ? 2O2 Net O3 O ? 2O2

• NO and NO2 in troposphere have a short residence
  time and are rained out
• N2O is naturally produced by soil bacteria, but
  more N2O is released from soils heavily
  fertilized with nitrate. 120 year residence time
  in troposphere
• These mechanisms may be responsible for 70 of O3
  destruction at tropopause

13
Catalyst Removal
.NO .OH ? HNO2 .NO2 .OH ? HNO3 .OOH .NO2
M ? HO2NO2 M .NO3 .NO2 M ? N2O5 M

• HNO2, HNO3, and N2O5 are reservoir species and do
  not catalytically destroy ozone because they are
  not radicals.
• 50 of NOx is stored in nitric acid reservoir

14
Two unlikely players

• Sherry Rowland
• Radiochemist (nuclear chemist)
• In Jan 1972 Rowland went to a meeting designed to
  bring chemists and meteorologists together
• No background in atmospheric chemistry, but
  wanted to apply his radiochemistry knowledge
• James Lovelock presented work about invention of
  electron capture gas chromatograph
• Measured chloroflurocarbons (CFCs) in atmosphere
• Found concentrations were 230 parts per trillion
• Rowland figured out that the amount of CFCs in
  atmosphere were equivalent to the total amount of
  CFCs produced
• It didnt appear that these chemicals broke down
  in the environment.

15
Two unlikely players

• Mario Molina
• Had just completed Ph.D. in photochemistry at UC
  Berkely
• Few Days before Christmas 1973 working as a
  faculty research position at UC Irvine under
  Sherry Rowland on the fate of CFCs in the
  atmosphere
• Determined CFCs migrate to stratosphere slowly
  then breakdown under UV light
• CF2Cl2 UV-C ? CF2Cl Cl

16

• Molina and Rowland thought of publishing work
  showing fate of CFCs
• On a hunch, Rowland suggested to find out the
  fate of the resulting Cl atom

17
Disturbing Results
.Cl O3 ? .ClO O2 .ClO O ? .Cl O2 Net O3
O ? 2O2

• They were unsure of the results because Molina
  predicted that at 1973 levels of CFCs, between 7
  and 13 of O3 would be depleted in 100 years.
• This would seriously affect biological organisms

18
Absorption of light by DNA compared to intensity
of light at the earths surface
Figure taken from Environmental Chemistry 2nd Ed.
By Colin Baird 1999
19
Skin Cancer Rates with UV Exposure
Figure taken from Environmental Chemistry 2nd Ed.
By Colin Baird 1999
20
Help in the Atmospheric Community?

• Molina and Rowland talked with Harold Johnston
  (UC Berkeley).
• Johnston was an atmospheric chemist
• They hoped he would show them the flaw in their
  work.
• He could find none, but didnt want to become
  involved because of the political issues he knew
  would follow the release of this work.

21
Releasing the work to the scientific community

• Molina and Rowland published work in Nature June
  28, 1974
• Molina and Rowland present their work at ACS
  meeting September 1974
• This work caused quite an uproar. Why?

22
CFCs

• Chlorofluorocarbons
• Contain C, H, Cl, F atoms
• CFC-xyz
• x C atoms 1 (omitted if x 0)
• y H atoms 1
• z F atoms
• Examples
• CF2Cl2 CFC-12

23
CFCs

• Low viscosity
• Low surface tension
• Low boiling point
• CFC-11 boils near room temperature
• Easily compressed at room temperature
• Chemical and Biologically inert
• Not corrosive
• Not toxic
• Prior to WWII, used ammonia as a refigerant, CFCs
  replaced ammonia as a refigerant
• DuPont obtained patents for many CFCs

24
CFC Uses

• Refrigerants
• Foam blowing agent
• Aerosol propellants
• Cleaning agents

25
Economic Impact of banning CFCs

• 1974
• CFC production 500 million
• 600,000 jobs with a payroll of 6.7 billion
• 1.5 million additional workers were indirectly
  dependent on CFCs
• There was a lot of opposition to banning CFCs
  even though some people thought they might harm
  the environment!

