Ozone Depletion

10.4Chapter 15

Ozone Layer and the UV Spectrum

• UV-A: unabsorbed, starting of skin cancer

• UV-B & UV-C (99% absorbed)– Damage to tissues– Damage to DNA (especially amphibians)

Ozone Depletion in Stratosphere

Ozone Protects earth from UV radiationPart of the electromagnetic spectrum with wavelengths just shorter than visible light

The Ozone Layer is an implicit global ecosystem service

Natural Closed-Loop Cycle

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O2 +UVC →2O

O2 +O→O3

O3 +UVBorC →O2 +O

Anthropogenic Ozone Destroying Compounds

• Chlorofluorocarbons (CFC’s)– Refrigeration– Air conditioners– Aerosol propellants– Blowing agents for foam products (Styrofoam)

CFC’s are stable, inert, nontoxic, nonflammable

• Others• Nitrogen Oxides• Bromines (soil fumigation)• Carbon tetrachloride

Chlorine is a Catalyst

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O3 +Cl→ClO+O2ClO+O→Cl +O2

O3 +O→2O2

1 Cl atom can destroy 100,000 ozone molecules

Time for Math!

A. Suppose that 200 CFC molecules entered the stratosphere. If one chlorine atom destroys 100,000 ozone molecules, how many ozone molecules would be destroyed by 200 CFC molecules?

B. If the same number of CFC molecules entered the stratosphere each year for the next 30 years how many total ozone molecules would be destroyed?

Time for Math

A. 2.0x107 O3molecules/yr

B. 6.0x108 O3 molecules over 30 years

Ozone Depletion in StratosphereOzone thinning/hole

First identified in 1985 over Antarctica

Caused by

human-produced bromine and chlorine containing chemicals

Ex: CFCs

Seasonal Ozone Depletion

• Antarctic• Depletion:

– August-November (late winter-early spring)

Video

Hole over Antarctica Requires Two Conditions

(1)Sunlight just returning to polar region

(2)Circumpolar vortex- a mass of cold air that circulates around the southern polar region

Isolates it from the warmer air in the rest of the planet

Polar stratospheric clouds formEnables Cl and Br to destroy ozone

Why an Ozone Hole?

• Polar winter causes a build-up of ice crystals mixed with nitrogen oxide (NO)

• Ideal surface for Cl2/Br2

• Sun reappears in Spring & UV breaks the Cl2 down again catalyzing ozone destruction

• The “hole” is present in the dark of the polar winter (because it is not being made)

Increased UV-so What!

marine phytoplankton (food webs)

amphibian populationsskin cancer (melanoma)Cataractsdamage to crops & forests

Recovery of Ozone Layer

• Montreal Protocol (1987)– Reduction of CFCs– Started using HCFCs (greenhouse gas)

• Satellite pictures in 2000 indicated that ozone layer was recovering

• Full recovery will not occur until 2050