1. "The Antarctic Ozone Hole"
Lesson 2:
"The Antarctic Ozone Hole"
The objectives for this lesson are to introduce the subject of the ozone hole.
Note: A misconception that many Australians have is that the ozone hole is over Australia. This will be discussed in this lesson.
Ozone depletion module prepared by Eugene C. Cordero

2. Learning objectives
To develop an understanding for the history of ozone depletion
To develop an understanding for what the ozone hole is.
To understanding how the ozone hole is produced.
Ozone depletion module prepared by Eugene C. Cordero

3. Early concerns about ozone depletion
History of ozone depletion.
The Concorde supersonic aircraft was originally built in the 1970’s to enable quick transport between Europe, the US and Asia (and someday Australia!). There were later proposals to produce a large fleet of these aircrafts. Because of the high speeds of these plans, they must fly high up in the atmosphere, where ozone concentrations are largest (note: normal aircraft fly at say 35,000ft (12km). Supersonic aircraft fly upward of 60,000 ft or near 20 km in altitude.
Early concerns arose that air exhaust from the Concorde may directly influence ozone levels. Current research now concludes that while supersonic aircraft do have an adverse affect on the ozone layer, it may not be that large. Regardless, this is what started scientist looking at the possibility of humans altering the ozone layer.
Ozone depletion module prepared by Eugene C. Cordero

4. History of Ozone Depletion
CFCs developed in 40’s and 50’s
Refrigerants, propellants, fire retardants
1970’s CFCs detected in atmosphere.
Many of these have long atmospheric lifetimes (10’s to 100’s of years)
1974 Rowland and Molina propose that CFC’s can destroy ozone in the stratosphere.
CFCs broken apart by UV radiation forming chlorine which can destroy ozone quickly:
O3 +Cl  ClO+ O2 (Catalytic Reaction)
ClO+O  Cl+O2
(advanced)
CFC’s are a class of chemicals that were developed in the 40 and 50s for many valuable applications including refrigerants, propellants (for spray cans) and fire retardants. One of the most remarkable and extremely valuable characterizes of CFC’s is that they do not react anything. Therefore, if you have some type of fire, you can safely use CFC’s to extinguish the fire, not worrying if your fire retardant is going to enhance the fire.
This characteristics of CFC’s also means that many CFC’s remain in the atmosphere for many years (10’s to 100’s). They have long atmospheric lifetimes. Therefore, if you release a CFC molecule into the atmosphere, it may bounce around in the atmosphere for many many years.
In 1974, two scientist (Rowland and Molina; who later received the Nobel Prize in chemistry) suggested that because CFC’s are around for a long time, they may eventually reach the upper atmosphere, where they can be broken apart by the sun. CFC’s contain chlorine, a chemical that can rapidly destroy ozone under certain conditions, and in some cases, a single chlorine atom may destroy thousands of ozone molecules through a self replicating cycles (catalytic reaction).
The catalytic cycle is self replicating…ozone gets converted by chlorine, producing ClO and more oxygen. The ClO then reacts with a free oxygen atom, thereby producing another Cl atom. The cycle can then continue again.
Ozone depletion module prepared by Eugene C. Cordero

5. History of ozone depletion
1970’s Supersonic Aircraft fleet under review.
1978: CFCs used in aerosols banned.
1985: British Antarctic Survey reports 40% loss of ozone over Antarctica during spring. (NASA confirms)
1987: Signing of the Montreal Protocol
International agreement to reduce CFC use
Later agreements agreed to completely phase out CFC and halons.
1996 Complete ban on industrial production of CFCs goes into effect.
After a period of intense scientific study, it was determined that indeed, CFC’s could be a large potential threat to our ozone layer. In 1978, CFC’s in aerosols (spray cans) were banned in the US, as pressure mounted. In 1985, the British Antarctic Survey reported that ozone levels over Antarctica were 40% below normal. The ‘discovery’ of the ozone hole, as we call it, helped drive the signing of the Montreal Protocol, and international agreement to reduce and eventually phase out CFC use worldwide. Later amendments to the Montreal Protocol agreed to a complete ban on these chemicals; at present, the industrialized nations no longer produce CFC’s, while developing nations have till 2010 to phase out their use. Presently, Australia, USA, Europe are using HCFC’s for auto refrigerants, a more ozone friendly gas. These chemicals will also be phased out in time, along further replacement gases to come into use.
Ozone depletion module prepared by Eugene C. Cordero

