Environmental Chemistry

Experiment E4

You probably (especially if you've visited experiment E1) know that Ozone, O₃, is a reactive allotrope of Oxygen.  While it is very corrosive and thus toxic to life, it does have beneficial uses in certain controlled situations such as a disinfectant.  Since 1930 it has been known that there is also a natural layer of Ozone high in the Earth's atmosphere, created when high energy radiation with wavelength < 240 nm (recall larger energy bundles have shorter wavelengths) strikes and dissociates O₂.   This high altitude Ozone layer is not harmful to life, but actually provides a beneficial service by blocking much of the high energy ultraviolet portion of sunlight which otherwise would be destructive to life.

Seemingly unrelated is another property shared by all gases:  when compressed their temperature rises, and when allowed to expand and rarify, they cool.  First demonstrated as a method of cooling in 1748 by William Cullen at the University of Glasgow, the cyclic compression and rarefaction was first used in a practical refrigerator built by Jacob Perkins in 1834.  Only a few gases can be easily compressed enough that when allowed to re-cool to room temperature, they also condense to liquids.  If such liquid is then allowed to flow and re-expand in a nearby chamber, it vaporizes, greatly expands in volume, cooling itself and the chamber as well.  Pumped outside the chamber and compressed to liquid again, this gas/liquid transfers heat from inside the chamber to outside.  Unfortunately the few gases which can be easily liquified are either poisonous (such as ammonia,  NH₃) or flammable ( such as ether,  CH₃CH₂OCH₂CH₃).  Following a number of deaths caused by leaking refrigerator gases, Thomas Midgely developed in 1928 the first of several non-toxic, non-flammable alternative refrigerants commonly called Freon.  Because of Freon's safer properties, it has been widely used in refrigerators, air conditioners, and other heat pumps.

About 1970 chemists discovered a chemical mechanism by which Freon might catalyze conversion of reactive Ozone to more stable O₂.  While this might be beneficial at ground level, the immediate worry was that leaked refrigerants might slowly mix in the atmosphere and over many years reach higher altitudes, there reducing the upper atmosphere Ozone.  This in turn could allow increased amounts of harmful ultraviolet light to reach ground level where it could increase skin cancers and destruction of plants.  Once this was verified in 1985, a world wide effort was undertaken to stop manufacturing and using substances found to catalyze Ozone conversion and to find better substitutes.  A protocol was established in Montreal in 1987 to limit production of ozone depleting chemicals.  That agreement seems to be working.  Between 1996 and 2002 the depletion of the ozone layer began to level off at latitudes higher than 40° in both hemispheres.  It is anticipated that the slow natural decomposition of the stable Freon and the re-establishment to traditional concentrations of unstable Ozone in the upper atmosphere may take 20 to 70 years.

There are already excellent resources about stratospheric Ozone available on the Internet which we shall not try to duplicate here.  Visit one of them to learn more about Ozone before proceeding to the experiment.

Dr. Glenn Carver and Owen Garrett at the Centre of Atmospheric Science at the University of Cambridge have developed The Ozone Hole.

A team of scientists at NASA's Goddard Space Flight Center has developed a more thorough electronic textbook on Stratospheric Ozone.

Investigation

Procedure

1. Access a current graph of the minima Ozone concentration over the earth's southern hemisphere at http://jwocky.gsfc.nasa.gov/eptoms/dataqual/ozone_v8.html.  Click on the small graphs to view larger versions.  Patience!  This may take some time.  If you encounter difficulty, you may need to be sure your browser can view png files.
2. When is the concentration lowest (the hole largest)?  Is the minimum always the same time each year?
3. Compare the 1972-1992 average with the current curve.  Is the Ozone depletion more or less now?  Is there still a problem?
4. Compare the current depletion with that a year ago.  Is the problem getting worse or better?
5. Estimate the amount of fluctuation from a smooth curve.  What might cause these short term fluctuations?  Which features are most significant: daily fluctuations, seasonal fluctuations, annual fluctuations?  Why?
6. Investigate what a person can do to help eliminate this problem.  What other substances might also catalyze the conversion of Ozone?

Communicating technical information such as observations and findings is a skill used by scientists but useful for most others.  If you need course credit, use your observations in your journal to construct a formal report.

References

• Glenn Carver, Owen Garrett, Ozone Hole Tour, Centre of Atmospheric Science, University of Cambridge.
• TOMS (Total Ozone Mapping Spectrometer) satellite web site, NASA.
• Team at NASA's Goddard Space Flight Center, Studying Earth's Environment From Space: Stratospheric Ozone, http://www.ccpo.odu.edu/SEES/index.html, Old Dominion University.