26
Actual Data?

• Molina and Rowland had no proof that O3 was
  actually declining
• All experiments were done in the lab
• All ground and satellite measurements indicated
  that O3 levels were fine
• O3 concentrations fluctuate with hour and season
• Temperature affects reaction rates
• Flux of UV affects formation and destruction of
  O3
• Hard to measure a small change above a large
  cyclical change (figure 35-60oN)

27
Actual Data

• Joe Farman professor at Cambridge
• Farman had been measuring O3 over Antarctica
  since 1957
• He almost lost funding because he hadnt really
  seen anything change in 25 years
• Noticed dip in O3 over South pole in Spring 1982
• Farman was skeptical
• His instruments were hard to keep calibrated
• O3 was lower in Spring of 1983 and 1984
• Farman was more confident because they had made
  extra sure that instruments were working
  correctly.
• Published work in Nature May 1985

28
Oops

• NASA had been measuring O3 over the earth with
  satellite imaging
• The satellites had a lot of problems so NASA
  scientist programmed the computers to ignore data
  that was erroneously low
• Thus they didnt notice the dip in O3 in
  Antarctic spring until Farman pointed it out.

29
More Study Needed

• DuPont and others wouldnt give up easily on CFCs
• High profile expeditions to Antarctica to study
  Ozone hole in October (Antarctica Spring)
  occurred in 1986 and 1987

Figure taken from Environmental Chemistry 2nd Ed.
By Colin Baird 1999
30
Antarctic Ozone Hole ChemistryCl reservoir
species (removal of Cl from catalytic cycle)
.Cl CH4 ? HCl .CH3 .ClO .OOH ? HOCl
O2 .ClO .NO2 M ? ClONO2 M

• 70 of stratospheric Cl is present as HCl

31
Antarctic Ozone Hole Chemistry

• Cold temperatures and polar vortex around
  Antarctica cause polar stratospheric clouds
  (PSCs)
• PSC are made of ice crystals that have an aqueous
  layer
• ClONO2 and HCl react with water on the surface of
  the ice crystal

HCl ClONO2 PSC surface ? Cl2 HNO3 H2O
ClONO2 PSC surface ? HOCl HNO3
32
In spring (late October) after months of darkness
Cl2 hn ? 2.Cl HOCl hn ? .Cl .OH .Cl O3 ?
.ClO O2 .ClO O ? .Cl O2 Net O3 O ?
2O2 Or 2.Cl 2O3 ? 2.ClO 2O2 .ClO .ClO ?
ClOOCl ClOOCl hn ? .ClOO .Cl .ClOO ? .Cl
O2 Net 2O3 hn ? 3O2
33
Figure taken from Environmental Chemistry 2nd Ed.
By Colin Baird 1999
34
Montreal Protocol

• September 1987 Protocol signed to reduce CFCs by
  50 worldwide
• November 1987 US lawmakers call for new
  negotiations to strengthen the Montreal Protocol
• February 1988 Three US Senators ask Du Pont to
  stop making CFCs (DuPont denied request)
• March 1988 US ratifies Montreal Protocol with a
  unanimous vote
• March 1989 US and European countries develop
  faster reductions of CFC

35
CFC production
36
(No Transcript)
37
Arctic O3

• Arctic is not as cold, but some ice crystals do
  form
• O3 hole is not as severe
• Figure taken in Norway

Figure taken from Environmental Chemistry 2nd Ed.
By Colin Baird 1999
38
CFC replacements

• Replacements contain one H atom
• Easily degraded in troposphere by OH.
• R-134a is commonly used

Figure taken from Environmental Chemistry 2nd Ed.
By Colin Baird 1999
39
(No Transcript)
40
Cool Comfort

• CE News Jan 7, 2008 page 14
• The race to find a replacement auto refrigerant
  heats up in Europe as a Ban on HFC-134A looms
• Heat Challenged The search is on for car air
  conditioner fluids that wont harm the
  environment.
• CO2 may become the replacement refrigerant.