Chapter 17 Section C: Depletion of Ozone in the Statosphere

- Ozone in the stratosphere prevents UV radiation emitted by the sun from reaching Earth's surface
- It is formed by a process which begins with the photodissociation
- Oxygen molecules are broken into two oxygen atoms by solar radiation
- The highly reactive O atoms combine with unbroken O2 to form ozone, O3
- If all stratospheric ozone were compressed into a single layer at STP on Earth, it would only be 3 mm thick
- Certain compounds called chlorofluorocarbons (CFCs) damage the ozone layer and cause it to thin
- CFCs, known by the trade name Freons, include CFCL3, CFCL2, C2F3Cl3, and C2F4Cl2
- They have been used as coolants in refrigerators and air conditioners in place of SO2 and NH3
- They are used in the manufacture of disposable foam products such as cups and plates
- They have also been used as aerosol in spray cans and as solvents to clean circuit boards
- CFCs diffuse unchanged to the stratosphere where UV radiation decomposes them
- A Cl atom is broken off and bonds to an O3 molecule, removing an ozone molecule from the stratosphere

Polar Ozone Holes
- In the mid-1980s evidence began to grow that an "Antarctic ozone hole" developed in the late winter
- In the stratosphere, a stream of air known as the "polar vortex" cricles Antarctica in winter
- Air trapped within this vortex becomes extremely cold during the polar night
- This condition leads to the formation of ice particles known as polar stratospheric clouds (PSCs)
- PSCs provide a surface for reactions converting HCl (from Earth) and ClONO2 to more reactive Cl molecules




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