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Ozone Depletion
Ozone (O3) occurs
naturally in the stratosphere because of intense ultra-violet (UV) radiation
on oxygen (O2) molecules. UV radiation also destroys ozone by the Chapman process,
but the balance of production and destruction leads to a constant level of ozone
(in the absence of CFCs and chlorine) that is much higher than in the
troposphere.
Figure 1 is the first published evidence of major ozone depletion over Antarctica.
This graph gives spring (in the Southern Hemisphere) ozone concentrations, in Dobson units, over a 30-year period. In the 1950's the ozone concentration was about 320 Dobson units but has dropped dramatically in recent years to about 200 as shown on this graph. In fact in more recent years it has dropped to 100 averaged over a fairly large area, and in some places it has dropped completely to zero - no ozone at all.
This peculiar behavior in ozone concentrations was not limited to Antarctica, but the effect was most pronounced in that region. Figure 2 gives observations from Switzerland plotted as the difference in average ozone levels between the period 1931-1969 and 1970-1986.
The graph shows that the reduction in ozone over this location in recent years is most pronounced in the winter months.
Scientists became concerned about the trend of these observations, but thought that some unaccounted-for drift or other problem had developed with their instruments. It wasn't until surface measurements (see photo of instrument) were compared with satellite measurements and observations were compared from different locations that it was realized that ozone concentrations were significantly depleted at certain times and in certain places.
Figure 3 shows data taken more recently from a vertical profile of partial pressure of ozone, in nanobars, as a function of height.
Recall that the troposphere extends from the surface to about 10 kilometers and the region beyond it is the stratosphere, which continues up to about 50 kilometers. These ozone measurements were taken in the lower stratosphere over the South Pole during the Southern Hemisphere spring in 1992. It is apparent from these plots that ozone concentrations drop dramatically over a very short period near the end of September to readings of essentially zero in the region around 15 to 20 kilometers above sea level. Complete destruction of the ozone layer occurred over a fairly narrow vertical range. Measurements of total ozone in vertical columns of the atmosphere are made by the Total Ozone Mapping Spectrometer (TOMS).
NEXT: Reactions and Compounds that Destroy Ozone