6. The Ozone Hole
First discovered in 1985: observations from Antarctica extend back into 1950’s.
Characterized as a rapid depletion of ozone over Antarctica during spring.
Ozone hole season, Spring (August – October)
Ozone hole located over mainly over Antarctica.
Ozone hole recovers by late December
Ozone hole caused by human chemicals (CFC’s)
Ozone hole not present in early 1970’s
Observations over Antarctica data back to the 1950’s. The discovery of the ozone hole occurred in It turns out that the research group measuring ozone had seen a sharp decline in ozone levels during October over Antarctica (see next overhead), but didn’t report it immediately because they worried that their instrument might be faulty. Finally, after a few years of testing, they published a paper documenting this rapid decline of ozone levels. However, at the time, there was no explanation for why ozone was declining.
During that same time period, NASA was also observing ozone levels globally. However, they didn’t expect to see signs of ozone depletion over Antarctica, and consequently, ‘missed’ the ozone hole due to a computer glitch. When they realized their mistake, and fixed up the computer problem, they also saw the clear signs of an emerging hole in the ozone layer.
The ozone hole is characterized by a rapid depletion of ozone over Antarctica during the springtime.
The science of the ozone hole was well understood by the early 1990s.
Ozone depletion module prepared by Eugene C. Cordero

7. Total Ozone: October 14, 1997 (Key Learning Figure)
Here is an example of the ozone hole on October 14, This graphic is a map of the Southern Hemisphere (see Australia etc.) where the colors represent values of total ozone given in Dobson Units. Dark colors, purple and dark blue represent low values of ozone, while red and white colors represent high values of total ozone.
On this particular day, the ozone hole was mainly over Antarctica, and over the tip of South America. In a manner similar to how the winds move clouds around the atmosphere, winds in the atmosphere also can move ozone around. So, every day a map of the ozone layer looks slightly different, as you will see in the animation. However, the ozone hole normally stays over Antarctica, and never comes over Australia.
Note: Total ozone is the amount of ozone from the ground to the top of the atmosphere. Total ozone, or sometimes called, column ozone is measured in Dobson Units (DU). Total ozone values below 220 DU are what we call the ozone hole.
Ozone depletion module prepared by Eugene C. Cordero

8. Where is the ozone hole?
Ozone hole largely restricted to areas over Antarctica
Ozone hole may pass over tip of South America
Ozone hole seldom comes near Australia.
Northern Hemisphere (NH) has no ozone hole like the Southern Hemisphere
However, recently NH has experienced the occurrence of ozone ‘mini holes’.
Summary of where the ozone hole is.
Note:
The ozone hole occurs over Antarctica, but not over the Arctic. The reason for this is that temperatures over the Arctic don’t get as cold as they do over Antarctica (remember our ozone hole recipe). However, in the last half decade or so, there have been periods during Northern Hemisphere winter when temperatures have been cold enough to allow chlorine chemistry and thus destroy ozone. These episodes are limited to small geographical locations which are nowhere as large as the Antarctic ozone hole.
A question often asked is why temperatures over Antarctica are so much colder than over the Arctic. The simple answer has to do with the larger amount of land mass located over the Northern Hemisphere. Because the Northern Hemisphere has so much land mass, and mountains (Rocky Mountains and Himalayas), strong weather disturbances do not allow temperatures to get so low during winter. In contrast, the Southern Hemisphere has a large amount of ocean at higher latitudes, and winter weather disturbances are not so strong. Thus the Antarctic region gets much colder than the arctic.
As we will see in our ozone hole recipe (coming up) , cold temperatures are one of the requirements for an ozone hole
Ozone depletion module prepared by Eugene C. Cordero

9. Satellite Total Ozone from 70’s and 90’s
(Key Learning Figure)
These are satellite maps of total ozone over the Southern Hemisphere for eight different years. (recall: the larger Total Ozone (Dobson units) the less UV radiation that hits the earth.
In the 1970’s ozone levels over the Southern Hemisphere were much higher than in the 1990s. For example, the red colors, often seen during the 1970’s indicate ozone values about 400 Dobson units. In the 1990, blue and purple colors over Antarctica, which represent ozone levels below 200 Dobson units, can be seen. The large difference between the 70’s and 90’s is clearly apparent.
Ozone depletion module prepared by Eugene C. Cordero

10. Ozone observations over Antarctica
This plot shows ozone values over Antarctica for October from the 50’s through the 90’s. The dramatic change in ozone values can be seen from the mid 1970’s through the 90’s. Ozone values below 220 Dobson Units, are termed the ozone hole. Different colors on the plot indicate different measurements techniques, either from balloon measurements (yellow) or from satellite.
Ozone depletion module prepared by Eugene C. Cordero

11. Is the ozone hole getting worse?
(Key Learning Figure)
Many students (and the public) have the perception that the ozone hole (and ozone in general) is getting much worse. This is likely due to press reports that usually say, ‘the ozone hole reached it’s largest size…’ The reality is that in the last decade, the size and severity of the ozone hole have stayed about the same, compared to rapid changes seen in the middle 1980’s.
Year to year variations in the depth and severity of the ozone hole is really a function of the atmospheric weather conditions (I.e. the colder the winter, the more greater the ozone loss). In addition, the ozone hole is about as bad as it can get. Between km, all the ozone is essentially destroyed during spring over Antarctica, so it couldn’t get much worse. Thus, we don’t expect the size and depth of the ozone hole to change dramatically in the future. Rather, it is expected that the size of the ozone hole is about as big as it’s going to get, and with time (10 or more years) and reduced chlorine levels, we should start to see the size of the ozone hole get smaller.
Thus, this graphic is intended to illustrate how in the 80’s, the size of the ozone hole increased rapidly, while in the 90’s and 2000, things stabilized.
Ozone depletion module prepared by Eugene C. Cordero

