Stratospheric
Ozone
Depletion
Loss of a Natural Sunscreen
Stratospheric Ozone Layer
• Distribution and Measurement
• Health Effects
UV-A, B, C and Atmospheric Absorption
Skin Cancer and Cataracts
• Chemical Processes
Chapman Cycle
Chain Catalytic Destruction
• CFCs
• Ozone Hole
• Loss Predictions and Mitigation
Ozone LayerStratosphere
Troposphere
Smog
Atmospheric Ozone
Visible UV
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Ozone Measurements
• Ozone varies with altitude and latitude
z
[O3]
Typical [O3]:
Approx. 300 DU
1 Dobson Unit (DU) =
2.7 × 1016 ozone
molecules in this column
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Poleward ozone transport
Circumpolar vortex (Winter)
Ozone accumulates in high latitudes
Ozone-poor air from troposphere near Equator
South Pole TroposphereStratosphere
Absorption of UV
• UV-C: completely absorbed by O2, O3
None reaches ground
• UV-B: partially absorbed by O3
Small amount reaches ground
• UV-A: not absorbed by atmosphere
All reaches ground
• UV-C: completely absorbed by O2, O3
None reaches ground
• UV-B: partially absorbed by O3
Small amount reaches ground
• UV-A: not absorbed by atmosphere
All reaches ground
Transmission of UV
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Health Effects of UV Exposure
• Skin tanning
• Premature aging of skin (by UV-A)
• Skin Cancer
Basal/squamous cell carcinoma
Melanoma
• Eye cataracts, retinal damage
• Damage to plants, microbes, phytoplankton
Basal cell carcinoma
Squamous cell carcinoma
Melanoma
Chapman Cycle: a “null” cycle (no net production nor destruction)
➡ Steady-state [O3]
O2 + (UV-C) ⇒ O + O
O + O2 + M ⇒ O3 + M
O3 + (UV-B) ⇒ O + O2
O + O3 ⇒ O2 + O2
O3 Production
O3 Destruction
Ozone Layer Photochemistry
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Catalytic Ozone Destruction
• Catalyst: substance that promotes a reaction but is not consumed
• Chain Catalytic Destruction Cycle:
X + O3 ⇒ XO + O2
XO + O ⇒ X + O2
Net: O + O3 ⇒ O2 + O2
X ≡ catalyst
Destructive Catalysts (“X”)
• Nitric oxide (NO)
Biogenic N2O from troposphere
NOX from supersonic transports
(SSTs) and spacecraft
• Hydroxyl radical (OH)
From water vapor and organics
Not significant as a source of ozone depletion
• Atomic Bromine (Br)
From methyl bromide (a soil fumigant)
Halon® fire extinguishers
• Atomic chlorine (Cl)
Volcanic HCl (not!)
Chlorofluorocarbons (CFCs) and chlorinated hydrocarbon solvents
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Chlorofluorocarbons (CFCs)
• Inert (non-reactive), non-soluble in troposphere
• Uses: refrigeration/air conditioning, foam blowing agent, inert gas
Troposphere
Stratosphere
1 yr.
10 – 50 yrs.
Diffusion time
• UV-C breaks up CFCs in the stratosphere at about 30 km altitude
Releases atomic chlorine within stratospheric ozone maximum!
z
[O3]
The Ozone Hole
• A sudden, localized, temporary deficiency in stratospheric ozone
• Antarctic hole (South Pole)
Starts in early spring (Sept.), lasts a couple of months (Nov.-Dec,.)
Formation coincides with sunrise after polar winter night
Depletion confined to circumpolar vortex region
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Polar Stratospheric Cloud (PSC)
Sunlight
Ozone Loss Predictions
• Old estimates: about 1 – 5% loss
Natural variability masks loss
Temporary losses due to volcanic activity
Dependent on rate of CFC phaseout
Global warming may extend loss
• Projections: ozone levels stabilize sometime after 2050 or later
Mitigation and Regulation
• CFCs have long atmospheric lifetimes
No short-term solutions
Long-term: find CFC substitutes
• Montreal Protocol (1987): international treaty to phase out CFC usage
Result is earlier-than-expected recovery of ozone layer
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CFC Production
CFC Sales
CFC substitutes
• Hydrochlorofluorocarbons, such as HCFC-22 (Freon®-22, R-22) break down in troposphere
Being phased out, since some still reaches the stratosphere
• Hydrofluorocarbons, such as HFC-134a, contain no chlorine
Current R-12 replacement
Disadvantages: toxic, carcinogenic
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