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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