Ozone Depletion

The Ozone “Hole” …What is it?

Schematic of the North-to-South Ozone Depletion: courtesy NOAA equator

Strong circumpolar winds (known as the “polar vortex”) tend to isolate air over Antarctica. Once the air within the polar vortex drops to -80 o C, Polar Stratospheric Clouds (crucial to ozone loss) begin to form.

The annual Antarctic ozone “hole” occurs during the Austral spring when ozone levels fall by over 60% during the worst years. Dobson units

Ozone Concentration (DU) at the Center of the Ozone Hole Data Courtesy geocities.com

Why Should We Care?

Washington Post Writers Group What can we expect?

Consequences from increased levels of ultraviolet radiation Damage to marine life? Cataracts? Crop destruction? Genetic damage?

The Chemistry of Ozone Depletion

Ironically, CFC molecules in the lower atmosphere are protected by the ozone layer and are therefore able to migrate intact up into the stratosphere. Catalytic reactions courtesy NASA

In 1973, Rowland & Molina reasoned that in the stratosphere high-energy photons (220 nm or less) can break the carbon-chlorine bonds of CFC molecules. CCl 3 F + photonCCl 2 F + Cl

Free chlorine atoms react with ozone molecules, taking one oxygen atom to form chlorine monoxide. Cl + O 3 ClO + O 2

When chlorine monoxide encounters a free atom of oxygen, it breaks up the chlorine monoxide and releases the chlorine atom back into the stratosphere to do its “dirty deed.” ClO + OCl + O 2

It is estimated that 1 chlorine atom can destroy over 100,000 ozone molecules. This causes ozone to be destroyed faster than it can be created naturally. When free chlorine atoms eventually react with gases such as methane (CH 4 ) they are bound up into molecules of hydrogen chloride (HCl) which can be carried into the troposphere.

Chlorine Monoxide and the Antarctic Ozone Hole Late August, 1996 Courtesy NOAA

Recipe for Ozone Loss Isolation of air during polar winter Cold temperatures (at or below -80 o C) Polar Stratospheric Clouds Conversion of inactive chlorine & bromine to more active forms Return of sunlight after the polar night