2st PART


Stratospheric Ozone Layer

The ozone layer is a layer of ozone particles dispersed between 11,80 and 18,64 miles (19 and 30 km) altitude in the stratosphere. The concentration of ozone in the ozone layer is usually below 10 parts per million ozone. The ozone layer is essential for life on Earth as it absorbs ultraviolet radiation (UV) from the Sun.

The unique physical properties of ozone allow the ozone layer to act as a sunscreen for our planet, providing an invisible filter to help protect all forms of life against danger ultraviolet rays. Most UV radiation entering the atmosphere is absorbed by ozone and prevented from reaching the Earth's surface.

Ozone is created in the stratosphere over the tropics and stratospheric winds transport around the Earth. Ozone (O3) is composed of three oxygen atoms (O). The ozone molecules are able to absorb ultraviolet rays and they decompose in oxygen (an oxygen molecule (O2)) and a free oxygen atom (O). when the solar radiation strikes an oxygen molecule (O2) that absorbs UV rays and creates two oxygen atoms (O) by dividing into two. If it comes in contact with another molecule of oxygen (O2), so they can regenerate a molecule of ozone (O3). This process is known as photolysis. Ozone is also naturally broken down in the stratosphere by sunlight and by a chemical reaction with various compounds containing nitrogen, hydrogen and chlorine. These chemicals occurs naturally in the atmosphere in very small amounts. Volcanic eruptions can alter the amount of ozone in the atmosphere as was the case during the eruption of Mount Pinatubo and Hudson, 1991. Then it's the same for the solar activity influence on concentration of stratospheric ozone.

The anomaly of the stratospheric ozone about latitude since 1978. Source NOAA

The Hole in the Ozone Layer

The diminishing ozone layer occurs when the natural balance between the production and destruction of stratospheric ozone is tilted in favor of the destruction. Although natural phenomena can cause temporary ozone loss, chlorine and bromine released from man-made compounds such as CFC are now accepted as the main cause of this depletion.

The "hole" in the ozone layer is not a hole in any real sense of the word, but a thinning of the amount of ozone in the stratosphere (From 11 to 18,64 miles (19 to 30 kilometers) above the surface of the Earth) and mostly over the Antarctic and Arctic during the end of the winter. However, in some regions, the thinning of the amount of ozone is more important than the other and it is in these areas where the term "hole in the ozone layer" is strictly used.

During the spring in the Southern Hemisphere, there is an ozone depletion in the stratosphere over Antarctica, with losses up to 60 %. This is because each winter the polar vortex isolates the air located in polar latitudes of the rest of the Earth's atmosphere and at very low temperatures in winter (below -112°F (-80°C)) because it's dark permanently. When it is very cold during the Polar Nights, PSC (Polar Stratospheric Clouds) form in the polar stratosphere. And as the sunlight returns to the pole during early spring then by the action of ultraviolet (UV) while the PSC that formed the winter destroys ozone and is not replaced as this season there is not new supply of atmospheric ozone. It is at the end of the spring atmospheric circulation changes, and the ozone that comes from the tropical atmosphere replenishes the polar atmosphere. The hole in the ozone layer over Antarctica is formed every year since the early 1970s.

Since 1970 the hole has grown, in the sense that more and more ozone is destroyed. But from the observations made by the SAGE and HALOE satellites, it was found that the rate of decrease of the ozone layer in the upper atmosphere has been reduced since 1997.


From the graph above, we have the data of two various instrumentations aboard the same satellite.
The total of the ozone is represented by a deformed column and of shaded off color.
The profile of the ozone is converted in volume to obtain a cutting of surface of pressure
by latitude. Both observable data concern the southern hemisphere, they
appear in a polar spelling projection, on a superficial topographic support.

A video giving an explanation of the ozone layer
Winters with low temperatures in Antarctica favor the concentration of these molecules in the atmosphere inert form. But as soon as the sunlight starts, with the action of UV rays, chlorine becomes active and reacts with ozone molecules. There is a catastrophic chain reaction : a single chlorine atom can destroy released 100.000 ozone molecules. However, the stratospheric ozone layer is our only protection from ultraviolet radiation.

The ozone hole in the southern hemisphere from
1979 to October 2010.
Source NASA


The ozone hole in the northern hemisphere
from 1979 to March 1997.

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