The Ozone Balance
Over Earth's lifetime, natural processes have regulated the balance of ozone in the stratosphere.
Scientists are finding that ozone levels change periodically as part of regular natural cycles such as
seasons, periods of solar activity, andchanges in wind direction.Concentrations are also affected by
isolated events that inject materials into the stratosphere, such as volcanic eruptions.
Polar regions reflect the greatest changes in ozone concentrations, especially the South Pole.The
topography of Antarctica is such that a stagnant whirpool of extremely cold stratospheric air forms
over the region during the long polar night.The air stays within this polar vortex all winter, becoming
cold enough to allow the formation of polar stratospheric clouds.
Polar stratospheric clouds speed up the natural process of ozone destruction by providing ice crystal
surfaces on which the destructive reactions take place.After the long polar winter, ozone within this
extremely cold vortex is very vulnerable to the arrivalof sunlight. As spring arrives, major ozone
losses occur. In the southern hemisphere, the area of most severe ozone depletion is localized above
Antarctica and is generally referred to as the ozone hole.Thehole appears in the southern spring,
following the continent's coldest season and polar night.
Ozone depletion over the Arctic is not as well defined as in Antarctica.The rugged topography results
in an air circulation pattern that is quite different from that of the South Pole, but expeditions have
shown that the atmospheric chemistry of the two polar regions is very similar.In the Northern
Hemisphere, the polar vortex is not as strong.It can break up and reform several times during the
course of winter.One air mass after another enters the polar darkness and soon emerges back into low
sunshine. Thus, a bit of ozone is lost from each parcel of air, rather than a large amount from one
parcel as in the southern hemisphere.
The end result is that we are losing ozone in both hemispheres. (26k
jpeg) Ozone levels in the atmosphere have been monitored from the ground since the 1950s and by
satellite since the 1970s.Regional total ozone levels measured from satellites over Antarctica have
decreased 30-50% since their monitoring began.
Since ozone is created and destroyed by solar UV radiation, there is some correlation of ozone
concentration with 11-year sunspot cycles.Sunspots emit high levels of electromagnetic radiation.
The increased UV radiation contributes to ozone production.Sunspot variations only account for 2 to 4
% of the total variation in ozone concentrations.Natural cycles in ozone variation are also associated
with the quasi-biennial oscillation in which tropical winds switch from easterly to westerly every 26
months.This cyclic change in wind direction accounts for approximately 3 % of the natural variation
in ozone concentration.
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