1. Changes in the Ozone Hole
By Scott Murray, Shahin () and Danielle Lafayette

2. What is Ozone? Also called trioxygen
Less stable than dioxygen (normal oxygen with two atoms)
Toxic in large doses
Formed by diatomic oxygen reacting with UV photons
Destroyed by reacting again with dioxygen
Ozone is also called trioxygen as it consists of three oxygen atoms bound together
It is considerably less stable that normal dioxygen
This is due to the amount of energy in the molecule that is left over after its formation
Ozone is actually toxic to humans and other animals in concentrations as little as .1 to 1 ppm
Ozone is most commonly formed in the atmosphere by diatomic oxygen reacting with ultraviolet photons
The photon breaks apart the dioxygen into 2 separate oxygen molecules
These two molecules then go on to bond with another dioxygen, creating O3.
It can be destroyed by reacting again with dioxygen, breaking it into 2 O2 molecules or by interactions with UV wavelength photons

3. The Ozone Layer Ozone is naturally present in our atmosphere
Around 10% in the troposphere
The remaining 90% in the stratosphere
Between 10 km and 50 km off the surface
This is the “ozone layer”
Relative low abundance of ozone in the stratosphere
Uneven abundance around the globe
Ozone is a gas that is naturally present in our atmosphere.
Ozone is found in two regions of the atmosphere.
About 10% of it is in the troposphere
This is the layer of the atmosphere that is closest to the earth’s surface.
The remaining 90% is in the stratosphere between 10km and 50km above the earth’s surface
This abundance of ozone in the stratosphere is by definition what we refer to as the “ozone layer”
There is actually a relatively low amount of ozone found in the stratosphere.
At its peak concentration there are only around 12,000 ozone molecules for every billion air molecules
The amount of ozone varies depending on several factors including
Stratospheric winds
Formation and destruction of ozone
Seasonal changes
Generally speaking (barring the ozone “hole” over Antarctica) ozone is less abundant around the equator and more abundant around the poles
The Ozone Layer

4. Ozone Layer Chemistry Video here: Click!
This video shows how oxygen atoms in the ozone layer interact with each other to form ozone
You can see that the O3 molecules absorb UV radiation and are broken apart in the process
This interaction with UV radiation is why the ozone layer is so important for life on earth because it helps block it from reaching us

5. What Does the Ozone Layer Do?
The ozone layer helps block UVB radiation
It absorbs around 97-99% of this UV radiation
Also helps moderate day and night temperature swings
The purpose of the ozone layer that is most relevant to humans is its capacity to absorb ultraviolet radiation
Specifically, ozone absorbs most of the Ultraviolet B or medium wavelength light at a range of 320 nm – 280 nm
The ozone layer absorbs a good 97-99% of this light
Ozone is also a greenhouse gas that helps moderate day and night temperature swings

6. What is The Ozone Hole?
The area above the Antarctic where ozone is depleted
Not a total absence of ozone, but a reduction
First discovered in 1985 by British scientists
Is at its worst in the spring of the Southern Hemisphere
The ozone hole is the area in the stratosphere above the Antarctic where the level of ozone has dropped to as low as 33% of their pre-1975 values.
The ozone hole is not a literal gap where there is no ozone present, but rather a drastic reduction in the levels as compared to the rest of the planet’s stratosphere
It was first discovered in 1985 by British Scientists
The depletion is strongest during the spring in the Southern Hemisphere and the ozone hole generally becomes more filled in during the fall.
Pictured: the ozone hole above the Antarctic

7. How the Ozone Hole is Formed
The 5 Step Process:
Emission
Halogen gases are emitted from natural and anthropogenic sources
Accumulation
Halogens accumulate in the troposphere
Transport
Halogens move up to the stratosphere via air currents
Conversion
Halogens interact with UV radiation
Reaction
Causes depletion
The first step in the formation of the ozone hole above Antarctica is the EMISSION of halogen gases by both human activities and natural processes.
These source gases contain the halogens chlorine and bromine
The famous CFC (chlorofluorocarbons) contain chlorine atoms
The second step is the ACCUMULATION of these gases in the troposphere
(Remember that’s the layer closest to the earth’s surface)
At this point the gases are UNREACTIVE
Small amounts of these gases also dissolve and are taken up in ocean waters
The third step is the TRANSPORT of the halogen gases from the troposphere into the stratosphere
This is caused by air currents
Once the halogen gases are in the stratosphere they are CONVERTED into their reactive form by interactions with UV radiation
The newly reactive halogens REACT chemically with the ozone already present to destroy it thus depleting the overall amount

8. The Role of Antarctic Atmospheric Conditions
Atmospheric conditions in Antarctica are such that they enhance the effect of ozone depletion
This is caused by:
Low temperatures that form PSCs
Relative isolation of the atmosphere
So at this point you might ask the question “why is there only a hole over Antarctica if the ozone depleting chemicals are present all over the globe?”
This is because the atmospheric conditions in Antarctica work in concert with the chemical reactions between the halogens and ozone
The low temperatures in Antarctica allow for the formation of polar stratospheric clouds (PSCs)
Chemical reactions on the surface of these clouds are very effective at generating the reactive form of the halogen gases that destroy ozone.
Different conditions in Antarctica allow for the formation of these clouds for more extended periods of time than the other pole
The atmosphere above Antarctica is also very isolated during the winter months
This is caused by the air currents surrounding the pole
Also this effect is more pronounced than in the Arctic