Background

Stratospheric ozone
          Most of the naturally occurring ozone could be found in the stratosphere (from 10 km –12 km to 50 km), which contains about 90% of the atmospheric ozone. Oxygen is the major source of ozone in the atmosphere. The ozone production from oxygen is illustrated in Figure 1.

Ozone depletion
          The ozone depletion is caused by CFCs (Chlorofluorocarbons - a compound consisting of chlorine, fluorine, and carbon) and other ozone-depleting substances (ODS) emitted into the atmosphere (Figure 2). Winds mix the air and evenly distribute these gases in the troposphere. However, CFCs are extremely stable and do not dissolve in rain. After several years, CFC and ODS molecules reach the stratosphere. Strong UV light breaks apart the ODS molecule. CFCs, HCFCs (hydrochlorofluorocarbons), carbon tetrachloride, methyl chloroform, and other gases release chlorine atoms, and halons and methyl bromide release bromine atoms. It is these atoms that actually destroy ozone, not the intact ODS molecule. It is estimated that one chlorine atom can destroy over 100,000 ozone molecules beforeit is removed from the stratosphere!

Figure 1. Stratospheric ozone production
(From: http://www.atmos.umd.edu/~owen/CHPI/IMAGES/ozonepro.html)
                                                                  

Figure 2. The ozone depletion scheme (From: http://www.epa.gov/ozone/science)

Ozone hole
          Each spring (in the Southern hemisphere - from September through November) the stratospheric ozone over Antarctica is rapidly destroyed by chemical processes, and thus the ozone hole is formed. The ozone hole was first discovered in 1985 by British scientists J. Farman, B. Gardiner, and J. Shanklin of the British Antarctic Survey. It is said that when the first measurements were taken in 1985, the drop in stratosphere ozone levels was so dramatic that at first the scientists thought their instruments were faulty. Replacement instruments were built and flown out, and it wasn't until they confirmed the earlier measurements, several months later, that the ozone depletion observed was accepted as genuine.

          Another story goes that the TOMS satellite data didn't show the dramatic loss of ozone because the satellite software processing the raw ozone data was programmed to treat very low values of ozone as bad readings! Later analysis of the raw data, when the results from the British Antarctic Survey team were published, confirmed their results and showed that the loss was rapid and large-scale; over most of the Antarctica continent (http://www.atm.ch.cam.ac.uk/tour/part1.html).

          The ozone hole is defined geographically as the area wherein the total ozone amount is less than 220 Dobson Units (http://www.theozonehole.com). Figure 3 below shows the Antarctic ozone hole measured by OSIRIS in 2002.

Figure 3. Antarctic ozone hole measured by Odin/OSIRIS on October 4-5, 2002.
I plotted this Figure back in 2004 using the software provided by
 Canadian OSIRIS Team members

Ozone Units
          The ozone concentration at a certain height is usually measured in Number Density as the number of molecules per cubic centimeter (mol/cm3). This is how the OSIRIS ozone data are provided. However, for studying the total stratospheric ozone amount it is more convenient to use Dobson Units (DU).

          G.M.B. Dobson (~1920 – 1960) was one of the first scientists who studied atmospheric ozone. If a column of air, 10 deg x 5 deg, extending from the earth surface to the end of the stratosphere (over any point on Earth) were to be compressed to standard temperature and pressure (STP) (0 degrees Celsius and 1 atmosphere pressure) and distributed evenly over the area, it would form a slab ~3 mm thick, which is defined as 300 DU. Consequently, 1 DU is a slab 0.01 mm thick, with a perimeter 10deg x 5deg, at STP.

Figure 4. Dobson Units (DU) (From: http://jwocky.gsfc.nasa.gov/dobson.html)

 

The Odin satellite
          “Odin is a Swedish-led international satellite mission involving Canada, Finland and France to address a number of important issues in astronomy and atmospheric sciences. The two instruments designed to achieve the scientific goals of the mission are the Canadian Optical Spectrograph and InfraRed Imaging System (OSIRIS) and the Swedish Sub-Millimeter Radiometer (SMR).” (From the interview with Prof. E.J. Llewellyn). Odin was launched on February 20, 2001, and since November 2001, OSIRIS operates in the normal regime and provides high quality datasets. The difference between Odin and other satellites that perform atmospheric measurements is the approach to ozone observation. “The Odin approach is to use scattered sunlight to act as a source against which we can measure ozone. As the sunlight travels through the atmosphere so it acquires the signature of the ozone column that it passes through and by making observations of the Earth's atmospheric limb we can recover this ozone height distribution. In this way Odin provides a global height profile map in 1 day. Here Odin/OSIRIS is leading the world.” (Prof. E.J. Llewellyn, http://www.space.gc.ca/asc/eng/apogee/2003/11_osiris.asp).

 

OSIRIS ozone data
          Optical Spectrograph and InfraRed Imager System (OSIRIS) provides stratospheric ozone profiles since November 2001. These profiles are given on a 2 km vertical grid between 10 and 48 km. An example of the OSIRIS and coincident ozonesonde profiles of the Antarctic ozone hole is shown in Figure 5.

Figure 5. OSIRIS and ozonesonde measurements of the ozone hole on 27.10.2002
(This plot was given to me by OSIRIS Team members)

          Two versions of OSIRIS ozone are presently available for use: version 1.2 covers the period from November 2001 until January 2005; version 2.4 covers the period from August 2004 until June 2005. I use version 1.2 which has longer time coverage.