Stratospheric Chemistry The Ozone Layer

1
Stratospheric Chemistry The Ozone Layer

• Chapter 2, Baird

2
Introduction The Ozone Layer

• Earths natural sunscreen
• Filters harmful UV rays before they reach us
• Appearance of a large hole over Antarctica in
  mid-1980s
• www.

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Ozone, O3

• Gas present in small conc throughout atmosphere
• Total overhead amount expressed in terms of
  Dobson Units
• 1DU 0.01 mm thickness of O3 when brought to
  ground level(1 atm) at 0oC
• Normal amount is 350 DU at temperate latitudes
  (Thus, only a 3.5 mm thick )
• RQ 2,3 AP 1 (Let Vol 1.8x1015L)

4
Ozone, O3

• O3 conc varies seasonally.
• Highest levels in early spring.
• Lowest levels in the fall.

5
Ozone Hole

• Began to notice in the late 70s over Antarctica.
  By 1980s there was a 50 drop from normal
  values.
• See Fig 2-2

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Minimum Concentration of Total Overhead O3 above
Antarctica in Sept-Oct of Recent Years-Fig 2-2
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Variation with Year of the Size of the Ozone
Hole-Now about the Size of North America
8
Total Overhead O3 for NonPolar Regions(60oN-60oS)
Showing Some WorldWide Depletion
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Chapter 2 Topics Emphasized

• The Production of the Ozone Layer
• How is it created and sustained?
• Ozone Depletion
• By what mechanism is it being depleted?

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Regions of the Atmosphere

• Earths atmosphere has evolved with time.
• (Early atmosphere high conc of CO2)
• Major change caused by photosynthesis
• 6CO2 6H2O ? C6H12O6 6O2

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Atmospheric Composition

• Present Composition
  N2 78
• O2 21
• Ar 1
• CO2 0.04
• H2O variable

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Atmospheric Layering

• Thermosphere (above 90 km)
• 90 km------------------Mesopause
• Mesosphere(50 90 km)
• 50 km-------------------Stratopause
  
• Stratosphere(15 50 km)
• 15 km-------------------Tropopause
• Troposphere (See Fig 2-5)

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Atmospheric Layering

• Slope of curve of temp vs altitude is lapse
  rate
• See Fig 2-5
• RQ 4

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Atmospheric Layering PicturedConceptsLapse
Rate, Temp. Inv.RQ 5
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Atmospheric Temperature Inversion

• This is a change from a neg. to a pos. lapse
  rate. The point at which the lapse rate changes
  marks a stable boundary between two physically
  distinct layers of air.
• Warm air over Cooler air.

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Atmospheric Temperatures Governing Factors

• Absorption of Solar Energy
• Peak intensity in Visible Region
• Heat Production inside Earth
• Radioactive decay
• Energy Emission from Earth
• Blackbody radiation in IR Region

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Environmental Concentration
Units for Gases

• Usually Pollutant gases have their conc.
  Expressed as either ppm or ppb.
• Assuming PVnRT, Pressure, Volume, and Mole
  Fractions are all the same. Thus,
• ppm is also ppmv.

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Chemistry of the Ozone Layer

• Light Absorption by Molecules
• Principles of Photochemistry

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The Electromagnetic Spectrum

• With Ranges of Environmental Interest (RQ5)

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Absorption Spectrum of O2

• Far UVphotons of the sun are absorbed by O2
• This causes the Photolysis
• O2 UV(?lt241nm)?2O
• Do a Calculation to prove this, if OO Energy is
  495 kJ/mol
• RQ7,8,9 2-1,2-2,2-3

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Results of O2 PhotolysisIn and Above
Stratosphere

• This filters all of the UV light from 120-240
  nm. This also filters some, but not all UV in the
  220-240 nm range
• No UV with ?lt220 nm reaches Earths
  surface(Protects life from UV damage)
• Now O3 can form.

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The Creation and Noncatalytic Destruction of Ozone

• In the Stratosphere, conc of O2 molecules is
  relatively large and conc of atomic O is
  relatively small.
• Thus, O3 can form via
• O O2 M ? O3 M 105 kJ
• Above reaction heats Stratosphere
• What role does M play?

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The Creation and Noncatalytic Destruction of Ozone

• Ozone does efficiently absorb UV light with
  ?lt320. (See Fig 2-8 next slide)
• It then dissociates
• O3 UV(?lt320nm)?O2 O
• The reaction O3 O?2O2 does occur somewhat but
  is slowed by a high activation energy.

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Absorption Spectrum of O3
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Solar UV Intensity Outside Atmosphere and at
Earths Surface
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Creation and Noncatalytic Destruction of O3 The
Chapman Cycle
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Chapman Cycle Condensed

• O
• ________________________
• ? UV-C O2
  ?
• O2 --------------?O-----------?O3
• ???
  _________??
• UV-B