12. Animation (Key Learning Figure)
Note: to see animation, you must be in Slide Show mode.
This is an animation of Southern Hemisphere ozone values from August through December. The colors indicate the total ozone given in Dobson Units (DU). Purple and blue indicate low values of ozone, while yellows and reds indicate high total ozone. The date is given at the top of the animation, ranging from Aug 1, 1997 through Nov 28, 1997.
Teacher Note: Interpreting the satellite data
This graphic shows real data from the Total Ozone Mapping Spectrometer (TOMS). TOMS is a NASA satellite dedicated to measuring total ozone on earth. The funny patterns of black (lightning-like) shading are actually areas where the satellite did not measure anything. TOMS flies in a polar orbit, taking 16 orbits per day. Those black strips are where the satellite didn’t take measurements, and thus there is no data.
In addition, in August, there is a big black circle over Antarctica. Gradually with time, this black hole gets smaller and smaller. No, this is not the ozone hole, but rather areas where TOMS couldn’t measure ozone. That is because, TOMS needs sunlight to measure total ozone. On Aug 1st, Antarctica was completely dark 24 hours per day, but as spring approaches, the sunlight gradually returns to the polar region. You can see this in the animation.
Therefore, students should try to ignore the black shading, and focus on the the blue and purple shading that appears in September and October. This changing circular-like shape is our ozone hole, which changes day to day, but stays concentrated over the pole. You may note again that the ozone hole never comes over Australia. However, students will see that the tip of the ozone hole does go over the southern tip of South America for a day or so a few times in Sep and Oct. Researchers in Ushuaia, Argentina (at the southern tip of South America) take measurements of ozone and UV to study the ozone hole.
The Australian continent is barely visible in the lower left part of the globe.
Ozone depletion module prepared by Eugene C. Cordero

13. Ozone Hole Recipe Ingredients: Chlorine gas Cold Temperatures (~-80C)
Instructions:
Allow cold temperatures to form Polar Stratospheric Clouds (1-2 weeks).
Allow time for polar stratospheric clouds to convert chlorine gas into ozone destroying chemicals. (1 month)
Bake ingredients with sunlight.
Presto, a delicious ozone hole!
Science interpretation
Chlorine gas is abundant in atmosphere due to CFC’s
Cold Temperatures (~-80C) only occur over Antarctica during the cold winter.
Polar Stratospheric Clouds allow ozone friendly chlorine to be transformed into ozone destroying chlorine.
Ozone depletion then starts when sun returns to Antarctica in the spring
Ozone hole grows from late August through till October.
We can explain what causes the ozone hole by considering an ozone hole recipe!
To make an ozone hole, you need three things.
Abundant chlorine (chlorine gas)
Cold temperatures (lower than 185K)
Sunlight to start chemistry.
Recall that to destroy ozone, you need chlorine. Well, chlorine comes from CFC’s, and measurements from satellites show us that there is plenty of chlorine in the atmosphere. (Note: CFC’s contain chlorine, but normally they are not reactive, or CFC’s don’t react with any other chemical. However, once CFC’s reach the upper atmosphere, high energy sunlight can break up the CFC’s, releasing chlorine. )
We then need very cold temperatures to allow a special type of chemistry to proceed (called, heterogeneous chemistry). If you have these very cold temperatures, then this special type of chemistry can happen. This is similar to the mixing of a cake. Put ingredients together in a bowl, and you are ready to bake a cake. In our case, you take Chlorine and cold temperatures together and you are ready to have an ozone hole.
However, just like baking a cake, the final thing you need to do, after mixing the ingredients, is to put the cake into an oven. For our case, the final thing that needs to happen is we need sunlight to start the ozone depletion.
So, during winter over Antarctica, there is plenty of chlorine around in the atmosphere (Satellite measurements show this) and temperatures get very cold, thus producing this special type of chemistry. Recall that at this time of year, there is no sunlight, it is dark 24 hours per day. In Spring (August through October), when the sunlight returns, ozone is destroyed very quickly (just like putting the cake into the oven). This produces very low values of total ozone, and we get the so called, ozone hole.
Ozone depletion module prepared by Eugene C. Cordero

14. Everywhere in Atmosphere
Ozone Hole Formation
Only during winter
Cold Temperatures
T~-80C
Only during spring
produces
Ozone
destroying
chemicals
Sunlight
Polar
Stratospheric
Clouds
and
produces
produces
and
(Key Learning Figure)
A graphical description of the ozone hole formation.
Chlorine gas
Ozone
Hole
Everywhere in Atmosphere

15. Summary The Ozone hole develops during spring over Antarctica.
The Ozone hole is produced by unique combination of weather (cold temps) and chemistry (chlorine).
Global ozone trends are negative except in the tropics.
Ozone depletion module prepared by Eugene C. Cordero