INTRODUCTION

OZONE
1) What is Ozone? 
Ozone is a naturally occurring molecule with the formula O₃.  The structural formula of ozone is:
 

Ozone is a pale blue gas with a characteristically pungent odor.  It is a strong oxidizing agent and more reactive than molecular oxygen.  Ozone is very unstable and hazardous to prepare.

2) Where is Ozone found?
Ozone is located in the troposphere and the stratosphere.  The stratosphere extends from about 15 km to 50 km. The stratosphere is often compared to the "troposphere", which is the atmosphere below about 15 km.  In the troposphere, ozone is a pollutant.  However, in the stratosphere, ozone is critical to the survival of the biosphere.  If stratospheric ozone were brought down to the Earth's surface under STP conditions, the layer would compress to a thickness of 2 to 5 mm.   (click here)

3) Why is Ozone important to the planet?
Ozone protects all life on earth by absorbing harmful ultra-violet radiation. These high energy frequencies, if not absorbed or filtered  by the ozone layer, may cause genetic mutations among lower organisms or melanoma, cataracts, and skin cancer among humans.

4) How is Ozone naturally formed and destroyed in the stratosphere? 
Sunlight plays a key role in the formation and destruction of ozone.  When ultra-violet light (<200 nm) strikes an oxygen molecule, the molecule splits into two oxygen atoms.
 

Each of these free oxygen atoms are then free to combine with either an oxygen molecule (to form ozone) or with oxygen, chlorine, hydrogen, or nitrogen atoms.  (click here for diagram)

When an ozone molecule absorbs sunlight of longer wavelength (200-300 nm), the ozone is destroyed. (click here for diagram)
 

CHLOROFLUOROCARBONS
5) What are CFC's? 
CFC's (ChloroFluoroCarbons) are non-reactive, non-flammable gases or volatile liquids.  They were first created  as refrigerants in 1928 (to replace toxic gases such as sulfur dioxide and ammonia) and produced commercially by DuPont in the 1930s.   CFC's are used in a multitude of commercial applications including refrigeration, air-conditioning, foams, and solvents.

6) What are the types of CFC's? 
CFC's are divided into groups I to V as shown in the following table:
 

7) How do Chlorofluorocarbons destroy stratospheric ozone? (click on steps for diagram)
step 1 Ultraviolet radiation strikes CFC molecule
step 2 Chlorine atom of CFC breaks away
step 3 Chlorine atom collides with ozone
step 4 Chlorine atom breaks up ozone to form ClO and O₂
step 5 Free oxygen atom collides with chlorine monoxide
step 6 Oxygen molecule released.  Chlorine radical repeats cycle.

 
OZONE HOLE
8) What caused the scientific interest in CFC's and Ozone? 
In 1974,  M.J. Molina and F.S. Rowland  linked CFC's with Ozone destruction in the presence of high frequency UV.  Although ground measurements of ozone began in 1956, comprehensive satellite measurements with Nimbus-7 didn't begin  until 1978.   Further studies indicated that within 60 years, the ozone layer would be depleted by approximately  7%.  As a result, the United States banned aerosol CFC's in 1978.   The ozone hole was first detected in 1985 by Farman, Gardinar and Shanklin but further examination of data indicated that the hole was evident as early as 1976.  (click here for table of selected studies on Ozone)

9) What are the common misconceptions about the Ozone Hole? 
a. CFC's are heavier than air so they can  not reach the stratosphere.
CFC's mix in the troposphere and reach the stratosphere due to air circulation.
b. Volcanoes and oceans cause ozone depletion.
Data do not support a connection between volcanic and oceanic activity and ozone depletion.
c. Ozone depletion only occurs above Antarctica.
Ozone levels vary by season and location.  Ozone depletion has been measured everywhere outside of the tropics.
d. There is no correlation between ozone depletion and higher levels of UV.
Only ozone absorbs UVB.  Therefore, as ozone levels decrease, UVB increases.
 
10) Why is the ozone hole more prevalent over Antarctica?
Between September and November,  the circumpolar vortex, a mass of cold air circulating around the polar region, creates stratospheric clouds.  These clouds contain ice crystals to which chlorine and other chemicals adhere.  The combination of the vortex and extremely cold temperatures (-80^oC) is the main reason the ozone layer is thinnest over Antarctica.
 

CLASSROOM APPLICATIONS
This STELLA version of the ozone cycle is appropiate as a teaching tool for modelling in meteorology, earth science, environmental chemistry and environmental science classes at the high school level.  A basic understanding of chemistry will be needed to understand certain aspects of the model.