Hemisphere Air Circulation and Ozone
Name: Julie P.
I read somewhere that the air masses in the northern hemisphere do not mix with
the air masses in the southern hemisphere. If that is the case, why is the ozone "hole"
primarily observed over Antarctica when most of the industrial pollution is presumably in
the northern hemisphere?
Although it is true that mixing of northern and southern hemisphere air masses is limited in
the troposphere, except within about 20 degrees of the equator, mixing of air in the
stratosphere is global. Air from the troposphere is mixed into the stratosphere, especially
via thunderstorms, and then is transported globally in the stratosphere. Stratospheric
circulations can be much larger in scale than those in the Troposphere.
It may take months or years before chlorofluorocarbons (CFCs) from, let's say Chicago, reach
the stratosphere over Antarctica,
but they will eventually. The level of CFCs over Antarctica therefore considerably lag the
production at the Earth's surface. Furthermore, removal of these chemicals will take much
longer, as they are very light and not easily broken down.
However, the reason that the ozone "hole" occurs over Antarctica is because of unique
meteorological conditions. A closed circulation sets up over Antarctica for a brief
period (a few months) during southern hemisphere late winter and early Spring. This
circulation allows the CFCs to interact with the same volume of air in the stratosphere for a
fairly long period of time, and thus results in greater opportunity to destroy more ozone
molecules. Ice cloud particles, which are common during this period of time, play a crucial
role in the chemical breakdown of the ozone. The colder the winter, the more ozone is likely
to be destroyed. All of the right conditions are in place at the same time to create the ozone
David R. Cook
Atmospheric Research Section
Environmental Research Division
Argonne National Laboratory
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Update: June 2012